Mitosis, Cell Development, and Meiosis PDF

Chapter 12, 21, 13: Mitosis, cell development, and meiosis  Outline Introduction to cell division. Eukaryotic chromosomes. Eukaryotic cell cycle and mitosis. Cell development. Meiosis. Regulation of the cell cycle....

Chapter 12, 21, 13: Mitosis, cell development, and meiosis  Outline Introduction to cell division. Eukaryotic chromosomes. Eukaryotic cell cycle and mitosis. Cell development. Meiosis. Regulation of the cell cycle. Readings: Chapter 12, p256 - p274. Chapter 21, p430 - p436, p441 - p443. Chapter 13, p276 - p294.  Learning outcomes By the end of this lesson, you should be able to 1. describe biological significance of cell division. 2. compare and contrast cell division in prokaryotic cells and eukaryotic cells. 3. distinguish between the following terms: replicated and unreplicated chromosomes; chromosome, chromatin, and chromatid; centrosome and centromere. 4. list the phases of the cell cycle and describe the events during each phase. 5. draw or describe the spindle apparatus, including MTOCs, kinetochore microtubules, polar microtubules, and aster microtubules. 6. list the phases of mitosis and describe the events characteristic of each phase. 7. compare cytokinesis in animals and plants.  Learning outcomes cont’ By the end of this lesson, you should be able to 8. explain how cells regulate their life cycle. 9. explain how homologous chromosomes are paired. 10. distinguish between the following terms: diploid and haploid; sex chromosomes and autosomes; somatic cells and gametes; genes and alleles; sister chromatids and nonsister chromatids. 11. describe the events that characterize each phase of meiosis. 12. describe a generalized animal life cycle and a generalize plant life cycle. 13. name and explain the three events that contribute to genetic variation in sexually reproducing organisms. 14. define nondisjunction and explain the consequences of nondisjunction.  Introduction to cell division Cell division is the process which a single cell becomes two daughter cells. This process must satisfy two requirements. 1. The two daughter cells must each receive the complete genome present in the single parent cell. What process copies the cell’s complete DNA sequence? 2. The parent cell must be large enough to divide in two and still contribute sufficient cytoplasmic components to each daughter cell. What are some examples of cytoplasmic components?  Introduction to cell division When or why might cell division happen? 1. Reproduction. 2. Growth and development. 3. Tissue renewal of old/damaged cells.  Cellular organization of the genetic material How is a genome organized in a prokaryotic cell? How is a genome organized in a eukaryotic cell? Where is the genome stored in eukaryotes and prokaryotes? Prokaryotes reproduce by a type of cell division called binary fission. 1. The circular DNA is attached by proteins to inside of the cell membrane. 2. DNA replication is initiated and occurs in both direction. 3. The daughter DNA is also attached to the cell membrane. 4. The cell elongates, the two DNA attachment sites move apart. 5. Cell division begins with the synthesis of new cell membrane and cell wall. 6. Continued synthesis results in two identical daughter cells.  Eukaryotic chromosomes Eukaryotic cells are more complex and larger than prokaryotic cells and have more genes. How many genes do you have in one of your cells? Where are most of your genes located in your cell? Where else would you find genes in your cell? How are genes stored in a human cell? What are the two major molecules that make up your chromosomes?  Chromatin and chromosomes Each DNA molecule is packaged with histone proteins into nucleosomes forming chromatin, which can be further looped, packaged, and condensed to form chromosomes. Chromosomes are formed at the beginning of mitosis and meiosis. The portrait formed by the number and shapes of chromosomes representative of a species is called its karyotype. What’s the benefit of an organized chromosome? What’s the benefit of an untangled chromatin? 11  Homologous chromosomes In humans, somatic cells have 46 chromosomes (23 pairs). Somatic cells have pairs of chromosomes (diploid), receiving one member of each pair from each parent. Two chromosomes in each pair are called homologous chromosomes which are matched by having the same 1. Length 2. Centromere position 3. Same gene locations – Different versions of a gene (i.e., alleles) may be found.  Sex chromosomes The human sex chromosomes X and Y differ in size and genetic composition. Only some parts of x and y are homologous. We inherit one sex chromosome from our mother and one from our father, which in the end determines gender. In humans, the presence of the Y chromosome determines the biological sex of the organism. – E.g., males have XY and females have XX. The 22 pairs of chromosomes that do not determine sex are called autosomes.  