Lectures 10_11_12_13.pdf

Chapter 10 | Cell Reproduction 279 10 | CELL REPRODUCTION Figure 10.1 A sea urchin begins life as a single diploid cell (zygote) that (a) divides through cell division to form two genetically identical daughter cells, visible here through scanning electron microscopy (SEM). After four rounds of cell division, (b) there are 16 cells, as seen in this SEM image. After many rounds of cell division, the individual develops into a complex, multicellular organism, as seen in this (c) mature sea urchin. (credit a: modification of work by Evelyn Spiegel, Louisa Howard; credit b: modification of work by Evelyn Spiegel, Louisa Howard; credit c: modification of work by Marco Busdraghi; scale-bar data from Matt Russell) Chapter Outline 10.1: Cell Division 10.2: The Cell Cycle 10.3: Control of the Cell Cycle 10.4: Cancer and the Cell Cycle 10.5: Prokaryotic Cell Division Introduction A human, like every sexually reproducing organism, begins life as a fertilized egg (embryo) or zygote. In our species, billions of cell divisions subsequently must occur in a controlled manner in order to produce a complex, multicellular human comprising trillions of cells. Thus, the original single-celled zygote is literally the ancestor of all cells in the body. However, once a human is fully grown, cell reproduction is still necessary to repair and regenerate tissues, and sometimes to increase our size! In fact, all multicellular organisms use cell division for growth and the maintenance and repair of cells and tissues. Cell division is closely regulated, and the occasional failure of this regulation can have life-threatening consequences. Single-celled organisms may also use cell division as their method of reproduction. 10.1 | Cell Division By the end of this section, you will be able to do the following: Describe the structure of prokaryotic and eukaryotic genomes Distinguish between chromosomes, genes, and traits Describe the mechanisms of chromosome compaction The continuity of life from one cell to another has its foundation in the reproduction of cells by way of the cell cycle. The cell cycle is an orderly sequence of events that describes the stages of a cell’s life from the division of a single parent cell to the production of two new genetically identical daughter cells. 280 Chapter 10 | Cell Reproduction Genomic DNA Before discussing the steps a cell must undertake to replicate and divide its DNA, a deeper understanding of the structure and function of a cell’s genetic information is necessary. A cell’s DNA, packaged as a double-stranded DNA molecule, is called its genome. In prokaryotes, the genome is composed of a single, double-stranded DNA molecule in the form of a loop or circle (Figure 10.2). The region in the cell containing this genetic material is called a nucleoid. Some prokaryotes also have smaller loops of DNA called plasmids that are not essential for normal growth. Bacteria can exchange these plasmids with other bacteria, sometimes receiving beneficial new genes that the recipient can add to their chromosomal DNA. Antibiotic resistance is one trait that often spreads through a bacterial colony through plasmid exchange from resistant donors to recipient cells. Figure 10.2 Prokaryotes, including both Bacteria and Archaea, have a single, circular chromosome located in a central region called the nucleoid. In eukaryotes, the genome consists of several double-stranded linear DNA molecules (Figure 10.3). Each species of eukaryotes has a characteristic number of chromosomes in the nuclei of its cells. Human body (somatic) cells have 46 chromosomes, while human gametes (sperm or eggs) have 23 chromosomes each. A typical body cell contains two matched or homologous sets of chromosomes (one set from each biological parent)—a configuration known as diploid. (Note: The letter n is used to represent a single set of chromosomes; therefore, a diploid organism is designated 2n.) Human cells that contain one set of chromosomes are called gametes, or sex cells; these are eggs and sperm, and are designated 1n, or haploid. Upon fertilization, each gamete contributes one set of chromosomes, creating a diploid cell containing matched pairs of chromosomes called homologous (“same knowledge”) chromosomes. Homologous chromosomes are the same length and have specific nucleotide segments called genes in exactly the same location, or locus. Genes, the functional units of chromosomes, determine specific characteristics by coding for specific proteins. Traits are the variations of those characteristics. For example, hair color is a characteristic with traits that are blonde, brown, or black, and many colors in between. This OpenStax book is available for free at http://cnx.org/content/col24361/1.8 Chapter 10 | Cell Reproduction 281 Figure 10.3 There are 23 pairs of homologous chromosomes in a female human somatic cell. The condensed chromosomes are viewed within the nucleus (top), removed from a cell during mitosis (also called karyokinesis or nuclear division) and spread out on a slide (right), and artificially arranged according to length (left); an arrangement like this is called a karyotype. In this image, the chromosomes were exposed to fluorescent stains for differentiation of the different chromosomes. A method of staining called “chromosome painting” employs fluorescent dyes that highlight chromosomes in different colors. (credit: National Human Genome Project/NIH) Each copy of a homologous pair of chromosomes originates from a different parent; therefore, the different genes (alleles) themselves are not identical, although they code for the same traits such as “hair color.” The variation of individuals within a species is due to the specific combination of the genes inherited from both parents. Even a slightly altered sequence of nucleotides within a gene can result in an alternative trait. For example, there are three possible gene sequences on the human chromosome that code for blood type: sequence A, sequence B, and sequence O. Because all diploid human cells have two copies of the chromosome that determines blood type, the blood type (the trait) is determined by the two alleles of the marker gene that are inherited. It is possible to have two copies of the same gene sequence on both homologous chromosomes, with one on each (for example, AA, BB, or OO), or two different sequences, such as AB, AO, or BO. Apparently minor variations of traits, such as blood type, eye color, and handedness, contribute to the natural variation found within a species, but even though they seem minor, these traits may be connected with the expression of other traits as of yet unknown. However, if the entire DNA sequence from any pair of human homologous chromosomes is compared, the difference is much less than one percent. The sex chromosomes, X and Y, are the single exception to the rule of homologous chromosome uniformity: Other than a small amount of homology that is necessary to accurately produce gametes, the genes found on the X and Y chromosomes are different. Eukaryotic Chromosomal Structure and Compaction If the DNA from all 46 chromosomes in a human cell nucleus were laid out end-to-end, it would measure approximately two meters; however, its diameter would be only 2 nm! Considering that the size of a typical human cell is about 10 µm (100,000 cells lined up to equal one meter), DNA must be tightly packaged to fit in the cell’s nucleus. At the same time, it must also be readily accessible for the genes to be expressed. For this reason, the long strands of DNA are condensed into compact chromosomes during certain stages of the cell cycle. There are a number of ways that chromosomes are compacted. In the first level of compaction, short stretches of the DNA double helix wrap around a core of eight histone proteins at regular intervals along the entire length of the chromosome (Figure 10.4). The DNA-histone complex is called chromatin. The beadlike, histone DNA complex is called a nucleosome, and DNA connecting the nucleosomes is called linker DNA. A DNA molecule in this form is about seven times shorter than the double helix without the histones, and the beads are about 10 nm in diameter, in contrast with the 2-nm diameter of a DNA double helix. The second level of compaction occurs as the nucleosomes and the linker DNA between them coil into a 30-nm chromatin fiber. This coiling further condenses the chromosome so that it is now about 50 times shorter than the extended form. 282 Chapter 10 | Cell Reproduction In the third level of compaction, a variety of fibrous proteins is used to “pack the chromatin.” These fibrous proteins also ensure that each chromosome in a non-dividing cell occupies a particular area of the nucleus that does not overlap with that of any other chromosome (see the top image in Figure 10.3). Figure 10.4 Double-stranded DNA wraps around histone proteins to form nucleosomes that create the appearance of “beads on a string.” The nucleosomes are coiled into a 30-nm chromatin fiber. When a cell undergoes mitosis, the chromosomes condense even further. DNA replicates in the S phase of interphase, which technically is not a part of mitosis, but must always precede it. After replication, the chromosomes are composed of two linked sister chromatids. When fully compact, the pairs of identically packed chromosomes are bound to each other by cohesin proteins. The connection between the sister chromatids is closest in a region called the centromere. The conjoined sister chromatids, with a diameter of about 1 µm, are visible under a light microscope. The centromeric region is highly condensed and thus will appear as a constricted area. This animation illustrates the different levels of chromosome packing. (This multimedia resource will open in a browser.) (http://cnx.org/content/m66477/1.3/#eip- id1165802978636) This OpenStax book is available for free at http://cnx.org/content/col24361/1.8 Chapter 10 | Cell Reproduction 283 10.2 | The Cell Cycle By the end of this section, you will be able to do the following: Describe the three stages of interphase Discuss the behavior of chromosomes during karyokinesis/mitosis Explain how the cytoplasmic content is divided during cytokinesis Define the quiescent G0 phase The cell cycle is an ordered series of events involving cell growth and cell division that produces two new daughter cells. Cells on the path to cell division proceed through a series of precisely timed and carefully regulated stages of growth, DNA replication, and nuclear and cytoplasmic