Cellular Reproduction: Cells from Cells PDF

8 Cellular Reproduction: Cells from Cells Why Cellular Reproduction Matters ◀ Having too few or too...

8 Cellular Reproduction: Cells from Cells Why Cellular Reproduction Matters ◀ Having too few or too many chromosomes is almost always fatal. ▲ In certain species of sea star, a severed arm may be able to regrow a whole new body. If stretched out, the DNA ▶ in any one of your cells would be taller than you. ▲ Every tumor is the result of a malfunction in cell division. 154  CHAPTER CONTENTS CHAPTER THREAD What Cell Reproduction Accomplishes 156 Life with and without Sex The Cell Cycle and Mitosis 157 BIOLOGY AND SOCIETY Virgin Birth of a Dragon 155 Meiosis, the Basis of Sexual Reproduction 164 THE PROCESS OF SCIENCE Do All Animals Have Sex? 171 EVOLUTION CONNECTION The Advantages of Sex 174 Life with and without Sex BIOLOGY AND SOCIETY Virgin Birth of a Dragon Zookeepers at the Chester Zoo in England were startled to discover that Flora, a female Komodo dragon— Varanus komodoensis, the largest living species of lizard, capable of growing up to 10 feet long—had laid a clutch of 25 eggs. It wasn’t surprising that a captive Komodo dragon would breed. In fact, Flora was at the zoo for that very reason: She was one of two female Komodo dragons that were part of a captive breeding program intended to help repopulate the species. What made Flora’s clutch of eggs so remarkable is that she had not yet been in the company of a male, let alone mated with one. As far as anyone knew, Komodo dragons, like the vast majority of animal species, cre- ate offspring only through sexual reproduction involving the union of a male’s sperm and a female’s egg. But despite Flora’s virginity, eight of her eggs developed normally and hatched into live, healthy Komodo dragons. DNA analysis confirmed that Flora’s offspring derived their genes solely from her. The new dragons must have resulted from parthenogenesis, the production of offspring by a female without involvement of a male. Parthenogenesis is one form of asexual reproduction, the creation of a new generation without participation of sperm and egg. Parthenogenesis is rare among vertebrates (animals with backbones), although it has been documented in species as diverse as sharks (includ- ing the hammerhead), domesticated birds, and now Komodo dragons. Soon, zoologists identified a second Komodo at a dif- ferent zoo who had also borne young by parthenogenesis. This The Komodo dragon. The Komodo is the world’s same ­Komodo dragon later had additional offspring via sexual largest lizard and is found in the wild only on three ­reproduction—indicating that this species is capable of switch- islands in Indonesia. ing between two reproductive modes. Biologists are investigating the evolutionary basis of this phenomenon and considering what implications it may have on efforts to repopulate this rare species. The ability of organisms to procreate is the one characteristic that best distinguishes living things from nonliving matter. All organisms—from bacteria to lizards to you—are the result of repeated cell divisions. The perpetuation of life therefore depends on cell division, the production of new cells. In this chapter, we’ll look at how individual cells are copied and then see how cell reproduction underlies the process of sexual ­reproduction. Throughout our discussion, we’ll consider examples of asexual and sexual reproduction among both plants and animals. 155  What Cell Reproduction Accomplishes Chapter 8 Cellular Reproduction: Cells from Cells When you hear the word reproduction, you probably reproduce by dividing in half, and the offspring are think of the birth of new organisms. But reproduction genetic replicas of the parent. Because it does not in- actually occurs much more often at the cellular level. volve fertilization of an egg by a sperm, this type of Consider the skin on your arm. Skin cells are reproduction is called asexual reproduction. constantly reproducing themselves and mov- Offspring produced by asexual reproduction ing outward toward the surface, replacing In certain inherit all of their chromosomes from a sin- species of sea dead cells that have rubbed off. This renewal gle parent and are thus genetic duplicates. star, a severed arm of your skin goes on throughout your life. may be able to Many multicellular organisms can repro- And when your skin is injured, additional regrow a whole duce asexually as well. For example, some cell reproduction helps heal the wound. new body. sea star species have the ability to grow new When a cell undergoes reproduction, or individuals from fragmented pieces. And if cell division, the two “daughter” cells that re- you’ve ever grown a houseplant from a clipping, sult are genetically identical to each other and to the you’ve observed asexual reproduction in plants. In original “parent” cell. (Biologists traditionally use the asexual reproduction, there is one simple principle of word daughter in this context to refer to offspring cells, inheritance: The lone parent and each of its offspring but of course cells lack gender.) Before the parent cell have identical genes. The type of cell division responsible splits into two, it duplicates its chromosomes, the struc- for asexual reproduction and for the growth and mainte- tures that contain most of the cell’s DNA. Then, during nance of multicellular organisms is called mitosis. CHECKPOINT cell division, each daughter cell receives one identical set Sexual reproduction is different; it requires fertiliza- Ordinary cell division of chromosomes from the original parent cell. tion of an egg by a sperm. The production of gametes— produces two daughter cells As summarized in Figure 8.1, cell division plays sev- egg and sperm—involves a special type of cell division that are genetically identical. eral important roles in the lives of organisms. For exam- called meiosis, which occurs only in reproductive Name three functions of this ple, within your body, millions of cells must divide every organs. As we’ll discuss later, a gamete has only half as type of cell division. Which of these functions occur in second to replace damaged or lost cells. Another function many chromosomes as the parent cell that gave rise to it. your body? of cell division is growth. All of the trillions of cells in In summary, two kinds of cell division are involved in occur in your body your body are the result of repeated cell divisions that be- the lives of sexually reproducing organisms: mitosis for gan in your mother’s body with a single fertilized egg cell. growth and maintenance and meiosis for reproduction. of an organism; only the first two an organism, asexual reproduction Answer: cell