Alleles are alternative versions of genes Gene loci Dominant allele P a B Homologous chromosomes P a b Recessive allele Genotype: PP aa Bb Homozygous Homozygous Heterozygous, for the for the with one dominant dominant recessive and one recessive allele allele allele Allele = different version of the same gene. Each allele resides on one of the pairs of homologous chromosomes. By convention, the dominant allele is indicated with a capital letter.  Chromosomes duplicate during the S phase A cell with one complete set of chromosomes (1 of each homologs) is haploid (1n), and a cell with two complete set of chromosomes is diploid (2n). – Humans have one pair of each of the 23 chromosomes, so humans are diploid organisms. In order for cell division to proceed, every chromosome in the parent cell must be duplicated so that each daughter cell receives a full set of chromosomes. How many Even though the DNA chromosomes are in each chromosome shown here? duplicates, the two a) 2 identical copies, b) 4 called sister chromatids, do not completely separate. Sister chromatids stay side by side, held together at the centromere.  Terminology of chromosome parts Before cell division, Key the genome of each Maternal set of cell will need to be chromosomes (n = 3) replicated. 2n = 6 Paternal set of Each chromosome will be replicated. chromosomes (n = 3) Each replicated Two sister chromatids chromosome of one replicated consists of two chromosome identical sister chromatids. Centromere Two nonsister chromatids in Pair of homologous a homologous pair chromosomes (one from each set) ? a. b. c. neither  ? a. b. c.  Eukaryotic cell cycle and mitosis Cell division in eukaryotes proceeds through a number of steps that make up the cell cycle. The cell cycle consists of two distinct stages. 1. Interphase describes the period between two successive M phases. 2. M phase describes the period during which the parent cell divides into two daughter cells. – The M phase includes What is the difference between mitosis and mitosis and cytokinesis. cytokinesis?  The stages of interphase During interphase, the cell makes many preparations for division including DNA replication and an increase in cell size and components. G0 phase. The “resting phase” because there is no active preparation taking place for cell division. Some cells remain in this phase. G1 phase. The first “gap” phase, where the cell size and protein content of the cell increases in preparation for the S phase. S phase. The “synthesis” phase, where the chromosomes are duplicated. G2 phase. The second “gap phase,” where the cell prepares for mitosis and cytokinesis. What type of cells remain in the G0 phase?  The stages of mitosis 1. Prophase 2. Prometaphase 3. Metaphase 4. Anaphase 5. Telophase  G2 of Interphase Prophase Prometaphase Centrosomes Chromatin Early mitotic Aster Centromere Fragments Nonkinetochore (with centriole (duplicated) spindle of nuclear microtubules pairs) envelope Nucleolus Nuclear Plasma Chromosome, consisting Kinetochore Kinetochore envelope membrane of two sister chromatids microtubule A nuclear envelope encloses the nucleus, nucleolus is visible. Two centrosomes (the microtubule-organizing centers for animal cells) have formed in animal cells. Chromosomes, duplicated during S phase, cannot be seen individually because they have not yet condensed (in loosely packed chromatin).  G2 of Interphase Prophase Prometaphase Centrosomes Chromatin Early mitotic Aster Centromere Fragments Nonkinetochore (with centriole (duplicated) spindle of nuclear microtubules pairs) envelope Nucleolus Nuclear Plasma Chromosome, consisting Kinetochore Kinetochore envelope membrane of two sister chromatids microtubule In the cytoplasm, microtubules begin to emerge from the two centrosomes, forming the mitotic spindle. Centrosomes move apart towards the opposite pole. In the nucleus, chromosomes coil and become compact and condensing into discrete chromosomes with sister chromatids joined together, and nucleoli disappear.  G2 of Interphase Prophase Prometaphase Centrosomes Chromatin Early mitotic Aster Centromere Fragments Nonkinetochore (with centriole (duplicated) spindle of nuclear microtubules pairs) envelope Nucleolus Nuclear Plasma Chromosome, consisting Kinetochore Kinetochore envelope membrane of two sister chromatids microtubule The nuclear envelope breaks down. The microtubules of the mitotic spindle attach to the chromosomes. Associated with the centromeres are two protein complexes called kinetochores. Each kinetochore is associated with each sister chromatids and forms the site of attachment for a single microtubule.  