division that ultimately produces two identical (clone) cells. The cell cycle has two major phases: interphase and the mitotic phase (Figure 10.5). During interphase, the cell grows and DNA is replicated. During the mitotic phase, the replicated DNA and cytoplasmic contents are separated, and the cell cytoplasm is typically partitioned by a third process of the cell cycle called cytokinesis. We should note, however, that interphase and mitosis (kayrokinesis) may take place without cytokinesis, in which case cells with multiple nuclei (multinucleate cells) are produced. Figure 10.5 The cell cycle in multicellular organisms consists of interphase and the mitotic phase. During interphase, the cell grows and the nuclear DNA is duplicated. Interphase is followed by the mitotic phase. During the mitotic phase, the duplicated chromosomes are segregated and distributed into daughter nuclei. Following mitosis, the cytoplasm is usually divided as well by cytokinesis, resulting in two genetically identical daughter cells. Interphase During interphase, the cell undergoes normal growth processes while also preparing for cell division. In order for a cell to move from interphase into the mitotic phase, many internal and external conditions must be met. The three stages of interphase are called G1, S, and G2. G1 Phase (First Gap) The first stage of interphase is called the G1 phase (first gap) because, from a microscopic point of view, little 284 Chapter 10 | Cell Reproduction change is visible. However, during the G1 stage, the cell is quite active at the biochemical level. The cell is accumulating the building blocks of chromosomal DNA and the associated proteins as well as accumulating sufficient energy reserves to complete the task of replicating each chromosome in the nucleus. S Phase (Synthesis of DNA) Throughout interphase, nuclear DNA remains in a semi-condensed chromatin configuration. In the S phase, DNA replication can proceed through the mechanisms that result in the formation of identical pairs of DNA molecules—sister chromatids—that are firmly attached to the centromeric region. The centrosome is also duplicated during the S phase. The two centrosomes of homologous chromosomes will give rise to the mitotic spindle, the apparatus that orchestrates the movement of chromosomes during mitosis. For example, roughly at the center of each animal cell, the centrosomes are associated with a pair of rod-like objects, the centrioles, which are positioned at right angles to each other. Centrioles help organize cell division. We should note, however, that centrioles are not present in the centrosomes of other eukaryotic organisms, such as plants and most fungi. G2 Phase (Second Gap) In the G2 phase, the cell replenishes its energy stores and synthesizes proteins necessary for chromosome manipulation and movement. Some cell organelles are duplicated, and the cytoskeleton is dismantled to provide resources for the mitotic phase. There may be additional cell growth during G2. The final preparations for the mitotic phase must be completed before the cell is able to enter the first stage of mitosis. The Mitotic Phase The mitotic phase is a multistep process during which the duplicated chromosomes are aligned, separated, and move into two new, identical daughter cells. The first portion of the mitotic phase is called karyokinesis, or nuclear division. As we have just seen, the second portion of the mitotic phase (and often viewed as a process separate from and following mitosis) is called cytokinesis—the physical separation of the cytoplasmic components into the two daughter cells. Revisit the stages of mitosis at this site (http://openstaxcollege.org/l/Cell_cycle_mito). Karyokinesis (Mitosis) Karyokinesis, also known as mitosis, is divided into a series of phases—prophase, prometaphase, metaphase, anaphase, and telophase—that result in the division of the cell nucleus (Figure 10.6). This OpenStax book is available for free at http://cnx.org/content/col24361/1.8 Chapter 10 | Cell Reproduction 285 Figure 10.6 Karyokinesis (or mitosis) is divided into five stages—prophase, prometaphase, metaphase, anaphase, and telophase. We should note that this is a continuous process, and that the divisions between the stages are not discrete. The pictures at the bottom were taken by fluorescence microscopy (hence, the black background) of cells artificially stained by fluorescent dyes: blue fluorescence indicates DNA (chromosomes) and green fluorescence indicates microtubules (spindle apparatus). (credit “mitosis drawings”: modification of work by Mariana Ruiz Villareal; credit “micrographs”: modification of work by Roy van Heesbeen; credit “cytokinesis micrograph”: Wadsworth Center/ New York State Department of Health; scale-bar data from Matt Russell) Prophase (the “first phase”): the nuclear envelope starts to dissociate into small vesicles, and the membranous organelles (such as the Golgi complex [Golgi apparatus] and the endoplasmic reticulum), fragment and disperse toward the periphery of the cell. The nucleolus disappears (disperses) as well, and the centrosomes begin to move to opposite poles of the cell. Microtubules that will form the mitotic spindle extend between the centrosomes, pushing them farther apart as the microtubule fibers lengthen. The sister chromatids begin to coil more tightly with the aid of condensin proteins and now become visible under a light microscope. Prometaphase (the “first change phase”): Many processes that began in prophase continue to advance. The remnants of the nuclear envelope fragment further, and the mitotic spindle continues to develop as more microtubules assemble and stretch across the length of the former nuclear area. Chromosomes become even more condensed and discrete. Each sister chromatid develops a protein structure called a kinetochore in its centromeric region (Figure 10.7). The proteins of the kinetochore attract and bind to the mitotic spindle microtubules. As the spindle microtubules extend from the centrosomes, some of these microtubules come into contact with and firmly bind to the kinetochores. Once a mitotic fiber attaches to a chromosome, the chromosome will be oriented until the kinetochores of sister chromatids face the opposite poles. Eventually, all the sister chromatids will be attached via their kinetochores to microtubules from opposing poles. Spindle microtubules that do not engage the chromosomes are called polar microtubules. These microtubules overlap each other midway between the two poles and contribute to cell elongation. Astral microtubules are located near the poles, aid in spindle orientation, and are required for the regulation of mitosis. 286 Chapter 10 | Cell Reproduction Figure 10.7 During prometaphase, mitotic spindle microtubules from opposite poles attach to each sister chromatid at the kinetochore. In anaphase, the connection between the sister chromatids breaks down, and the microtubules pull the chromosomes toward opposite poles. Metaphase (the “change phase”): All the chromosomes are aligned in a plane called the metaphase plate, or the equatorial plane, roughly midway between the two poles of the cell. The sister chromatids are still tightly attached to each other by cohesin proteins. At this time, the chromosomes are maximally condensed. Anaphase (“upward phase”): The cohesin proteins degrade, and the sister chromatids separate at the centromere. Each chromatid, now called a single chromosome, is pulled rapidly toward the centrosome to which its microtubule is attached. The cell becomes visibly elongated (oval shaped) as the polar microtubules slide against each other at the metaphase plate where they overlap. Telophase (the “distance phase”): the chromosomes reach the opposite poles and begin to decondense (unravel), relaxing once again into a stretched-out chromatin configuration. The mitotic spindles are depolymerized into tubulin monomers that will be used to assemble cytoskeletal components for each daughter cell. Nuclear envelopes form around the chromosomes, and nucleosomes appear within the nuclear area. Cytokinesis Cytokinesis, or “cell motion,” is sometimes viewed as the second main stage of the mitotic phase, during which cell division is completed via the physical separation of the cytoplasmic components into two daughter cells However, as we have seen earlier, cytokinesis can also be viewed as a separate phase, which may or may not take place following mitosis. If cytokinesis does take place, cell division is not complete until the cell components have been apportioned and completely separated into the two daughter cells. Although the stages of mitosis are similar for most eukaryotes, the process of cytokinesis is quite different for eukaryotes that have cell walls, such as plant cells. In animal cells, cytokinesis typically starts during late anaphase. A contractile ring composed of actin filaments forms just inside the plasma membrane at the former metaphase plate. The actin filaments pull the equator of the cell inward, forming a fissure. This fissure is called the cleavage furrow. The furrow deepens as the actin ring contracts, and eventually the membrane is cleaved in two (Figure 10.8). In plant cells, a new cell wall must form between the daughter cells. During interphase, the Golgi apparatus accumulates enzymes, structural proteins, and glucose molecules prior to breaking into vesicles and dispersing throughout the dividing cell. During telophase, these Golgi vesicles are transported on microtubules to form a phragmoplast (a vesicular structure) at the metaphase plate. There, the vesicles fuse and coalesce from the center toward the cell walls; this structure is called a cell plate. As more vesicles fuse, the cell plate enlarges until it merges with the cell walls at the periphery of the cell. Enzymes use the glucose that has accumulated between the membrane layers to build a new cell wall. The Golgi membranes become parts of the plasma membrane on either side of the new cell wall (Figure 10.8). This OpenStax book is available for free at http://cnx.org/content/col24361/1.8 Chapter 10 | Cell Reproduction 287 Figure 10.8 During cytokinesis in animal cells, a ring of actin filaments forms at the metaphase plate. The ring contracts, forming a cleavage furrow, which divides the cell in two. In plant cells, Golgi vesicles coalesce at the former metaphase plate, forming a phragmoplast. A cell plate formed by the fusion of the vesicles of the phragmoplast grows from the center toward the cell walls, and the membranes