replacement, growth of Another vital function of cell division is reproduc- The remainder of the chapter is divided into two main tion. Many single-celled organisms, such as amoebas, sections, one devoted to each type of cell division. ▶ Figure 8.1 Three FUNCTIONS OF CELL DIVISION BY MITOSIS f­ unctions of cell division by mitosis. Cell Replacement Growth via Cell Division Colorized SEM 810× LM 590× The cells Division of a human of an early kidney cell into two human embryo 156 The Cell Cycle and Mitosis The Cell Cycle and Mitosis Almost all of the genes of a eukaryotic cell— Most of the time, the chromosomes ex- around 21,000 in humans—are located on ist as thin fibers that are much longer than chromosomes in the cell nucleus. (The If stretched out, the nucleus they are stored in. In fact, if main exceptions are genes on small DNA the DNA in any one fully extended, the DNA in just one of ­molecules found in mitochondria and of your cells would be your cells would be more than 6 feet long! chloroplasts.) Because chromosomes are taller than you. Chromatin in this state is too thin to be the lead players in cell division, we’ll focus seen using a light microscope. As a cell on them before turning our attention to the cell as a whole. Species Number of chromosomes in body cells Eukaryotic Chromosomes Indian muntjac deer 6 Each eukaryotic chromosome contains one very long Koala 16 DNA molecule, typically bearing thousands of genes. The number of chromosomes in a eukaryotic cell Opossum 22 depends on the species (Figure 8.2). For example, human body cells have 46 chromosomes, while the Giraffe 30 body cells of a dog have 78 and those of a koala bear have 16. Chromosomes are made up of a material Mouse 40 called chromatin, fibers composed of roughly equal amounts of DNA and protein molecules. The protein Human 46 molecules help organize the chromatin and help Duck-billed platypus 54 control the activity of its genes. Bison 60 Dog 78 ▶ Figure 8.2 The number of chromosomes in the cells of selected mammals. Notice that humans have 46 chromosomes and that the number of chromosomes Red viscacha rat 102 does not correspond to the size or complexity of an organism. Asexual Reproduction LM 250× Fragmentation and regeneration of a sea star. The sea star on the right lost and replaced an arm. The severed arm grew into the new Reproduction of an African violet from a Reproduction of an amoeba sea star on the left. clipping (large leaf) 157 Chapter 8 prepares to divide, its chromatin fibers coil up, forming cell’s tiny nucleus. Think of a DNA as a length of yarn; Cellular Reproduction: compact ­chromosomes that become visible under a light a chromosome is then like a skein of yarn, one very Cells from Cells ­microscope (­Figure 8.3). long piece that is folded into a tight package for easier Such long molecules of DNA can fit into the tiny handling. nucleus because within each chromosome the DNA is packed into an elaborate, multilevel system of ­coiling and folding. A crucial aspect of DNA packing is the association of the DNA with small proteins called ▼ Figure 8.4 DNA packing in a eukaryotic chromosome. ­histones. Why is it necessary for a cell’s chromo- Successive levels of coiling of DNA and associated proteins somes to be compacted in this way? Imagine that your ultimately result in highly compacted chromosomes. The fuzzy appearance of the final chromosome at the bottom arises from ­belongings are spread out around your room. If you the ­intricate twists and folds of the chromatin fibers. had to move, you would gather up all your things and put them in small containers. Similarly, a cell must compact its DNA before it can move it to a new cell. Figure 8.4 presents a simplified model of DNA packing. First, histones attach to the DNA. In electron micrographs, the combination DNA double helix of DNA and histones has the appearance of beads on a string. Each “bead,” called a nucleosome, consists of DNA wound around several histone molecules. Histones When not dividing, the DNA of active genes takes on this lightly packed arrangement. When preparing to divide, chromosomes condense even more: the beaded string itself wraps, loops, and folds into a tight, com- “Beads on pact structure, as you can see in the chromosome at a string” the bottom of the figure. Viewed as a whole, Figure 8.4 gives a sense of how successive levels of coiling and folding enable a huge amount of DNA to fit into a TEM 130,000× Nucleosome ▼ Figure 8.3 A plant cell just before division, with chromosomes colored by stains. LM 1,400× Duplicated chromosomes TEM 9,000× (sister chromatids) Chromosomes Centromere 158  Information its normal functions within the organism. For example, The Cell Cycle and Mitosis Duplicating Flow during interphase a cell in your stomach lining might make and release enzyme molecules that aid in digestion. Chromosomes While in interphase, a cell roughly doubles everything in Think of the chromosomes as being its cytoplasm. It increases its supply of proteins, increases like a detailed instruction manual the number of many of its organelles (such as mitochon- on how to run a cell; during divi- dria and ribosomes), and grows in size. Typically, inter- sion, the original cell must pass on phase lasts for at least 90% of the cell cycle. Chromosome a copy of the manual to the new cell duplication From the standpoint of cell reproduction, the most while also retaining a copy for itself. important event of interphase is chromosome duplica- Before a cell begins the division tion, when the DNA in the nucleus is precisely doubled. process, it must therefore duplicate Sister The period when this occurs is called the S phase (for chromatids all of its chromosomes. The DNA DNA synthesis). The interphase periods before and after molecule of each chromosome the S phase are called the G1 and G2 phases, respectively is copied through the process of (G stands for gap). During G1, each chromosome is sin- DNA replication (discussed gle, and the cell performs its normal functions. ­During in detail in ­Chapter 10), G2 (after DNA duplication during the S phase), each CHECKPOINT and new histone protein chromosome in the cell consists of two identical sister 1. A duplicated chromosome molecules attach as needed. Chromosome chromatids, and the cell prepares to divide. consists of two sister distribution to The part of the cell cycle when the cell is actually di- _________ joined together The result is that—at this daughter cells viding is called the mitotic (M) phase. It includes two at the _________. point—each chromosome 2. What are the two consists of two copies called overlapping stages, mitosis and cytokinesis. In ­mitosis, the broadest divisions of ▲ Figure 8.5 Duplication sister ­chromatids, which nucleus and its contents, most importantly the duplicated the cell cycle? What two and distribution of a contain identical genes. At chromosomes, divide and are evenly distributed, forming processes are involved in single chromosome. the bottom of Figure 8.4, the During cell reproduction, two daughter nuclei. During cytokinesis, the cytoplasm the actual duplication of the cell duplicates (along with all the organelles) is divided in two. The com- the cell? two sister chromatids are each chromosome and bination of mitosis and ­cytokinesis produces two geneti- mitosis and cytokinesis joined together most tightly distributes the two copies 2. interphase and the mitotic phase; at a narrow “waist” called the cally identical daughter cells, each fully equipped with a Answers: 1. chromatids; centromere to the daughter cells. centromere. nucleus, ­cytoplasm, ­organelles, and plasma membrane. When the cell divides, the sister chromatids of a dupli- ▼ Figure 8.6 The eukaryotic cell cycle. The cell cycle extends from the “birth” of a cell (just cated chromosome separate from each other (­ Figure 8.5). after the point indicated by the dark blue arrow at the bottom of the cycle), resulting from cell Once separated from its sister, each chromatid is con- reproduction, to the time the cell itself divides in two. (During interphase, the chromosomes sidered a full-fledged chromosome, and it is identical to are diffuse masses of thin fibers; they do not actually appear in the rodlike form you see here.) the original chromosome. One of the new chromosomes S phase goes to one daughter cell, and the other goes to the other (DNA synthesis; chromosome duplication) daughter cell. In this way, each daughter cell receives a complete and identical set of chromosomes. A dividing human skin cell, for example, has 46 duplicated chromo- somes, and each of the two daughter cells that result from Interphase: metabolism and it has 46 single chromosomes. growth (90% of time) The Cell Cycle G1 (first gap) G2 (second gap) Mitotic The rate at which a cell divides depends on its role (M) phase: within the organism’s body. Some cells divide once a day, cell division others less often, and some highly specialized cells, such (10% of time) as mature muscle cells, do not divide at all. The cell cycle is the ordered sequence of events that Cytokinesis extends from the time a cell is first formed from a divid- (division of Mitosis ing parent cell until its own division into two cells. Think cytoplasm) (division of nucleus) of the cell cycle as the “lifetime” of a cell, from its “birth” to its own reproduction. As Figure 8.6 shows, most of the cell cycle is spent in interphase. Interphase is a time when a cell goes about its usual business, performing 159 Chapter 8 Cellular Reproduction: Mitosis and Cytokinesis chromosomes depicted in blue. The drawings in the top row include details that are not visible in the mi- Cells from Cells Figure 8.7 illustrates the cell cycle for an animal cell crographs. In these cells, we illustrate just four chro- using drawings, descriptions, and photomicrographs. mosomes to keep the process a bit simpler to follow; The micrographs running along the bottom row of remember that one of your cells actually contains the page show dividing cells from a salamander, with 46 chromosomes. The text within the figure describes ▼ Figure 8.7 Cell reproduction: A dance of the chromosomes. After the chromosomes duplicate during interphase, the elaborately choreographed stages of mitosis—prophase, metaphase, anaphase, and telophase—distribute the duplicate sets of chromosomes to two separate nuclei. Cytokinesis then divides the cytoplasm, yielding two genetically identical daughter cells. INTERPHASE PROPHASE Fragments of Centrosomes Uncondensed Mitotic spindle forming Centromere nuclear envelope Condensed chromosome chromosome Nuclear Plasma Duplicated chromosome, consisting Spindle envelope membrane of two sister chromatids tracks Interphase is the period of cell growth when the During prophase, changes occur in both nucleus centromere. In the cytoplasm, the mitotic spindle cell makes new molecules and organelles. At the and cytoplasm. In the nucleus, the chromatin fibers begins to form. Late in prophase, the nuclear point shown here, late interphase (G2), the coil, so that the chromosomes become thick enough envelope breaks into pieces. The spindle tracks attach cytoplasm contains two centrosomes. Within the to be seen individually with a light microscope. Each to the centromeres of the chromosomes and move nucleus, the chromosomes are duplicated, but chromosome exists as two identical sister chromatids the chromosomes toward the center of the cell. they cannot be distinguished individually because joined together at the narrow “waist” of the they are still in the form of loosely packed chromatin fibers. LM 375× 160  the events occurring at each stage. Study this figure care- stages run into each other and vary from individual to The Cell Cycle fully (it has a lot of information and it’s important!) and individual; so it is with the stages of mitosis. and Mitosis notice the striking changes in the nucleus and other cel- The chromosomes are the stars of the mitotic drama, lular structures. and their movements depend on the mitotic spindle, a Biologists distinguish four main stages of mitosis: football-shaped structure of microtubule tracks (colored prophase, metaphase, anaphase, and telophase. The green in the figure) that guides the separation of the two timing of these stages is not precise, and they overlap sets of daughter chromosomes. The tracks of spindle a bit. Think of stages in your own life—infancy, child- microtubules grow from structures within the cytoplasm hood, adulthood, old age—and you’ll realize that the called centrosomes. METAPHASE ANAPHASE TELOPHASE Condensed chromosomes align Nuclear envelope Cleavage forming furrow Separated chromosomes The mitotic spindle is now fully formed. The Anaphase begins suddenly when the sister Telophase begins when the two groups of centromeres of all the chromosomes line up chromatids of each chromosome separate. Each is chromosomes have reached opposite ends of the between the two poles of the spindle. For each now considered a full-fledged (daughter) cell. Telophase is the reverse of prophase: Nuclear chromosome, the tracks of the mitotic spindle chromosome. The chromosomes move toward envelopes form, the chromosomes uncoil, and the attached to the two sister chromatids pull toward opposite poles of the cell as the spindle tracks spindle disappears. Mitosis, the division of one opposite poles. This tug of war keeps the shorten. Simultaneously, the tracks not attached to nucleus into two genetically identical daughter chromosomes in the middle of the cell. chromosomes lengthen, pushing the poles farther nuclei, is now finished. Cytokinesis, the division of apart and elongating the cell. the cytoplasm, usually occurs with telophase. In animals, a cleavage furrow pinches the cell in two, producing two daughter cells. 