Kinetochores There is one kinetochore on each side of the centromere. This arrangement ensures that each sister chromatid is attached to a microtubule radiating from one of the poles of the cell. Photos of mitosis in an animal cell, part 1 10 µ m G2 of Interphase Prophase Prometaphase What is stained blue? What is stained green? What is stained red?  Metaphase Anaphase Telophase and Cytokinesis Metaphase Cleavage Nucleolus plate furrow forming Daughter Nuclear Spindle Centrosome at chromosomes one spindle pole envelope forming Chromosomes are at the cell equator, or the metaphase plate. Kinetochores of sister chromatids face opposite poles. The entire structure formed by kinetochore and nonkinetochore microtubules is called a spindle.  Metaphase Anaphase Telophase and Cytokinesis Metaphase Cleavage Nucleolus plate furrow forming Daughter Nuclear Spindle Centrosome at chromosomes one spindle pole envelope forming Sister chromatids separate at the centromeres. The two liberated daughter chromosomes (V shaped) are moved to opposite poles of the cell as kinetochore microtubules shorten. Poles of the cell move farther apart, elongating the cell. By the end of anaphase, the two ends of the cell have equivalent and complete sets of chromosomes.  Metaphase Anaphase Telophase and Cytokinesis Metaphase Cleavage Nucleolus plate furrow forming Daughter Nuclear Spindle Centrosome at chromosomes one spindle pole envelope forming During telophase, each pole consists of a complete set of chromosomes. The microtubules of the mitotic spindle break down and disappear, and the nuclear envelopes begin to reform around each set of chromosomes, creating two new nuclei. Once the nuclear envelope is reformed, the chromosomes decondense and become less visible. Photos of mitosis in an animal cell, part 2 10 µm Metaphase Anaphase Telophase and Cytokinesis  Cytokinesis As mitosis ends, cytokinesis begins and the parent cell’s cytoplasm divides into two daughter cells. What would happen if mitosis occurred but no cytokinesis?  Cytokinesis for animal cells In animal cells, cytokinesis occurs as a cleavage furrow forms in the middle from a contracting ring of microfilaments until the parent cell is pinched into two cells. Cleavage furrow is like pulling of a drawstring. Cytokinesis Cleavage furrow Contracting ring of microfilaments Daughter cells Cleavage furrow  Summary - mitosis Mitosis produces genetically identical cells for many purposes. Reproduction (asexual) of single-celled organisms. Growth and development of multicellular organisms from a fertilized egg into an adult. Tissue renewal (e.g., wound healing) of actively dividing tissues such as the skin or lining of the intestine. 100 µm 200 µm 20 µm (a) Reproduction. An amoeba, (b) Growth and development. (c) Tissue renewal. These a single-celled eukaryote, is This micrograph shows a dividing bone marrow cells dividing into two cells. Each sand dollar embryo shortly (arrow) will give rise to new new cell will be an individual after the fertilized egg divided, blood cells (LM). organism forming two cells (LM).  Find the different stages of mitosis in the onion root tip Which cell is in metaphase? A)1 B) 2 C) 3 D) 4 E) 5 Which cell is in anaphase? A)1 B) 2 C) 3 D) 4 E) 5 Which cell is in telophase? A)1 B) 2 C) 3 D) 4 E) 5 4 1 3 5 2 35 Video - mitosis in animal cells 36 Some cells reproduce, some cells differentiate Cells adopt different fates as they divide Different transcription regulators generate different cell types Signal 1 Signal 2 Signal 3 Transcription factor 1 Transcription factor 2 Transcription factor 3 Cell death affects tissue differentiation Induced pluripotent stem cells (iPSCs) and directed differentiation  Meiosis Meiosis takes place in two sets of divisions, called meiosis I and meiosis II. The two cell divisions result in four daughter cells, rather than the two daughter cells in mitosis. Each daughter cell has only half as many chromosomes as the parent cell (haploid daughter cells). Each daughter cell is genetically unique due to Crossing over. Independent assortment of chromosomes.  The stages of meiosis - overview INTERPHASE MEIOSIS I MEIOSIS II Sister chromatids 1 2 3 A pair of A pair of homologous duplicated chromosomes homologous in a diploid chromosomes parent cell Results in two Results in four Like mitosis, meiosis is preceded by interphase haploid cells with haploid cells. Chromosomes duplicate during the S phase. duplicated chromosomes.  The stages of meiosis - meiosis I Division in meiosis I occurs in the following phases – Prophase I (and *prometaphase I). – Metaphase I. – Anaphase I. – Telophase I and cytokinesis. * We will include the prometaphase I stage in prophase I.  