of the vesicles fuse to form a plasma membrane that divides the cell in two. G0 Phase Not all cells adhere to the classic cell-cycle pattern in which a newly formed daughter cell immediately enters the preparatory phases of interphase, closely followed by the mitotic phase, and cytokinesis. Cells in G0 phase are not actively preparing to divide. The cell is in a quiescent (inactive) stage that occurs when cells exit the cell cycle. Some cells enter G0 temporarily due to environmental conditions such as availability of nutrients, or stimulation by growth factors. The cell will remain in this phase until conditions improve or until an external signal triggers the onset of G1. Other cells that never or rarely divide, such as mature cardiac muscle and nerve cells, remain in G0 permanently. Which of the following is the correct order of events in mitosis? a. Sister chromatids line up at the metaphase plate. The kinetochore becomes attached to the mitotic spindle. The nucleus reforms and the cell divides. Cohesin proteins break down and the sister chromatids separate. b. The kinetochore becomes attached to the mitotic spindle. Cohesin proteins break down and the sister chromatids separate. Sister chromatids line up at the metaphase plate. The nucleus reforms and the cell divides. c. The kinetochore becomes attached to the cohesin proteins. Sister chromatids line up at the metaphase plate. The kinetochore breaks down and the sister chromatids separate. The nucleus reforms and the cell divides. d. The kinetochore becomes attached to the mitotic spindle. Sister chromatids line up at the metaphase plate. Cohesin proteins break down and the sister chromatids separate. The nucleus reforms and the cell divides. 300 Chapter 10 | Cell Reproduction KEY TERMS anaphase stage of mitosis during which sister chromatids are separated from each other binary fission prokaryotic cell division process cell cycle ordered sequence of events through which a cell passes between one cell division and the next cell cycle ordered series of events involving cell growth and cell division that produces two new daughter cells cell plate structure formed during plant cell cytokinesis by Golgi vesicles, forming a temporary structure (phragmoplast) and fusing at the metaphase plate; ultimately leads to the formation of cell walls that separate the two daughter cells cell-cycle checkpoint mechanism that monitors the preparedness of a eukaryotic cell to advance through the various cell-cycle stages centriole rod-like structure constructed of microtubules at the center of each animal cell centrosome centromere region at which sister chromatids are bound together; a constricted area in condensed chromosomes chromatid single DNA molecule of two strands of duplicated DNA and associated proteins held together at the centromere cleavage furrow constriction formed by an actin ring during cytokinesis in animal cells that leads to cytoplasmic division condensin proteins that help sister chromatids coil during prophase cyclin one of a group of proteins that act in conjunction with cyclin-dependent kinases to help regulate the cell cycle by phosphorylating key proteins; the concentrations of cyclins fluctuate throughout the cell cycle cyclin-dependent kinase (Cdk) one of a group of protein kinases that helps to regulate the cell cycle when bound to cyclin; it functions to phosphorylate other proteins that are either activated or inactivated by phosphorylation cytokinesis division of the cytoplasm following mitosis that forms two daughter cells. diploid cell, nucleus, or organism containing two sets of chromosomes (2n) FtsZ tubulin-like protein component of the prokaryotic cytoskeleton that is important in prokaryotic cytokinesis (name origin: Filamenting temperature-sensitive mutant Z) G0 phase distinct from the G1 phase of interphase; a cell in G0 is not preparing to divide G1 phase (also, first gap) first phase of interphase centered on cell growth during mitosis G2 phase (also, second gap) third phase of interphase during which the cell undergoes final preparations for mitosis gamete haploid reproductive cell or sex cell (sperm, pollen grain, or egg) gene physical and functional unit of heredity, a sequence of DNA that codes for a protein. genome total genetic information of a cell or organism haploid cell, nucleus, or organism containing one set of chromosomes (n) histone one of several similar, highly conserved, low molecular weight, basic proteins found in the chromatin of all eukaryotic cells; associates with DNA to form nucleosomes homologous chromosomes chromosomes of the same morphology with genes in the same location; diploid This OpenStax book is available for free at http://cnx.org/content/col24361/1.8 Chapter 10 | Cell Reproduction 301 organisms have pairs of homologous chromosomes (homologs), with each homolog derived from a different parent interphase period of the cell cycle leading up to mitosis; includes G1, S, and G2 phases (the interim period between two consecutive cell divisions) karyokinesis mitotic nuclear division kinetochore protein structure associated with the centromere of each sister chromatid that attracts and binds spindle microtubules during prometaphase locus position of a gene on a chromosome metaphase stage of mitosis during which chromosomes are aligned at the metaphase plate metaphase plate equatorial plane midway between the two poles of a cell where the chromosomes align during metaphase mitosis (also, karyokinesis) period of the cell cycle during which the duplicated chromosomes are separated into identical nuclei; includes prophase, prometaphase, metaphase, anaphase, and telophase mitotic phase period of the cell cycle during which duplicated chromosomes are distributed into two nuclei and cytoplasmic contents are divided; includes karyokinesis (mitosis) and cytokinesis mitotic spindle apparatus composed of microtubules that orchestrates the movement of chromosomes during mitosis nucleosome subunit of chromatin composed of a short length of DNA wrapped around a core of histone proteins oncogene mutated version of a normal gene involved in the positive regulation of the cell cycle origin (also, ORI) region of the prokaryotic chromosome where replication begins (origin of replication) p21 cell-cycle regulatory protein that inhibits the cell cycle; its levels are controlled by p53 p53 cell-cycle regulatory protein that regulates cell growth and monitors DNA damage; it halts the progression of the cell cycle in cases of DNA damage and may induce apoptosis prometaphase stage of mitosis during which the nuclear membrane breaks down and mitotic spindle fibers attach to kinetochores prophase stage of mitosis during which chromosomes condense and the mitotic spindle begins to form proto-oncogene normal gene that when mutated becomes an oncogene quiescent refers to a cell that is performing normal cell functions and has not initiated preparations for cell division retinoblastoma protein (Rb) regulatory molecule that exhibits negative effects on the cell cycle by interacting with a transcription factor (E2F) S phase second, or synthesis, stage of interphase during which DNA replication occurs septum structure formed in a bacterial cell as a precursor to the separation of the cell into two daughter cells telophase stage of mitosis during which chromosomes arrive at opposite poles, decondense, and are surrounded by a new nuclear envelope tumor suppressor gene segment of DNA that codes for regulator proteins that prevent the cell from undergoing uncontrolled division 302 Chapter 10 | Cell Reproduction CHAPTER SUMMARY 10.1 Cell Division Prokaryotes have a single circular chromosome composed of double-stranded DNA, whereas eukaryotes have multiple, linear chromosomes composed of chromatin wrapped around histones, all of which are surrounded by a nuclear membrane. The 46 chromosomes of human somatic cells are composed of 22 pairs of autosomes (matched pairs) and a pair of sex chromosomes, which may or may not be matched. This is the 2n or diploid state. Human gametes have 23 chromosomes, or one complete set of chromosomes; a set of chromosomes is complete with either one of the sex chromosomes, X or Y. This is the n or haploid state. Genes are segments of DNA that code for a specific functional molecule (a protein or RNA). An organism’s traits are determined by the genes inherited from each parent. Duplicated chromosomes are composed of two sister chromatids. Chromosomes are compacted using a variety of mechanisms during certain stages of the cell cycle. Several classes of protein are involved in the organization and packing of the chromosomal DNA into a highly condensed structure. The condensing complex compacts chromosomes, and the resulting condensed structure is necessary for chromosomal segregation during mitosis. 10.2 The Cell Cycle The cell cycle is an orderly sequence of events. Cells on the path to cell division proceed through a series of precisely timed and carefully regulated stages. In eukaryotes, the cell cycle consists of a long preparatory period, called interphase, during which chromosomes are replicated. Interphase is divided into G1, S, and G2 phases. The mitotic phase begins with karyokinesis (mitosis), which consists of five stages: prophase, prometaphase, metaphase, anaphase, and telophase. The final stage of the cell division process, and sometimes viewed as the final stage of the mitotic phase, is cytokinesis, during which the cytoplasmic components of the daughter cells are separated either by an actin ring (animal cells) or by cell plate formation (plant cells). 10.3 Control of the Cell Cycle Each step of the cell cycle is monitored by internal controls called checkpoints. There are three major checkpoints in the cell cycle: one near the end of G1, a second at the G2/M transition, and the third during metaphase. Positive regulator molecules allow the cell cycle to advance to the next stage of cell division. Negative regulator molecules monitor cellular conditions and can halt the cycle until specific requirements are met. 10.4 Cancer and the Cell Cycle Cancer is the result of unchecked cell division caused by a breakdown of the mechanisms that regulate the cell cycle. The loss of control begins with a change in the DNA sequence of a gene that codes for one of the regulatory molecules. Faulty instructions lead to a protein that does not function as it should. Any disruption of the monitoring system can allow other mistakes to be passed on to the daughter cells. Each successive cell division will give rise to daughter cells with even more accumulated damage. Eventually, all checkpoints become nonfunctional, and rapidly reproducing cells crowd out normal cells, resulting in a tumor or leukemia (blood cancer). 