161 M08_SIMO2368_05_GE_CH08.indd 161 25/09/15 10:11 AM Chapter 8 ▼ Figure 8.8 Cytokinesis in animal and plant cells. Cellular Reproduction: Wall of Cell plate Daughter Cells from Cells SEM 100× parent cell forming nucleus LM 640× Cleavage furrow Vesicles containing Cell wall cell wall material Cell plate New cell wall Cleavage furrow Contracting ring of microfilaments Daughter cells Daughter cells (a) Animal cell cytokinesis (b) Plant cell cytokinesis CHECKPOINT Cytokinesis, the division of the cytoplasm into two Cytokinesis in a plant cell occurs differently. Vesicles An organism called a cells, usually begins during telophase, overlapping the containing cell wall material collect at the middle of plasmodial slime mold is one huge cytoplasmic mass end of mitosis. In animal cells, the cytokinesis process is the cell. The vesicles fuse, forming a membranous disk with many nuclei. Explain known as cleavage. The first sign of cleavage is the ap- called the cell plate. The cell plate grows outward, ac- how a variation in the cell pearance of a cleavage furrow, an indentation at the equa- cumulating more cell wall material as more vesicles join cycle could cause this tor of the cell. A ring of microfilaments in the cytoplasm it. Eventually, the membrane of the cell plate fuses with “monster cell” to arise. just under the plasma membrane contracts, like the pull- the plasma membrane, and the cell plate’s contents join without cytokinesis. Answer: Mitosis occurs repeatedly ing of a drawstring on a hooded sweatshirt, deepening the the parental cell wall. The result is two daughter cells furrow and pinching the parent cell in two (Figure 8.8a). (Figure 8.8b). Cancer Cells: Dividing Out of Control For a plant or animal to grow and maintain its tissues the cell will switch into a permanently nondividing state. normally, it must be able to control the timing of cell di- Some of your nerve and muscle cells, for example, are vision—speeding up, slowing down, or turning the arrested this way. If the go-ahead signal is received process off/on as needed. The sequential events and the G1 checkpoint is passed, the cell will of the cell cycle are directed by a cell cycle usually complete the rest of the cycle. ­control system that consists of specialized Every tumor is proteins within the cell. These proteins the result of a What Is Cancer? integrate information from the environ- malfunction in cell Cancer, which currently claims the lives of ment and from other body cells and send division. one out of every five people in the United “stop” and “go-ahead” signals at certain key States and other industrialized nations, is a points during the cell cycle. For example, the disease of the cell cycle. Cancer cells do not cell cycle normally halts within the G1 phase of respond normally to the cell cycle control system; interphase unless the cell receives a go-ahead signal they divide excessively and may invade other tissues of via certain control proteins. If that signal never arrives, the body. If unchecked, cancer cells may continue to divide 162 until they kill the host. Cancer cells are thus referred to as of high-energy radiation, which often harm cancer cells The Cell Cycle ­“immortal” since, unlike other human cells, they will never more than normal cells. Radiation therapy is often effective and Mitosis cease dividing. In fact, thousands of laboratories around the against malignant tumors that have not yet spread. How- world today use HeLa cells, a laboratory strain of human ever, there is sometimes enough damage to normal body cells that were originally obtained from a woman named cells to produce side effects, such as nausea and hair loss. Henrietta Lacks, who died of cervical cancer in 1951. Chemotherapy, the use of drugs to disrupt cell divi- The abnormal behavior of cancer cells begins when a sion, is used to treat widespread or metastatic tumors. single cell undergoes genetic changes (mutations) in one Chemotherapy drugs work in a variety of ways. Some pre- or more genes that encode for proteins in the cell cycle vent cell division by interfering with the mitotic spindle. control system. These changes cause the cell to grow ab- For example, paclitaxel (trade name Taxol) freezes the normally. The immune system generally recognizes and spindle after it forms, keeping it from functioning. Pacli- destroys such cells. However, if the cell evades destruction, taxel is made from a chemical discovered in the bark of it may proliferate to form a tumor, an abnormally grow- the Pacific yew, a tree found mainly in the northwestern ing mass of body cells. If the abnormal cells remain at the United States. It has fewer side effects than many other original site, the lump is called a benign tumor. Benign anticancer drugs and seems to be effective against some tumors can cause problems if they grow large and disrupt hard-to-treat cancers of the ovary and breast. Another certain organs, such as the brain, but often they can be drug, vinblastine, prevents the mitotic spindle from form- completely removed by surgery and are rarely deadly. ing in the first place. Vinblastine was first obtained from In contrast, a malignant tumor is one that has the the periwinkle plant, which is native to the tropical rain potential to spread into neighboring tissues and other forests of Madagascar. Given these examples, preserving parts of the body, forming new tumors (Figure 8.9). A biodiversity may be the key to discovering the next gen- malignant tumor may or may not have actually begun eration of lifesaving anticancer drugs. to spread, but once it does, it will soon displace nor- mal tissue and interrupt organ function. An individual Cancer Prevention and Survival with a malignant tumor is said to have cancer. The Although cancer can strike anyone, there are certain life- spread of cancer cells beyond their original site is called style changes you can make to reduce your chances of ­metastasis. Cancers are named according to where they developing cancer or increase your chances of surviving it. originate. Liver cancer, for example, always begins in Not smoking, exercising adequately (usually defined as at liver tissue and may spread from there. least 150 minutes of moderate exercise each week), avoid- ing overexposure to the sun, and eating a high-fiber, low-fat Cancer Treatment diet can all help reduce the likelihood of getting cancer. Once a tumor starts