Prophase I Prophase I Metaphase I Each replicated chromosome condenses Centrosome into two sister chromatids (with centriole pair) held together at the Centromere centromere. Sister Chiasmata (with kinetochore) chromatids Spindle Synapsis: the homologous Metaphase plate chromosomes pair with each other side by side, gene for gene. Each pair, with four chromatids, is called a tetrad (or a bivalent). Homologous chromosomes Within the tetrads are X- shaped structures called Fragments Microtubule chiasmata where nonsister of nuclear attached to envelope kinetochore chromatids exchange DNA segments by crossing over events.  Crossing over creates genetic variation Through the process of crossing over, meiosis allows homologous chromosomes of maternal origin and paternal origin to undergo an exchange of DNA segments. The process of crossing over is random, resulting in completely unique chromosomes when meiosis is complete. No nucleotides are gained or lost in this process.  Metaphase I Prophase I Metaphase I In metaphase I, tetrads Centrosome align at the cell equator, (with centriole pair) with one chromosome Centromere facing each pole. Sister Chiasmata (with kinetochore) chromatids Spindle Microtubules are Metaphase plate attached to the kinetochore of one chromosome of each tetrad. Homologous How is this different from chromosomes metaphase of mitosis? Fragments Microtubule of nuclear attached to envelope kinetochore  Independent assortment of chromosomes Each homologous pair of chromosomes aligns at the cell equator independently of the other pairs. There is an equal probability of the paternal chromosome or the maternal chromosome facing a given pole. Possibility 1 Possibility 2 Two equally probable arrangements of chromosomes at metaphase I Metaphase II Daughter cells Combination 1 Combination 2 Combination 3 Combination 4  Number of possible combinations when chromosomes assort independently The number of combinations for chromosomes packaged into gametes is 2n, where n = haploid number of chromosomes (or pairs of chromosomes) For humans (n = 23), the number of possible combinations produced by orientation of human chromosomes at metaphase I of meiosis is 223, or about 8.4 million. Each gamete that you produce contains 1 of 8.4 million possible combinations of chromosomes.  Anaphase I Telophase I and Anaphase I In anaphase I, pairs of Cytokinesis homologous chromosomes separate as they are pulled Sister chromatids remain attached in opposite directions. Sister chromatids remain attached at the centromere and move as one unit toward the pole. Homologous Cleavage chromosomes furrow separate  Telophase I and cytokinesis Telophase I and Anaphase I In the beginning of Cytokinesis telophase I, each half of the cell has a haploid set of Sister chromatids remain attached chromosomes. A nuclear envelope forms around chromosomes in some species. Each chromosome still consists of two sister chromatids. Cytokinesis usually occurs Homologous Cleavage chromosomes furrow simultaneously, forming separate two haploid daughter cells.  At the end of meiosis I Cytokinesis at the end of meiosis I occurs by a cleavage furrow in animal cells. In plant cells, a cell plate forms. No chromosome replication occurs between the end of meiosis I and the beginning of meiosis II because the chromosomes are already duplicated.  The stages of meiosis - meiosis II Division in meiosis II occurs in the following phases. – Prophase II (*prometaphase II). – Metaphase II. – Anaphase II. – Telophase II and cytokinesis. Meiosis II is very similar to mitosis. * We will include the prometaphase II stage in prophase II.  Prophase II Prophase II Metaphase II In prophase II, spindle forms. nuclear envelope, breaks up again. In late prophase II, chromosomes (each still composed of two chromatids) move toward the metaphase plate.  Metaphase II Prophase II Metaphase II In metaphase II, the sister chromatids are arranged at the metaphase plate. Because of crossing over in meiosis I, the two sister chromatids of each chromosome are no longer genetically identical.  Anaphase II Telephase II and Anaphase II Cytokinesis In anaphase II, the sister chromatids separate. The sister chromatids of each chromosome now move as two newly individual chromosomes toward opposite poles. Sister chromatids Haploid daughter cells separate forming  Telophase II and cytokinesis Telephase II and Anaphase II Cytokinesis In telophase II, the chromosomes arrive at opposite poles. Nuclei form, and the chromosomes begin de- condensing. Cytokinesis separates the cytoplasm, and four haploid daughter cells are produced. Sister chromatids separate Haploid daughter cells Each daughter cell is genetically forming distinct from the other cells and from the parent cell. 58  Comparison of mitosis and meiosis