10.5 Prokaryotic Cell Division In both prokaryotic and eukaryotic cell division, the genomic DNA is replicated and then each copy is allocated into a daughter cell. In addition, the cytoplasmic contents are divided evenly and distributed to the new cells. However, there are many differences between prokaryotic and eukaryotic cell division. Bacteria have a single, circular DNA chromosome but no nucleus. Therefore, mitosis (karyokinesis) is not necessary in bacterial cell division. Bacterial cytokinesis is directed by a ring composed of a protein called FtsZ. Ingrowth of membrane and cell wall material from the periphery of the cells results in the formation of a septum that eventually constructs the separate cell walls of the daughter cells. VISUAL CONNECTION QUESTIONS 1. Figure 10.6 Which of the following is the correct order of events in mitosis? This OpenStax book is available for free at http://cnx.org/content/col24361/1.8 Chapter 10 | Cell Reproduction 303 a. Sister chromatids line up at the metaphase 2. Figure 10.13 Rb and other proteins that negatively plate. The kinetochore becomes attached to regulate the cell cycle are sometimes called tumor the mitotic spindle. The nucleus reforms and suppressors. Why do you think the name tumor the cell divides. Cohesin proteins break suppressor might be appropriate for these proteins? down and the sister chromatids separate. 3. Figure 10.14 Human papillomavirus can cause b. The kinetochore becomes attached to the cervical cancer. The virus encodes E6, a protein that mitotic spindle. Cohesin proteins break binds p53. Based on this fact and what you know down and the sister chromatids separate. about p53, what effect do you think E6 binding has Sister chromatids line up at the metaphase on p53 activity? plate. The nucleus reforms and the cell a. E6 activates p53 divides. b. E6 inactivates p53 c. The kinetochore becomes attached to the c. E6 mutates p53 cohesin proteins. Sister chromatids line up d. E6 binding marks p53 for degradation at the metaphase plate. The kinetochore breaks down and the sister chromatids separate. The nucleus reforms and the cell divides. d. The kinetochore becomes attached to the mitotic spindle. Sister chromatids line up at the metaphase plate. Cohesin proteins break down and the sister chromatids separate. The nucleus reforms and the cell divides. REVIEW QUESTIONS 4. A diploid cell has_______ the number of 9. Which of the following events does not occur chromosomes as a haploid cell. during some stages of interphase? a. one-fourth a. DNA duplication b. half b. organelle duplication c. twice c. increase in cell size d. four times d. separation of sister chromatids 5. An organism’s traits are determined by the specific 10. The mitotic spindles arise from which cell combination of inherited _____. structure? a. cells. a. centromere b. genes. b. centrosome c. proteins. c. kinetochore d. chromatids. d. cleavage furrow 6. The first level of DNA organization in a eukaryotic 11. Attachment of the mitotic spindle fibers to the cell is maintained by which molecule? kinetochores is a characteristic of which stage of a. cohesin mitosis? b. condensin a. prophase c. chromatin b. prometaphase d. histone c. metaphase d. anaphase 7. Identical copies of chromatin held together by cohesin at the centromere are called _____. 12. Unpacking of chromosomes and the formation of a new nuclear envelope is a characteristic of which a. histones. stage of mitosis? b. nucleosomes. a. prometaphase c. chromatin. b. metaphase d. sister chromatids. c. anaphase d. telophase 8. Chromosomes are duplicated during what stage of the cell cycle? 13. Separation of the sister chromatids is a a. G1 phase characteristic of which stage of mitosis? b. S phase c. prophase d. prometaphase 304 Chapter 10 | Cell Reproduction a. prometaphase 21. Which negative regulatory molecule can trigger b. metaphase cell suicide (apoptosis) if vital cell cycle events do not c. anaphase occur? d. telophase a. p53 14. The chromosomes become visible under a light b. p21 microscope during which stage of mitosis? c. retinoblastoma protein (Rb) a. prophase d. cyclin-dependent kinase (Cdk) b. prometaphase 22. ___________ are changes to the order of c. metaphase nucleotides in a segment of DNA that codes for a d. anaphase protein. 15. The fusing of Golgi vesicles at the metaphase a. Proto-oncogenes plate of dividing plant cells forms what structure? b. Tumor suppressor genes c. Gene mutations a. cell plate d. Negative regulators b. actin ring 23. A gene that codes for a positive cell-cycle c. cleavage furrow regulator is called a(n) _____. d. mitotic spindle a. kinase inhibitor. 16. At which of the cell-cycle checkpoints do external b. tumor suppressor gene. forces have the greatest influence? c. proto-oncogene. a. G1 checkpoint d. oncogene. b. G2 checkpoint 24. A mutated gene that codes for an altered version c. M checkpoint of Cdk that is active in the absence of cyclin is a(n) d. G0 checkpoint _____. a. kinase inhibitor. 17. What is the main prerequisite for clearance at the b. tumor suppressor gene. G2 checkpoint? c. proto-oncogene. a. cell has reached a sufficient size d. oncogene. b. an adequate stockpile of nucleotides c. accurate and complete DNA replication 25. Which molecule is a Cdk inhibitor that is d. proper attachment of mitotic spindle fibers to controlled by p53? kinetochores a. cyclin b. anti-kinase 18. If the M checkpoint is not cleared, what stage of c. Rb mitosis will be blocked? d. p21 a. prophase b. prometaphase 26. Which eukaryotic cell-cycle event is missing in c. metaphase binary fission? d. anaphase a. cell growth b. DNA duplication 19. Which protein is a positive regulator that c. karyokinesis phosphorylates other proteins when activated? d. cytokinesis a. p53 27. FtsZ proteins direct the formation of a _______ b. retinoblastoma protein (Rb) that will eventually form the new cell walls of the c. cyclin daughter cells. d. cyclin-dependent kinase (Cdk) a. contractile ring b. cell plate 20. Many of the negative regulator proteins of the cell c. cytoskeleton cycle were discovered in what type of cells? d. septum a. gametes b. cells in G0 c. cancer cells d. stem cells CRITICAL THINKING QUESTIONS 28. Compare and contrast a human somatic cell to a chromosomes, and genes? human gamete. 30. Eukaryotic chromosomes are thousands of times 29. What is the relationship between a genome, longer than a typical cell. Explain how chromosomes This OpenStax book is available for free at http://cnx.org/content/col24361/1.8 Chapter 10 | Cell Reproduction 305 can fit inside a eukaryotic nucleus. cell-cycle regulators negative regulators. 31. Briefly describe the events that occur in each 38. What steps are necessary for Cdk to become phase of interphase. fully active? 32. Chemotherapy drugs such as vincristine (derived 39. Rb is a negative regulator that blocks the cell from Madagascar periwinkle plants) and colchicine cycle at the G1 checkpoint until the cell achieves a (derived from autumn crocus plants) disrupt mitosis requisite size. What molecular mechanism does Rb by binding to tubulin (the subunit of microtubules) employ to halt the cell cycle? and interfering with microtubule assembly and 40. Outline the steps that lead to a cell becoming disassembly. Exactly what mitotic structure is cancerous. targeted by these drugs and what effect would that have on cell division? 41. Explain the difference between a proto-oncogene and a tumor-suppressor gene. 33. Describe the similarities and differences between the cytokinesis mechanisms found in animal cells 42. List the regulatory mechanisms that might be lost versus those in plant cells. in a cell producing faulty p53. 34. List some reasons why a cell that has just 43. p53 can trigger apoptosis if certain cell-cycle completed cytokinesis might enter the G0 phase events fail. How does this regulatory outcome benefit instead of the G1 phase. a multicellular organism? 35. What cell-cycle events will be affected in a cell 44. Name the common components of eukaryotic cell that produces mutated (non-functional) cohesin division and binary fission. protein? 45. Describe how the duplicated bacterial 36. Describe the general conditions that must be met chromosomes are distributed into new daughter cells at each of the three main cell-cycle checkpoints. without the direction of the mitotic spindle. 37. Compare and contrast the roles of the positive Chapter 11 | Meiosis and Sexual Reproduction 307 11 | MEIOSIS AND SEXUAL REPRODUCTION Figure 11.1 Each of us, like the organisms shown above, begins life as a fertilized egg (zygote). After trillions of cell divisions, each of us develops into a complex, multicellular organism. (credit a: modification of work by Frank Wouters; credit b: modification of work by Ken Cole, USGS; credit c: modification of work by Martin Pettitt) Chapter Outline 11.1: The Process of Meiosis 11.2: Sexual Reproduction Introduction The ability to reproduce is a basic characteristic of all organisms: Hippopotamuses give birth to hippopotamus calves; Joshua trees produce seeds from which Joshua tree seedlings emerge; and adult flamingos lay eggs that hatch into flamingo chicks. However, unlike the organisms shown above, offspring may or may not resemble their parents. For example, in the case of most insects such as butterflies (with a complete metamorphosis), the larval forms rarely resemble the adult forms. Although many unicellular organisms and a few multicellular organisms can produce genetically identical clones of themselves through asexual reproduction, many single-celled organisms and most multicellular organisms reproduce regularly using another method—sexual reproduction. This highly evolved method involves the production by parents of two haploid cells and the fusion of two haploid cells to form a single, genetically recombined diploid cell—a genetically unique organism. Haploid cells that are part of the sexual reproductive cycle are produced by a type of cell division called meiosis. Sexual reproduction, involving both meiosis and fertilization, introduces variation into offspring that may account for the evolutionary success of sexual reproduction. The vast majority of eukaryotic organisms, both multicellular and unicellular, can or must employ some form of meiosis and fertilization to reproduce. In most plants and animals, through thousands of rounds of mitotic cell division, diploid cells (whether produced by asexual or sexual reproduction) will develop into an adult organism. 