growing in the body, how can it be Seven types of cancer can be easily detected: skin and oral CHECKPOINT treated? There are three main types of cancer treatment. (via physical exam), breast (via self-exams or mammograms What differentiates a benign Surgery to remove a tumor is usually the first step. For for higher-risk women and women 50 and older), prostate tumor from a malignant many benign tumors, surgery may be sufficient. If it is not, (via rectal exam), cervical (via Pap smear), testicular (via tumor? doctors turn to treatments that attempt to stop cancer cells self-exam), and colon (via colonoscopy). Regular visits to malignant tumor can spread. at its point of origin, whereas a from dividing. In radiation therapy, parts of the body that the doctor can help identify tumors early, which is the best Answer: A benign tumor remains have cancerous tumors are exposed to concentrated beams way to increase the chance of successful treatment. ▼ Figure 8.9 Growth and metastasis of a malignant ­tumor of the breast. Lymph vessels Tumor Blood vessel Tumor in Glandular another part tissue of the body A tumor grows from a Cancer cells invade Metastasis: Cancer cells spread single cancer cell. neighboring tissue. through lymph and blood vessels to other parts of the body. 163  Meiosis, the Basis of Sexual Chapter 8 Cellular Reproduction: Cells from Cells Reproduction Only maple trees produce more maple trees, only goldfish Sexual reproduction depends on the cellular pro- make more goldfish, and only people make more people. cesses of meiosis and fertilization. But before discussing These simple facts of life have been recognized for thou- these processes, we need to return to chromosomes and sands of years and are reflected in the age-old saying, the role they play in the life cycle of sexually reproduc- “Like begets like.” But in a strict sense, “Like begets like” ing organisms. applies only to asexual reproduction, where offspring inherit all their DNA from a single parent. Asexual off- spring are exact genetic replicas of that one parent and of each other, and their appearances are Homologous Chromosomes very similar. If we examine cells from different individuals of a single The family photo in ­Figure 8.10 species—sticking to one sex, for now—we find that makes the point that in a sexually they have the same number and types of chromosomes. reproducing species, like does not Viewed with a microscope, your chromosomes would exactly beget like. You probably look exactly like those of Angelina Jolie (if you’re a ­resemble your biological par- woman) or Brad Pitt (if you’re a man). ents more closely than you A typical body cell, called a somatic cell, has 46 chro- resemble strangers, but you mosomes in humans. A technician can break open a hu- do not look exactly like your man cell in metaphase of mitosis, stain the chromosomes parents or your siblings— with dyes, take a picture with the aid of a microscope, and unless you are an identi- arrange the chromosomes in matching pairs by size. The cal twin. Each offspring of resulting display is called a karyotype (­Figure 8.11). Notice ▲ Figure 8.10 The ­varied sexual reproduction inherits in the figure that each chromosome is duplicated, with products of sexual a unique combination of genes from its two sister chromatids joined along their length; within the ­reproduction. Every child two parents, and this combined set of genes programs white box, for example, the left “stick” is actually a pair of ­inherits a unique combina- sister chromatids stuck together (as shown in the drawing tion of genes from his or a unique combination of traits. As a result, sexual re- her parents and displays a production can produce tremendous variety among to the left). Notice also that almost every chromosome has unique ­combination of traits. offspring. a twin that resembles it in length and centromere position; in the figure, the white box surrounds one set of twin chro- mosomes. The two chromosomes of such a matching pair, ▼ Figure 8.11 Pairs of homologous chromosomes in a human male called h­ omologous chromosomes, carry genes controlling karyotype. This karyotype shows 22 completely homologous pairs (autosomes) and a 23rd pair that consists of an X chromosome and a Y chromosome (sex the same inherited characteristics. For example, if a gene chromosomes). With the exception of X and Y, the homologous chromosomes influencing freckles is located at a particular place on one of each pair match in size, centromere position, and staining pattern. chromosome—within the yellow band in the drawing in Figure 8.11, for instance—then the homologous chromo- LM 3,600× Pair of homologous chromosomes some has that same gene in the same location. However, the two homologous chromosomes may have different versions of the same gene. Let’s restate this concept be- cause it often confuses students: A pair of homologous chromosomes has two nearly identical chromosomes, each Centromere of which consists of two identical sister chromatids after chromosome duplication. In human females, the 46 chromosomes fall neatly into 23 homologous pairs. For a male, however, the chromosomes in one pair do not look alike. This non- One duplicated chromosome matching pair, only partly homologous, is the male’s sex Sister chromosomes­. ­Sex chromosomes determine a person’s chromatids sex (male versus female). In mammals, males have one X chromosome and one Y chromosome. Females have 164 two X chromosomes. (Other organisms have different sys- Haploid gametes (n = 23) Meiosis, the Basis of tems; in this chapter, we focus on humans.) The remaining Sexual Reproduction chromosomes (44 in humans), found in both males and n Egg cell females, are called autosomes. For both autosomes and sex chromosomes, you inherited one chromosome of each pair from your mother and the other from your father. n Sperm cell Gametes and the Life MEIOSIS FERTILIZATION Cycle of a Sexual Organism The life cycle of a multicellular organism Multicellular Diploid is the sequence of stages leading from the diploid adults zygote (2n = 46) 2n (2n = 46) adults of one generation to the adults of the next. Having two sets of chromosomes, one ◀ Figure 8.12 The human inherited from each parent, is a key factor in MITOSIS life cycle. In each generation, the life cycle of humans and all other species that and development the doubling of chromosome Key reproduce sexually. ­Figure 8.12 shows the human number that results from Haploid (n) life cycle, emphasizing the number of chromosomes. fertilization is offset by the Humans (as well as most other animals and many Diploid (2n) halving of chromosome number during meiosis. plants) are ­diploid organisms because all body cells contain pairs of homologous chromosomes. In other words, all your chromosomes come in matching sets. Producing haploid gametes by meiosis keeps the chro- This is similar to shoes in your closet: You may have mosome ­number from doubling in every generation. 