MITOSIS MEIOSIS I Parent cell Prophase Site of Prophase I (before chromosome duplication) crossing over Duplicated Chromosome Chromosome Tetrad formed chromosome duplication duplication by synapsis of (two sister homologous chromatids) 2n = 4 chromosomes Metaphase Metaphase I Chromosomes Tetrads (homologous align at the pairs) align at the metaphase plate metaphase plate Anaphase Homologous Anaphase I Telophase chromosomes Telophase I separate during anaphase I; sister Sister chromatids chromatids Daughter Haploid separate during remain together cells of n=2 anaphase meiosis I No further MEIOSIS II 2n 2n chromosomal Daughter cells of mitosis duplication; sister chromatids n n n n separate during anaphase II Daughter cells of meiosis II  Mitosis/meiosis review 1. Does this diagram represent a stage in A) mitosis or B) meiosis? Why? 2. Which stage does this diagram represent? A) metaphase I or B) metaphase II. Why? 3. How many chromosomes are in this cell? Will the daughter cells of this division have the same number of chromosomes? A) yes or B) no. Why?  Cytoplasmic division In meiosis, the division of the cytoplasm differs between a. Female b. Male males and females. In females, most of the cytoplasm is retained in the DNA oocyte, which can develop into replication an egg, and the other products, the polar bodies, receive only small amounts of First meiotic division cytoplasm. Second In males, the cytoplasm meiotic division divides equally, resulting in sperm cells. During sperm development, most of the cytoplasm is eliminated and Polar bodies what remains is mostly the nucleus in the sperm head and Oocyte Sperm cells a flagellum.  Polar body exclusion Blue- DNA; Green – Microfilament (F-actin) 62 Somatic cell nuclear transfer  Regulation of the cell cycle Cell division cannot occur all the time because uncontrolled division can lead to cancer. Progression through the cell cycle is controlled by proteins called cyclins which appear and disappear in a cyclical fashion. Cyclins regulate the activities of enzymes called cyclin- dependent kinases (CDKs) If cyclins are present, it binds to CDKs and activates the enzyme promoting cell division.  Progression Through the Cell Cycle Cyclin levels were measured in the cell cycle of sea urchin embryos. Analyze and interpret this graph Interphase – Mitosis – Interphase – Mitosis – Interphase  Cyclin-CDK Complexes There are several different cyclins and CDKs that act as specific steps of the cell cycle. The G1/S cyclin-CDK complex is active at the end of G1. It prepares the cell for DNA replication The S cyclin-CDK complex is involved in the initiation of DNA synthesis. How? The M cyclin-CDK complex helps prepare the cell for mitosis. How?  Cells have many cell-cycle checkpoints, where they can Checkpoints pause the cell cycle if something is not right. Three major checkpoints: 1. DNA damage checkpoint at G1: checks for damaged DNA before it enters S phase 2. DNA replication checkpoint at G2: Checks for the presence of unreplicated DNA before it enters the M phase. 3. Spindle assembly checkpoint at M: checks for all chromosomes being attached to the spindle before the cell progresses with mitosis.  Apoptosis There are three possibilities for a cell during its lifecycle. 1. Remain alive and functioning, but non-dividing (G0). 2. Grow and divide. 3. Die. The organism needs to maintain a balance between cell division and cell death. Cells typically divide only about 50-70 times and then dies. Most normal cells undergo “programmed cell death” called apoptosis. Cells undergoing apoptosis dismantles itself internally in a very orderly fashion. During apoptosis, enzymes are produced within the cell that breaks down proteins and nucleic acids and and kills the cell.  Cancer cells are out of control Cancer cells do not heed the normal signals that regulate the cell cycle. Cancer cells divide rapidly and continually. Cancer cells do not exhibit contact inhibition. – Cell division is not inhibited when cells contact other cells. Cancer cells do not respond to signals for apoptosis if they are given a continual supply of nutrients. How do cancer cells form? The normal genes that participate in cell division and other cellular process can become cancer-causing genes (oncogenes) if they become mutated. Accumulations of many oncogenes in a cells can lead to cancer. Cancer cells can go on dividing indefinitely A striking example is a line of cells called HeLa cells. These were cervical cancer cells taken on Feb. 8, 1951 from a woman named Henrietta Lacks. HeLa cells are still alive and have been used for research into cancer, AIDS, effects of radiation and toxic substances, gene mapping, and many other scientific pursuits http://www.radiolab.org/story/91716-henriettas-tumor/