308 Chapter 11 | Meiosis and Sexual Reproduction 11.1 | The Process of Meiosis By the end of this section, you will be able to do the following: Describe the behavior of chromosomes during meiosis, and the differences between the first and second meiotic divisions Describe the cellular events that take place during meiosis Explain the differences between meiosis and mitosis Explain the mechanisms within the meiotic process that produce genetic variation among the haploid gametes Sexual reproduction requires the union of two specialized cells, called gametes, each of which contains one set of chromosomes. When gametes unite, they form a zygote, or fertilized egg that contains two sets of chromosomes. (Note: Cells that contain one set of chromosomes are called haploid; cells containing two sets of chromosomes are called diploid.) If the reproductive cycle is to continue for any sexually reproducing species, then the diploid cell must somehow reduce its number of chromosome sets to produce haploid gametes; otherwise, the number of chromosome sets will double with every future round of fertilization. Therefore, sexual reproduction requires a nuclear division that reduces the number of chromosome sets by half. Most animals and plants and many unicellular organisms are diploid and therefore have two sets of chromosomes. In each somatic cell of the organism (all cells of a multicellular organism except the gametes or reproductive cells), the nucleus contains two copies of each chromosome, called homologous chromosomes. Homologous chromosomes are matched pairs containing the same genes in identical locations along their lengths. Diploid organisms inherit one copy of each homologous chromosome from each parent. Meiosis is the nuclear division that forms haploid cells from diploid cells, and it employs many of the same cellular mechanisms as mitosis. However, as you have learned, mitosis produces daughter cells whose nuclei are genetically identical to the original parent nucleus. In mitosis, both the parent and the daughter nuclei are at the same “ploidy level”—diploid in the case of most multicellular most animals. Plants use mitosis to grow as sporophytes, and to grow and produce eggs and sperm as gametophytes; so they use mitosis for both haploid and diploid cells (as well as for all other ploidies). In meiosis, the starting nucleus is always diploid and the daughter nuclei that result are haploid. To achieve this reduction in chromosome number, meiosis consists of one round of chromosome replication followed by two rounds of nuclear division. Because the events that occur during each of the division stages are analogous to the events of mitosis, the same stage names are assigned. However, because there are two rounds of division, the major process and the stages are designated with a “I” or a “II.” Thus, meiosis I is the first round of meiotic division and consists of prophase I, prometaphase I, and so on. Likewise, Meiosis II (during which the second round of meiotic division takes place) includes prophase II, prometaphase II, and so on. Meiosis I Meiosis is preceded by an interphase consisting of G1, S, and G2 phases, which are nearly identical to the phases preceding mitosis. The G1 phase (the “first gap phase”) is focused on cell growth. During the S phase—the second phase of interphase—the cell copies or replicates the DNA of the chromosomes. Finally, in the G2 phase (the “second gap phase”) the cell undergoes the final preparations for meiosis. During DNA duplication in the S phase, each chromosome is replicated to produce two identical copies—sister chromatids that are held together at the centromere by cohesin proteins, which hold the chromatids together until anaphase II. Prophase I Early in prophase I, before the chromosomes can be seen clearly with a microscope, the homologous chromosomes are attached at their tips to the nuclear envelope by proteins. As the nuclear envelope begins to break down, the proteins associated with homologous chromosomes bring the pair closer together. Recall that in mitosis, homologous chromosomes do not pair together. The synaptonemal complex, a lattice of proteins between the homologous chromosomes, first forms at specific locations and then spreads outward to cover the entire length of the chromosomes. The tight pairing of the homologous chromosomes is called synapsis. In This OpenStax book is available for free at http://cnx.org/content/col24361/1.8 Chapter 11 | Meiosis and Sexual Reproduction 309 synapsis, the genes on the chromatids of the homologous chromosomes are aligned precisely with each other. The synaptonemal complex supports the exchange of chromosomal segments between homologous nonsister chromatids—a process called crossing over. Crossing over can be observed visually after the exchange as chiasmata (singular = chiasma) (Figure 11.2). In humans, even though the X and Y sex chromosomes are not completely homologous (that is, most of their genes differ), there is a small region of homology that allows the X and Y chromosomes to pair up during prophase I. A partial synaptonemal complex develops only between the regions of homology. Figure 11.2 Early in prophase I, homologous chromosomes come together to form a synapse. The chromosomes are bound tightly together and in perfect alignment by a protein lattice called a synaptonemal complex and by cohesin proteins at the centromere. Located at intervals along the synaptonemal complex are large protein assemblies called recombination nodules. These assemblies mark the points of later chiasmata and mediate the multistep process of crossover—or genetic recombination—between the nonsister chromatids. Near the recombination nodule, the double-stranded DNA of each chromatid is cleaved, the cut ends are modified, and a new connection is made between the nonsister chromatids. As prophase I progresses, the synaptonemal complex begins to break down and the chromosomes begin to condense. When the synaptonemal complex is gone, the homologous chromosomes remain attached to each other at the centromere and at chiasmata. The chiasmata remain until anaphase I. The number of chiasmata varies according to the species and the length of the chromosome. There must be at least one chiasma per chromosome for proper separation of homologous chromosomes during meiosis I, but there may be as many as 25. Following crossover, the synaptonemal complex breaks down and the cohesin connection between homologous pairs is removed. At the end of prophase I, the pairs are held together only at the chiasmata (Figure 11.3). These pairs are called tetrads because the four sister chromatids of each pair of homologous chromosomes are now visible. The crossover events are the first source of genetic variation in the nuclei produced by meiosis. A single crossover event between homologous nonsister chromatids leads to a reciprocal exchange of equivalent DNA between a maternal chromosome and a paternal chromosome. When a recombinant sister chromatid is moved into a gamete cell it will carry some DNA from one parent and some DNA from the other parent. The recombinant chromatid has a combination of maternal and paternal genes that did not exist before the crossover. Crossover events can occur almost anywhere along the length of the synapsed chromosomes. Different cells undergoing meiosis will therefore produce different recombinant chromatids, with varying combinations of maternal and parental genes. Multiple crossovers in an arm of the chromosome have the same effect, exchanging segments of DNA to produce genetically recombined chromosomes. 310 Chapter 11 | Meiosis and Sexual Reproduction Figure 11.3 Crossover occurs between nonsister chromatids of homologous chromosomes. The result is an exchange of genetic material between homologous chromosomes. Prometaphase I The key event in prometaphase I is the attachment of the spindle fiber microtubules to the kinetochore proteins at the centromeres. Kinetochore proteins are multiprotein complexes that bind the centromeres of a chromosome to the microtubules of the mitotic spindle. Microtubules grow from microtubule-organizing centers (MTOCs). In animal cells, MTOCs are centrosomes located at opposite poles of the cell. The microtubules from each pole move toward the middle of the cell and attach to one of the kinetochores of the two fused homologous chromosomes. Each member of the homologous pair attaches to a microtubule extending from opposite poles of the cell so that in the next phase, the microtubules can pull the homologous pair apart. A spindle fiber that has attached to a kinetochore is called a kinetochore microtubule. At the end of prometaphase I, each tetrad is attached to microtubules from both poles, with one homologous chromosome facing each pole. The homologous chromosomes are still held together at the chiasmata. In addition, the nuclear membrane has broken down entirely. Metaphase I During metaphase I, the homologous chromosomes are arranged at the metaphase plate—roughly in the midline of the cell, with the kinetochores facing opposite poles. The homologous pairs orient themselves randomly at the equator. For example, if the two homologous members of chromosome 1 are labeled a and b, then the chromosomes could line up a-b or b-a. This is important in determining the genes carried by a gamete, as each will only receive one of the two homologous chromosomes. (Recall that after crossing over takes place, homologous chromosomes are not identical. They contain slight differences in their genetic information, causing each gamete to have a unique genetic makeup.) The randomness in the alignment of recombined chromosomes at the metaphase plate, coupled with the crossing over events between nonsister chromatids, are responsible for much of the genetic variation in This OpenStax book is available for free at http://cnx.org/content/col24361/1.8 Chapter 11 | Meiosis and Sexual Reproduction 311 the offspring. To clarify this further, remember that the homologous chromosomes of a sexually reproducing organism are originally inherited as two separate sets, one from each parent. Using humans as an example, one set of 23 chromosomes is present in the egg donated by the mother. The father provides the other set of 23 chromosomes in the sperm that fertilizes the egg. Every cell of the multicellular offspring has