46 shoes, but they are organized as 23 pairs, with the To illustrate, ­Figure 8.13 tracks one pair of homologous members of each pair being nearly identical to each ­chromosomes. Each of the chromosomes is duplicated other. The total number of chromosomes, 46 in humans, during interphase (before mitosis). The first division, is the diploid number ­(abbreviated 2n). The gametes, meiosis I, ­segregates the two chromosomes of the homol- egg and sperm cells, are not diploid. Made by meiosis in ogous pair, packaging them in separate (haploid) daughter an ovary or testis, each gamete has a single set of chro- cells. But each chromosome is still doubled. Meiosis II mosomes: 22 autosomes plus a sex chromosome, either separates the sister chromatids. Each of the four daughter X or Y. A cell with a single chromosome set is called a cells is haploid and contains only a single chromosome haploid cell; it has only one member of each pair of ho- from the pair of homologous chromosomes. mologous chromosomes. To visualize the ­haploid state, imagine your closet containing only one shoe from each pair. For humans, the haploid number, n, is 23. ▼ Figure 8.13 How meiosis halves In the human life cycle, a haploid sperm fuses with chromosome number. a haploid egg in a process called ­fertilization. The ­resulting fertilized egg, called a zygote, is diploid. It 2 Homologous 1 Chromosomes chromosomes has two sets of chromosomes, one set from each duplicate. separate. parent. The life cycle is completed as a sexually mature adult develops from the zygote. Mitotic 3 Sister cell division ensures that all somatic cells of the chromatids separate. human body receive a copy of all of the zygote’s 46 chromosomes. Thus, every one of the tril- lions of cells in your body can trace its ances- Pair of homologous A pair of Sister chromosomes in homologous chromatids of try back through mitotic divisions to the single diploid parent cell chromosomes one duplicated zygote produced when your ­father’s sperm and chromosome your mother’s egg fused about nine months before you were born ­(although you probably don’t want to INTERPHASE BEFORE MEIOSIS MEIOSIS I MEIOSIS II dwell on those details!). Haploid daughter cells 165  The second difference of meiosis compared with Chapter 8 Cellular Reproduction: The Process of Meiosis mitosis is an exchange of genetic material—pieces of Cells from Cells Meiosis, the process of cell division that produces hap- chromosomes—between homologous chromosomes. loid gametes in diploid organisms, resembles mitosis, This exchange, called crossing over, occurs during the but with two important differences. The first difference first prophase of meiosis. We’ll look more closely at is that, during meiosis, the number of chromosomes is crossing over later. For now, study Figure 8.14, including cut in half. In meiosis, a cell that has duplicated its chro- the text below it, which describes the stages of meiosis mosomes undergoes two consecutive divisions, called in detail for a hypothetical animal cell containing four meiosis I and meiosis II. Because one duplication of the chromosomes. chromosomes is followed by two divisions, each of the As you go through Figure 8.14, keep in mind the dif- four daughter cells resulting from meiosis has a haploid ference between homologous chromosomes and sister set of chromosomes—half as many chromosomes as the chromatids: The two chromosomes of a homologous starting cell. pair are individual chromosomes that were inherited ▼ Figure 8.14 The stages MEIOSIS I: HOMOLOGOUS CHROMOSOMES SEPARATE of meiosis. INTERPHASE PROPHASE I METAPHASE I ANAPHASE I Sister chromatids Sites of crossing over Spindle tracks attached remain attached Centrosomes to chromosome Spindle Nuclear Sister Pair of Centromere envelope Uncondensed chromatids homologous chromosomes chromosomes Chromosomes duplicate. Homologous chromosomes Pairs of homologous Pairs of homologous pair up and exchange segments. chromosomes line up. chromosomes split up. As with mitosis, meiosis is preceded Prophase I As the chromosomes Metaphase I At metaphase I, the Anaphase I The attachment between by an interphase during which the coil up, special proteins cause the homologous pairs are aligned in the the homologous chromosomes of each chromosomes duplicate. Each homologous chromosomes to stick middle of the cell. The sister pair breaks, and the chromosomes now chromosome then consists of two together in pairs. The resulting chromatids of each chromosome migrate toward the poles of the cell. In identical sister chromatids. The structure has four chromatids. are still attached at their centro- contrast to mitosis, the sister chroma- chromosomes consist of uncon- Within each set, chromatids of the meres, where they are anchored to tids migrate as a pair instead of densed chromatin fibers. homologous chromosomes spindle tracks. Notice that for each splitting up. They are separated not exchange corresponding chromosome pair, the spindle from each other but from their segments—they “cross over.” tracks attached to one homologous homologous partners. Crossing over rearranges genetic chromosome come from one pole information. of the cell, and the tracks attached As prophase I continues, the to the other chromosome come chromosomes coil up further, a from the opposite pole. With this spindle forms, and the homolo- arrangement, the homologous gous pairs are moved toward the chromosomes are poised to move center of the cell. toward opposite poles of the cell. 166  LM 900× from different parents, one from the mother and Meiosis, the Basis of one from the father. The members of a pair of Sexual Reproduction homologous chromosomes in Figure 8.14 (and later figures) are identical in size and shape but colored in the illustrations differ- CHECKPOINT ently (red versus blue) to remind you that If a single diploid somatic they differ in this way. In the interphase just cell with 18 chromosomes before meiosis, each chromosome dupli- undergoes meiosis and cates to form sister chromatids that remain produces sperm, the result together until anaphase of meiosis II. Before will be _________ sperm, Meiosis II in each with _________ crossing over occurs, sister chromatids are a lily cell chromosomes. (Provide two identical and carry the same versions of all numbers.) their genes. Answer: four; nine MEIOSIS II: SISTER CHROMATIDS SEPARATE TELOPHASE I AND CYTOKINESIS PROPHASE II METAPHASE II ANAPHASE II TELOPHASE II AND CYTOKINESIS Cleavage furrow Sister chromatids