copies of the original two sets of homologous chromosomes. In prophase I of meiosis, the homologous chromosomes form the tetrads. In metaphase I, these pairs line up at the midway point between the two poles of the cell to form the metaphase plate. Because there is an equal chance that a microtubule fiber will encounter a maternally or paternally inherited chromosome, the arrangement of the tetrads at the metaphase plate is random. Thus, any maternally inherited chromosome may face either pole. Likewise, any paternally inherited chromosome may also face either pole. The orientation of each tetrad is independent of the orientation of the other 22 tetrads. This event—the random (or independent) assortment of homologous chromosomes at the metaphase plate—is the second mechanism that introduces variation into the gametes or spores. In each cell that undergoes meiosis, the arrangement of the tetrads is different. The number of variations is dependent on the number of chromosomes making up a set. There are two possibilities for orientation at the metaphase plate; the possible number of alignments therefore equals 2n in a diploid cell, where n is the number of chromosomes per haploid set. Humans have 23 chromosome pairs, which results in over eight million (223) possible genetically-distinct gametes just from the random alignment of chromosomes at the metaphase plate. This number does not include the variability that was previously produced by crossing over between the nonsister chromatids. Given these two mechanisms, it is highly unlikely that any two haploid cells resulting from meiosis will have the same genetic composition (Figure 11.4). To summarize, meiosis I creates genetically diverse gametes in two ways. First, during prophase I, crossover events between the nonsister chromatids of each homologous pair of chromosomes generate recombinant chromatids with new combinations of maternal and paternal genes. Second, the random assortment of tetrads on the metaphase plate produces unique combinations of maternal and paternal chromosomes that will make their way into the gametes. 312 Chapter 11 | Meiosis and Sexual Reproduction Figure 11.4 Random, independent assortment during metaphase I can be demonstrated by considering a cell with a set of two chromosomes (n = 2). In this case, there are two possible arrangements at the equatorial plane in metaphase I. The total possible number of different gametes is 2n, where n equals the number of chromosomes in a set. In this example, there are four possible genetic combinations for the gametes. With n = 23 in human cells, there are over eight million possible combinations of paternal and maternal chromosomes. Anaphase I In anaphase I, the microtubules pull the linked chromosomes apart. The sister chromatids remain tightly bound together at the centromere. The chiasmata are broken in anaphase I as the microtubules attached to the fused kinetochores pull the homologous chromosomes apart (Figure 11.5). Telophase I and Cytokinesis In telophase, the separated chromosomes arrive at opposite poles. The remainder of the typical telophase events may or may not occur, depending on the species. In some organisms, the chromosomes “decondense” and nuclear envelopes form around the separated sets of chromatids produced during telophase I. In other organisms, cytokinesis—the physical separation of the cytoplasmic components into two daughter cells—occurs without reformation of the nuclei. In nearly all species of animals and some fungi, cytokinesis separates the cell contents via a cleavage furrow (constriction of the actin ring that leads to cytoplasmic division). In plants, a cell plate is formed during cell cytokinesis by Golgi vesicles fusing at the metaphase plate. This cell plate will ultimately lead to the formation of cell walls that separate the two daughter cells. Two haploid cells are the result of the first meiotic division of a diploid cell. The cells are haploid because at each pole, there is just one of each pair of the homologous chromosomes. Therefore, only one full set of the chromosomes is present. This is why the cells are considered haploid—there is only one chromosome set, even though each chromosome still consists of two sister chromatids. Recall that sister chromatids are merely duplicates of one of the two homologous chromosomes (except for changes that occurred during crossing over). In meiosis II, these two sister chromatids will separate, creating four haploid daughter cells. This OpenStax book is available for free at http://cnx.org/content/col24361/1.8 Chapter 11 | Meiosis and Sexual Reproduction 313 Review the process of meiosis, observing how chromosomes align and migrate, at Meiosis: An Interactive Animation (http://openstaxcollege.org/l/animal_meiosis). Meiosis II In some species, cells enter a brief interphase, or interkinesis, before entering meiosis II. Interkinesis lacks an S phase, so chromosomes are not duplicated. The two cells produced in meiosis I go through the events of meiosis II in synchrony. During meiosis II, the sister chromatids within the two daughter cells separate, forming four new haploid gametes. The mechanics of meiosis II are similar to mitosis, except that each dividing cell has only one set of homologous chromosomes, each with two chromatids. Therefore, each cell has half the number of sister chromatids to separate out as a diploid cell undergoing mitosis. In terms of chromosomal content, cells at the start of meiosis II are similar to haploid cells in G2, preparing to undergo mitosis. Prophase II If the chromosomes decondensed in telophase I, they condense again. If nuclear envelopes were formed, they fragment into vesicles. The MTOCs that were duplicated during interkinesis move away from each other toward opposite poles, and new spindles are formed. Prometaphase II The nuclear envelopes are completely broken down, and the spindle is fully formed. Each sister chromatid forms an individual kinetochore that attaches to microtubules from opposite poles. Metaphase II The sister chromatids are maximally condensed and aligned at the equator of the cell. Anaphase II The sister chromatids are pulled apart by the kinetochore microtubules and move toward opposite poles. Nonkinetochore microtubules elongate the cell. 314 Chapter 11 | Meiosis and Sexual Reproduction Figure 11.5 The process of chromosome alignment differs between meiosis I and meiosis II. In prometaphase I, microtubules attach to the fused kinetochores of homologous chromosomes, and the homologous chromosomes are arranged at the midline of the cell (the metaphase plate) in metaphase I. In anaphase I, the homologous chromosomes separate. In prometaphase II, microtubules attach to the kinetochores of sister chromatids, and the sister chromatids are arranged at the midpoint of the cells in metaphase II. In anaphase II, the sister chromatids separate. Telophase II and Cytokinesis The chromosomes arrive at opposite poles and begin to decondense. Nuclear envelopes form around the chromosomes. If the parent cell was diploid, as is most commonly the case, then cytokinesis now separates the two cells into four unique haploid cells. The cells produced are genetically unique because of the random assortment of paternal and maternal homologs and because of the recombination of maternal and paternal segments of chromosomes (with their sets of genes) that occurs during crossover. The entire process of meiosis is outlined in Figure 11.6. This OpenStax book is available for free at http://cnx.org/content/col24361/1.8 Chapter 11 | Meiosis and Sexual Reproduction 315 Figure 11.6 An animal cell with a diploid number of four (2n = 4) proceeds through the stages of meiosis to form four haploid daughter cells. Comparing Meiosis and Mitosis Mitosis and meiosis are both forms of division of the nucleus in eukaryotic cells. They share some similarities, but also exhibit a number of important and distinct differences that lead to very different outcomes (Figure 11.7). Mitosis is a single nuclear division that results in two nuclei that are usually partitioned into two new cells. The nuclei resulting from a mitotic division are genetically identical to the original nucleus. They have the same number of sets of chromosomes: one set in the case of haploid cells and two sets in the case of diploid cells. In contrast, meiosis consists of two nuclear divisions resulting in four nuclei that are usually partitioned into four new, genetically distinct cells. The four nuclei produced during meiosis are not genetically identical, and they contain one chromosome set only. This is half the number of chromosome sets in the original cell, which is 316 Chapter 11 | Meiosis and Sexual Reproduction diploid. The main differences between mitosis and meiosis occur in meiosis I, which is a very different nuclear division than mitosis. In meiosis I, the homologous chromosome pairs physically meet and are bound together with the synaptonemal complex. Following this, the chromosomes develop chiasmata and undergo crossover between nonsister chromatids. In the end, the chromosomes line up along the metaphase plate as tetrads—with kinetochore fibers from opposite spindle poles attached to each kinetochore of a homolog to form a tetrad. All of these events occur only in meiosis I. When the chiasmata resolve and the tetrad is broken up with the homologs moving to one pole or another, the ploidy level—the number of sets of chromosomes in each future nucleus—has been reduced from two to one. For this reason, meiosis I is referred to as a reductional division. There is no such reduction in ploidy level during mitosis. Meiosis II is analogous to a mitotic division. In this case, the duplicated chromosomes (only one set of them) line up on the metaphase plate with divided kinetochores attached to kinetochore fibers from opposite poles. During anaphase II, as in mitotic anaphase, the kinetochores divide and one sister chromatid—now referred to as a chromosome—is pulled to one pole while the other sister chromatid is pulled to the other pole. If it were not for the fact that there had been crossover, the two products of each individual meiosis II division would be identical (as in mitosis). Instead, they are different because there has always been at least one crossover per chromosome. Meiosis II is not a reduction division because although there are fewer copies of the genome in the resulting cells, there is still one set of chromosomes, as there was at the end of meiosis I. Figure 11.7 Meiosis and mitosis are both