Haploid daughter cells separate forming Two haploid cells form; During another round of cell division, the sister chromatids finally separate; chromosomes are still doubled. four haploid daughter cells result, containing single chromosomes. Telophase I and Cytokinesis In The Process of Meiosis II Meiosis II is essentially the In anaphase II, the centromeres of sister chromatids telophase I, the chromosomes arrive same as mitosis. The important difference is that meiosis II separate, and the sister chromatids of each pair move at the poles of the cell. When they starts with a haploid cell that has not undergone chromo- toward opposite poles of the cell. In telophase II, nuclei finish their journey, each pole has a some duplication during the preceding interphase. form at the cell poles, and cytokinesis occurs at the haploid chromosome set, although During prophase II, a spindle forms and moves the same time. There are now four haploid daughter cells, each chromosome is still in duplicate chromosomes toward the middle of the cell. During each with single chromosomes. form. Usually, cytokinesis occurs along metaphase II, the chromosomes are aligned as they are in with telophase I, and two haploid mitosis, with the tracks attached to the sister chromatids of daughter cells are formed. each chromosome coming from opposite poles. 167 Chapter 8 Cellular Reproduction: Review: Comparing Mitosis parent cell. Meiosis, needed for sexual reproduction, yields genetically unique haploid daughter cells—cells with only Cells from Cells and Meiosis one member of each homologous chromosome pair. For both mitosis and meiosis, the You have now learned the two ways chromosomes duplicate only once, that cells of eukaryotic organisms di- in the preceding interphase. vide (Figure 8.15). Mitosis—which ­Mitosis involves one division provides for growth, tissue repair, of the nucleus and cytoplasm and asexual reproduction—­ Parent cell Parent cell (duplication, then division produces daughter cells that (before chromosome duplication) (before chromosome duplication) in half), producing two are genetically identical to the 2n = 4 2n = 4 ▶ Figure 8.15 Comparing MITOSIS MEIOSIS mitosis and meiosis. The events unique to meiosis occur during meiosis I: In prophase I, Prophase Prophase I duplicated homologous Chromosome Chromosome MEIOSIS I chromosomes pair along their duplication duplication lengths, and crossing over (during preceding (during preceding occurs between homologous interphase) interphase) (nonsister) chromatids. In metaphase I, pairs of Duplicated chromosome (two sister chromatids) homologous chromosomes (rather than individual chromosomes) are aligned at the center of the cell. During anaphase I, sister chromatids Homologous chromosomes come of each chromosome stay together in pairs. Site of crossing over together and go to the same between homologous (nonsister) chromatids pole of the cell as homologous chromosomes separate. At the end of meiosis I, there are Metaphase Metaphase I two haploid cells, but each chromosome still has two sister chromatids. Individual chromosomes Homologous pairs align at the middle align at the middle of the cell. of the cell. Anaphase Anaphase I Telophase Telophase I Chromosomes with two sister chromatids Sister chromatids separate during Homologous anaphase. chromosomes separate during Haploid 2n Daughter cells 2n anaphase I; Daughter n=2 of mitosis sister chromatids cells of meiosis I remain attached. MEIOSIS II Sister chromatids separate during anaphase II. n n n n Daughter cells of meiosis II 168 diploid cells. Meiosis entails two nuclear and cytoplas- of meiosis I affects the resulting gametes. Once again, Meiosis, the Basis of mic divisions (duplication, division in half, then division our example is from a hypothetical diploid organ- Sexual Reproduction in half again), yielding four haploid cells. ism with four chromosomes (two pairs of homolo- Figure 8.15 compares mitosis and meiosis, tracing gous chromosomes), with colors used to differentiate these two processes for a diploid parent cell with four ­homologous chromosomes (red for chromosomes chromosomes. As before, homologous chromosomes inherited from the mother and blue for chromosomes are those matching in size. (Imagine that the red chro- from the father). mosomes were inherited from the mother and the blue When aligned during metaphase I, the side-by-side CHECKPOINT chromosomes from the father.) Notice that all the events orientation of each homologous pair of chromosomes is True or false: Both mitosis unique to meiosis occur during meiosis I. Meiosis II is a matter of chance—either the red or blue chromosome and meiosis are preceded virtually identical to mitosis in that it separates sister may be on the left or right. Thus, in this example, there are by chromosome duplication. chromatids. But unlike mitosis, meiosis II yields daugh- two possible ways that the chromosome pairs can align Answer: true ter cells with a haploid set of chromosomes. during metaphase I. In possibility 1, the chromosome pairs are oriented with both red chromosomes on the same side (blue/red and blue/red). In this case, each of the The Origins of Genetic gametes produced at the end of meiosis II has only red or only blue chromosomes (combinations a and b). In pos- Variation sibility 2, the chromosome pairs are oriented differently As we discussed earlier, offspring that result from sexual (blue/red and red/blue). This arrangement produces gam- reproduction are genetically different from their parents etes with one red and one blue chromosome (combina- and from one another. How does meiosis produce such tions c and d). Thus, with the two possible arrangements genetic variation? shown in this example, the organism will produce gametes with four different combinations of chromosomes. For a Independent Assortment of Chromosomes species with more than two pairs of chromosomes, such Figure 8.16 illustrates one way in which meiosis con- as humans, every chromosome pair orients independently tributes to genetic variety. The figure shows how the ar- of all the others at metaphase I. (Chromosomes X and Y rangement of homologous chromosomes at metaphase behave as a homologous pair in meiosis.) ◀ Figure 8.16 Results of POSSIBILITY 1 POSSIBILITY 2 alternative arrangements of chromosomes at metaphase of meiosis I. The arrangement Two equally probable arrangements of chromosomes at metaphase I of chromosomes at metaphase of determines which chromosomes meiosis I will be packaged together in the haploid gametes. Metaphase of meiosis II Gametes Combination a Combination b Combination c Combination d Because possibilities 1 and 2 are equally likely, the four possible types of gametes will be made in approximately equal numbers. 169 Chapter 8 For any species, the total number of chromosome are closely paired all along their lengths, with a precise Cellular Reproduction: combinations that can appear in gametes is 2n, where gene-by-gene alignment. Cells from Cells n represents the haploid number. For the hypothetical The exchange of segments between nonsister organism in Figure 8.16, n = 2, so the number of chro- ­chromatids—one maternal chromatid and one pa- mosome combinations is 22, or 4. For a human (n = 23), ternal chromatid of a homologous pair—adds to the there are 223, or about 8 million, possible chromosome genetic variety resulting from sexual reproduction. combinations! This means that every gamete a person In ­Figure 8.18, if there were no crossing over, meiosis produces contains one of about 8 million possible com- could produce only two types of gametes: the ones binations of maternal and paternal chromosomes. When ending up with chromosomes that exactly match the you consider that a human egg cell with about 8 million parents’ chromosomes, either all blue or all red (as possibilities is fertilized at random by a human sperm in Figure 8.16). With crossing over, gametes arise cell with about 8 million possibilities (Figure 8.17), you can see that a single man and a single woman can produce zygotes with 64 trillion combinations of ▼ Figure 8.18 The results of crossing over during chromosomes! ­ eiosis for a single pair of homologous chromosomes. m A real cell has multiple pairs of homologous chromosomes that Crossing Over produce a huge variety of recombinant chromosomes in the So far, we have focused on genetic variety in gametes gametes. and zygotes at the whole-chromosome level. We’ll now take a closer look at crossing over, the exchange of Prophase I Duplicated pair of of meiosis homologous corresponding segments between nonsister chroma- chromosomes tids of homologous chromosomes, which occurs dur- ing ­prophase I of meiosis. Figure 8.18 shows crossing ▼ Figure 8.17 The process over between two homologous chromosomes and the of fertilization: a close-up view. Here you see many ­resulting gametes. At the time that crossing over begins Homologous (nonsister) very early in prophase I, homologous chromosomes chromatids exchange human sperm contacting an corresponding segments, egg. Only one sperm can remaining attached at the add its chromosomes to Site of crossing over crossover points. ­produce a zygote. Metaphase I Spindle Sister chromatids remain joined at their centromeres. Metaphase II Gametes Recombinant chromosomes combine genetic Colorized LM 1320× information originally derived from different parents. Recombinant chromosomes 170 with chromosomes that are partly from the mother Because most chromosomes contain thousands of Meiosis, the Basis of and partly from the father. These chromosomes are genes, a single crossover event can affect many genes. Sexual Reproduction called “recombinant” because they result from genetic When we also consider that multiple crossovers can oc- recombination, the production of gene combina- cur in each pair of homologous chromosomes, it’s not tions different from those carried by the parental surprising that gametes and the offspring that result CHECKPOINT chromosomes. from them are so incredibly varied. Name two events during meiosis that contribute to genetic variety among gametes. During what stages of meiosis does each occur? Life with and without Sex THE PROCESS OF SCIENCE metaphase I of homologous chromosomes at orientation/assortment of the pairs prophase I and independent homologous chromosomes during Do All Animals Have Sex? In a simple but elegant experiment, the research- Answer: crossing over between ers compared the sequences of a particular gene in As discussed in the Biology and Society section, some bdelloid and non-bdelloid rotifers. Their results were species such as Komodo dragons can reproduce via striking. Among non-bdelloid rotifers that reproduce both sexual and asexual routes. Although some animal sexually, the two homologous versions of the gene species can reproduce asexually, very few animals re- were nearly identical, differing by only 0.5% on aver- produce only asexually. In fact, evolutionary biologists age. In contrast, the two versions of the same gene have traditionally considered asexual reproduction an in bdelloid rotifers differed by 3.5–54%. These data evolutionary dead end (for reasons we’ll discuss in the provided strong evidence that bdelloid rotifers Evolution Connection section at the end of the chapter). have evolved for millions of years with- To investigate a case in which asexual reproduc- out any sexual reproduction. tion seemed to be the norm, researchers from Harvard University studied a group of animals called bdelloid rotifers (Figure 8.19). This class of nearly microscopic ▶ Figure 8.19 freshwater invertebrates includes more than 300 known A bdelloid rotifer. species. Despite hundreds of years of observations, no LM 300× one had ever found bdelloid rotifer males or evidence of sexual reproduction. But the possibility remained that bdelloids had sex very infrequently or that the males were impossible to recognize by appearance. Thus, the ­Harvard research team posed the following question: Does this entire class of animals reproduce solely by asexual means? The researchers formed the hypothesis that bdel- loid rotifers have indeed thrived for millions of years without sexually reproducing. But how could this hypothesis be tested? In most species, the two ver- sions of a gene in a pair of homologous chromo- somes are very similar due to the constant trading of genes during sexual reproduction. If a species has survived without sex for millions of years, the researchers reasoned, then changes in the DNA sequences of homologous genes should accumulate independently, and the two versions of the genes should have significantly ­diverged from each other over time. This led to the ­prediction that bdelloid rotifers would display much more variation in their pairs of homologous genes than most organisms. 171 Chapter 8 Cellular Reproduction: When Meiosis Goes Awry Abnormal egg cell with extra Cells from Cells chromosome So far, our discussion of meiosis has focused on the process as it normally and correctly occurs. But what happens when there is an error in the process? Such a mistake can result in genetic abnormalities that range from mild to severe to fatal. n+1 How Accidents during Meiosis Can Alter Chromosome Number Normal sperm cell Within the human body, meiosis occurs repeatedly as the testes or ovaries produce gametes. Almost always, Abnormal zygote with extra chromosome chromosomes are distributed to daughter cells without n (normal)