preceded by one cycle of DNA replication; however, meiosis includes two nuclear divisions. The four daughter cells resulting from meiosis are haploid and genetically distinct. The daughter cells resulting from mitosis are diploid and identical to the parent cell. This OpenStax book is available for free at http://cnx.org/content/col24361/1.8 Chapter 11 | Meiosis and Sexual Reproduction 317 The Mystery of the Evolution of Meiosis Some characteristics of organisms are so widespread and fundamental that it is sometimes difficult to remember that they evolved like other simple traits. Meiosis is such an extraordinarily complex series of cellular events that biologists have had trouble testing hypotheses concerning how it may have evolved. Although meiosis is inextricably entwined with sexual reproduction and its advantages and disadvantages, it is important to separate the questions of the evolution of meiosis and the evolution of sex, because early meiosis may have been advantageous for different reasons than it is now. Thinking outside the box and imagining what the early benefits from meiosis might have been is one approach to uncovering how it may have evolved. Meiosis and mitosis share obvious cellular processes, and it makes sense that meiosis evolved from mitosis. The difficulty lies in the clear differences between meiosis I and mitosis. Adam Wilkins and Robin Holliday summarized the unique events that needed to occur for the evolution of meiosis from mitosis. These steps are homologous chromosome pairing and synapsis, crossover exchanges, sister chromatids remaining attached during anaphase, and suppression of DNA replication in interphase. They argue that the first step is the hardest and most important and that understanding how it evolved would make the evolutionary process clearer. They suggest genetic experiments that might shed light on the evolution of synapsis. There are other approaches to understanding the evolution of meiosis in progress. Different forms of meiosis exist in single-celled protists. Some appear to be simpler or more “primitive” forms of meiosis. Comparing the meiotic divisions of different protists may shed light on the evolution of meiosis. Marilee Ramesh and colleagues compared the genes involved in meiosis in protists to understand when and where meiosis might have evolved. Although research is still ongoing, recent scholarship into meiosis in protists suggests that some aspects of meiosis may have evolved later than others. This kind of genetic comparison can tell us what aspects of meiosis are the oldest and what cellular processes they may have borrowed from in earlier cells. Click through the steps of this interactive animation to compare the meiotic process of cell division to that of mitosis at How Cells Divide (http://openstaxcollege.org/l/how_cells_dvide). 11.2 | Sexual Reproduction By the end of this section, you will be able to do the following: Explain that meiosis and sexual reproduction are highly evolved traits Identify variation among offspring as a potential evolutionary advantage of sexual reproduction Describe the three different life-cycle types among sexually reproducing multicellular organisms. 1. Adam S. Wilkins and Robin Holliday, “The Evolution of Meiosis from Mitosis,” Genetics 181 (2009): 3–12. 2. Marilee A. Ramesh, Shehre-Banoo Malik and John M. Logsdon, Jr, “A Phylogenetic Inventory of Meiotic Genes: Evidence for Sex in Giardia and an Early Eukaryotic Origin of Meiosis,” Current Biology 15 (2005):185–91. 318 Chapter 11 | Meiosis and Sexual Reproduction Sexual reproduction was likely an early evolutionary innovation after the appearance of eukaryotic cells. It appears to have been very successful because most eukaryotes are able to reproduce sexually and, in many animals, it is the only mode of reproduction. And yet, scientists also recognize some real disadvantages to sexual reproduction. On the surface, creating offspring that are genetic clones of the parent appears to be a better system. If the parent organism is successfully occupying a habitat, offspring with the same traits should be similarly successful. There is also the obvious benefit to an organism that can produce offspring whenever circumstances are favorable by asexual budding, fragmentation, or by producing eggs asexually. These methods of reproduction do not require another organism of the opposite sex. Indeed, some organisms that lead a solitary lifestyle have retained the ability to reproduce asexually. In addition, in asexual populations, every individual is capable of reproduction. In sexual populations, the males are not producing the offspring themselves, so hypothetically an asexual population could grow twice as fast. However, multicellular organisms that exclusively depend on asexual reproduction are exceedingly rare. Why are meiosis and sexual reproductive strategies so common? These are important (and as yet unanswered) questions in biology, even though they have been the focus of much research beginning in the latter half of the 20th century. There are several possible explanations, one of which is that the variation that sexual reproduction creates among offspring is very important to the survival and reproduction of the population. Thus, on average, a sexually reproducing population will leave more descendants than an otherwise similar asexually reproducing population. The only source of variation in asexual organisms is mutation. Mutations that take place during the formation of germ cell lines are also the ultimate source of variation in sexually reproducing organisms. However, in contrast to mutation during asexual reproduction, the mutations during sexual reproduction can be continually reshuffled from one generation to the next when different parents combine their unique genomes and the genes are mixed into different combinations by crossovers during prophase I and random assortment at metaphase I. The Red Queen Hypothesis Genetic variation is the outcome of sexual reproduction, but why are ongoing variations necessary, even under seemingly stable environmental conditions? Enter the Red Queen hypothesis, first proposed by Leigh Van Valen in 1973. The concept was named in reference to the Red Queen's race in Lewis Carroll's book, Through the Looking-Glass. All species coevolve (evolve together) with other organisms. For example, predators evolve with their prey, and parasites evolve with their hosts. Each tiny advantage gained by favorable variation gives a species a reproductive edge over close competitors, predators, parasites, or even prey. However, survival of any given genotype or phenotype in a population is dependent on the reproductive fitness of other genotypes or phenotypes within a given species. The only method that will allow a coevolving species to maintain its own share of the resources is to also continually improve its fitness (the capacity of the members to produce more reproductively viable offspring relative to others within a species). As one species gains an advantage, this increases selection on the other species; they must also develop an advantage or they will be outcompeted. No single species progresses too far ahead because genetic variation among the progeny of sexual reproduction provides all species with a mechanism to improve rapidly. Species that cannot keep up become extinct. The Red Queen’s catchphrase was, “It takes all the running you can do to stay in the same place.” This is an apt description of coevolution between competing species. Life Cycles of Sexually Reproducing Organisms Fertilization and meiosis alternate in sexual life cycles. What happens between these two events depends on the organism’s “reproductive strategy.” The process of meiosis reduces the chromosome number by half. Fertilization, the joining of two haploid gametes, restores the diploid condition. Some organisms have a multicellular diploid stage that is most obvious and only produce haploid reproductive cells. Animals, including humans, have this type of life cycle. Other organisms, such as fungi, have a multicellular haploid stage that is most obvious. Plants and some algae have alternation of generations, in which they have multicellular diploid and haploid life stages that are apparent to different degrees depending on the group. Nearly all animals employ a diploid-dominant life-cycle strategy in which the only haploid cells produced by the 3. Leigh Van Valen, “A New Evolutionary Law,” Evolutionary Theory 1 (1973): 1–30 This OpenStax book is available for free at http://cnx.org/content/col24361/1.8 Chapter 11 | Meiosis and Sexual Reproduction 319 organism are the gametes. Early in the development of the embryo, specialized diploid cells, called germ cells, are produced within the gonads (such as the testes and ovaries). Germ cells are capable of mitosis to perpetuate the germ cell line and meiosis to produce haploid gametes. Once the haploid gametes are formed, they lose the ability to divide again. There is no multicellular haploid life stage. Fertilization occurs with the fusion of two gametes, usually from different individuals, restoring the diploid state (Figure 11.8). Figure 11.8 In animals, sexually reproducing adults form haploid gametes from diploid germ cells. Fusion of the gametes gives rise to a fertilized egg cell, or zygote. The zygote will undergo multiple rounds of mitosis to produce a multicellular offspring. The germ cells are generated early in the development of the zygote. Most fungi and algae employ a life-cycle type in which the “body” of the organism—the ecologically important part of the life cycle—is haploid. The haploid cells that make up the tissues of the dominant multicellular stage are formed by mitosis. During sexual reproduction, specialized haploid cells from two individuals—designated the (+) and (−) mating types—join to form a diploid zygote. The zygote immediately undergoes meiosis to form four haploid cells called spores. Although these spores are haploid like the “parents,” they contain a new genetic combination from two parents. The spores can remain dormant for various time periods. Eventually, when conditions are favorable, the spores form multicellular haploid structures through many rounds of mitosis (Figure 11.9). 322 Chapter 11 | Meiosis and Sexual Reproduction KEY TERMS alternation of generations life-cycle type in which the diploid and haploid stages alternate chiasmata (singular, chiasma) the structure that forms at the crossover points after genetic material is exchanged cohesin proteins that form a complex that seals sister chromatids together at their centromeres until anaphase II of meiosis crossover exchange of genetic material between nonsister chromatids resulting in chromosomes that incorporate genes from both parents of the organism fertilization union of two haploid cells from two individual organisms gametophyte a multicellular haploid life-cycle stage that produces gametes germ cells specialized cell line that produces gametes, such as eggs or sperm interkinesis (also, interphase II) brief period of rest between meiosis I and meiosis II life cycle the sequence of events in the development of an organism and the production of cells that produce offspring meiosis a nuclear division process that results in four haploid cells meiosis I first round of meiotic cell division; referred to as reduction division because the ploidy level is reduced from diploid to haploid meiosis II second round of meiotic cell division following meiosis I; sister chromatids are separated into individual chromosomes, and the result is four unique haploid cells recombination nodules protein assemblies formed on the synaptonemal complex that mark the points of crossover events and mediate the multistep process of genetic recombination between nonsister chromatids reduction division nuclear division that produces daughter nuclei each having one-half as many chromosome sets as the parental nucleus; meiosis I is a reduction division somatic cell all the cells of a multicellular organism except the gametes or reproductive cells spore haploid cell that can produce a haploid multicellular organism or can fuse with another spore to form a diploid cell sporophyte a multicellular diploid life-cycle stage that produces haploid spores by meiosis synapsis formation of a close association between homologous chromosomes during prophase I synaptonemal complex protein lattice that forms between homologous chromosomes during prophase I, supporting crossover tetrad two duplicated homologous chromosomes (four chromatids) bound together by chiasmata during prophase I CHAPTER SUMMARY 11.1 The Process of Meiosis Sexual reproduction requires that organisms produce cells that can fuse during fertilization to produce offspring. In most animals, meiosis is used to produce haploid eggs and sperm from diploid parent cells so that the fusion of an egg and sperm produces a diploid zygote. As with mitosis, DNA replication occurs prior to meiosis during the S-phase of the cell cycle so that each chromosome becomes a pair of sister chromatids. In This OpenStax book is available for free at http://cnx.org/content/col24361/1.8 Chapter 11 | Meiosis and Sexual Reproduction 323 meiosis, there are two rounds of nuclear division resulting in four nuclei and usually four daughter cells, each with half the number of chromosomes as the parent cell. The first division separates homologs, and the second—like mitosis—separates chromatids into individual chromosomes. Meiosis generates variation in the daughter nuclei during crossover in prophase I as well as during the random alignment of tetrads at metaphase I. The cells that are produced by meiosis are genetically unique. Meiosis and mitosis share similar processes, but have distinct outcomes. Mitotic divisions are single nuclear divisions that produce genetically identical daughter nuclei (i.e., each daughter nucleus has the same number of chromosome sets as the original cell). In contrast, meiotic divisions include two nuclear divisions that ultimately produce four genetically different daughter nuclei that have only one chromosome set (instead of the two sets of chromosomes in the parent cell). The main differences between the two nuclear division processes take place during the first division of meiosis: homologous chromosomes pair, crossover, and exchange homologous nonsister chromatid segments. The homologous chromosomes separate into different nuclei during meiosis I, causing a reduction of ploidy level in the first division. The second division of meiosis is similar to a mitotic division, except that the daughter cells do not contain identical genomes because of crossover and chromosome recombination in prophase I. 11.2 Sexual Reproduction Nearly all eukaryotes undergo sexual reproduction. The variation introduced into the reproductive cells by meiosis provides an important advantage that has made sexual reproduction evolutionarily successful. Meiosis and fertilization alternate in sexual life cycles. The process of meiosis produces unique reproductive cells called gametes, which have half the number of chromosomes as the parent cell. When two haploid gametes fuse, this restores the diploid condition in the new zygote. Thus, most sexually reproducing organisms alternate between haploid and diploid stages. However, the ways in which reproductive cells are produced and the timing between meiosis and fertilization vary greatly. VISUAL CONNECTION QUESTIONS 1. Figure 11.9 If a mutation occurs so that a fungus it still be able to reproduce? is no longer able to produce a minus mating type, will REVIEW QUESTIONS 2. Meiosis usually produces ________ daughter 6. Which of the following is not true in regard to cells. crossover? a. two haploid a. Spindle microtubules guide the transfer of b. two diploid DNA across the synaptonemal complex. c. four haploid b. Nonsister chromatids exchange genetic d. four diploid material. c. Chiasmata are formed. 3. What structure is most important in forming the d. Recombination nodules mark the crossover tetrads? point. a. centromere b. synaptonemal complex 7. What phase of mitotic interphase is missing from c. chiasma meiotic interkinesis? d. kinetochore a. G0 phase 4. At which stage of meiosis are sister chromatids b. G1 phase separated from each other? c. S phase a. prophase I d. G2 phase b. prophase II 8. The part of meiosis that is similar to mitosis is c. anaphase I ________. d. anaphase II a. meiosis I 5. At metaphase I, homologous chromosomes are b. anaphase I connected only at what structures? c. meiosis II a. chiasmata d. interkinesis b. recombination nodules 9. If a muscle cell of a typical organism has 32 c. microtubules chromosomes, how many chromosomes will be in a d. kinetochores gamete of that same organism? 324 Chapter 11 | Meiosis and Sexual Reproduction a. 8 diploid multicellular stage? b. 16 a. asexual life cycles c. 32 b. most animal life cycles d. 64 c. most fungal life cycles d. alternation of generations 10. Which statement best describes the genetic content of the two daughter cells in prophase II of 15. What is the ploidy of the most conspicuous form meiosis? of most fungi? a. haploid with one copy of each gene a. diploid b. haploid with two copies of each gene b. haploid c. diploid with two copies of each gene c. alternation of generations d. diploid with four copies of each gene d. asexual 11. The pea plants used in Mendel’s genetic 16. A diploid, multicellular life-cycle stage that gives inheritance studies were diploid, with 14 rise to haploid cells by meiosis is called a ________. chromosomes in somatic cells. Assuming no crossing over events occur, how many unique gametes could a. sporophyte one pea plant produce? b. gametophyte a. 28 c. spore b. 128 d. gamete c. 196 17. Hydras and jellyfish both live in a freshwater lake d. 16,384 that is slowly being acidified by the runoff from a 12. How do telophase I and telophase II differ during chemical plant built upstream. Which population is meiosis in animal cells? predicted to be better able to cope with the changing a. Cells remain diploid at the end of telophase environment? I, but are haploid at the end of telophase II. a. jellyfish b. Daughter cells form a cell plate to divide b. hydra during telophase I, but divide by cytokinesis c. The populations will be equally able to cope. during telophase II. d. Both populations will die. c. Cells enter interphase after telophase I, but 18. Many farmers are worried about the decreasing not after telophase II. genetic diversity of plants associated with d. Chromosomes can remain condensed at the generations of artificial selection and inbreeding. Why end of telophase I, but decondense after is limiting random sexual reproduction of food crops telophase II. concerning? 13. What is a likely evolutionary advantage of sexual a. Mutations during asexual reproduction reproduction over asexual reproduction? decrease plant fitness. a. Sexual reproduction involves fewer steps. b. Consumers do not trust identical-appearing b. There is a lower chance of using up the produce. resources in a given environment. c. Larger portions of the plant populations are c. Sexual reproduction results in variation in susceptible to the same diseases. the offspring. d. Spores are not viable in an agricultural d. Sexual reproduction is more cost-effective. setting. 14. Which type of life cycle has both a haploid and CRITICAL THINKING QUESTIONS 19. Describe the process that results in the formation meiosis and mitosis? of a tetrad. 23. Why would an individual with a mutation that 20. Explain how the random alignment of prevented the formation of recombination nodules be homologous chromosomes during metaphase I considered less fit than other members of its contributes to the variation in gametes produced by species? meiosis. 24. Does crossing over occur during prophase II? 21. What is the function of the fused kinetochore From an evolutionary perspective, why is this found on sister chromatids in prometaphase I? advantageous? 22. In a comparison of the stages of meiosis to the 25. List and briefly describe the three processes that stages of mitosis, which stages are unique to meiosis lead to variation in offspring with the same parents. and which stages have the same events in both 26. Animals and plants both have diploid and haploid This OpenStax book is available for free at http://cnx.org/content/col24361/1.8 Chapter 11 | Meiosis and Sexual Reproduction 325 cells. How does the animal life cycle differ from the dominant life cycle and organisms with an alternation alternation of generations exhibited by plants? of generations life cycle? 27. Explain why sexual reproduction is beneficial to a 29. How do organisms with haploid-dominant life population but can be detrimental to an individual cycles ensure continued genetic diversification in offspring. offspring without using a meiotic process to make gametes? 28. How does the role of meiosis in gamete production differ between organisms with a diploid-

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