Topic 8.2 Nuclear Division PDF

8.2: Nuclear division 8.2.1: The cell cycle Syllabus: The life of a cell to be described as consisting of three phases: (1) nuclear division (mitosis or meiosis); (2) cell division (cytokinesis); (3) interphase [consisting of three subphases: G1, S and G2]. Candidates are...

8.2: Nuclear division 8.2.1: The cell cycle Syllabus: The life of a cell to be described as consisting of three phases: (1) nuclear division (mitosis or meiosis); (2) cell division (cytokinesis); (3) interphase [consisting of three subphases: G1, S and G2]. Candidates are not expected to know what controls the transition from one phase to another of the cell cycle although candidates should appreciate the fact that the length of the various phases depends on the type of cell. Nuclear Division Dr M. Ellul Learning outcomes To describe the cell cycle as consisting of 3 different phases. To appreciate that the length of various phases depends on the type of cells. The Cell Cycle: The cell is a dynamic structure that is constantly trying to keep up with the changes in its environment. Most cells go through a series of changes to maintain homeostasis. One of these changes deals with reproduction of the cell. Cells need to reproduce when their surface area can no longer supply the much larger volume with nutrients and rid itself of metabolic wastes. The cell cycle has three phases: Interphase Nuclear division (mitosis or meiosis) Cell division (cytokinesis) The length of the cell cycle is very variable depending on environmental conditions, the cell type and the organism. For example, onion root tip cells divide once every 20 hours (roughly) but human intestine epithelial cells divide once every 10 hours (roughly). 2 Nuclear Division Dr M. Ellul The cell cycle entails the following 4 events: ↳ Gl (normal stage Interphase · Sc Synthesis ( · G2 < proofreading and repair ( The first part of the cell cycle is called Interphase. Interphase is considered the resting stage of the cell. This definition can be misleading since the cell is not really resting. During interphase the cell is carrying out its everyday activities. This part of interphase is called the G1 phase. G stands for gap. During this phase, cells grows in size and makes the RNA, enzymes and other proteins required for growth. Cellular organelles increase in number. It may last several days or several years depending on the type of cell involved. When the surface area to volume imbalance occurs the S phase begins. Here the genetic material (DNA) replicates itself. This allows the cell to contain enough material to fully supply 2 cells upon division. The G2 phase soon begins. During this phase, the cell continues to grow and the new DNA that has been synthesised is checked and any errors are usually repaired. Other preparations for cell division are made (eg. production of tubulin protein, which is used to make microtubules for the mitotic spindle). 3 Nuclear Division Dr M. Ellul G0 Phase Not all cells follow the typical cell cycle pattern, in which a newly generated daughter cell experiences the preparatory phases of interphase first, followed by the mitotic phase. Cells in G 0 phase are not actively preparing for division. The cell is in an inactive state, which occurs when cells exit the cell cycle. Some cells reach G0 briefly before transitioning to G1 in response to an external signal. Other cells that never or seldom divide, such mature heart muscle and nerve cells, remain in G0 indefinitely. The graph shows how cellular DNA content changes over the course of a typical cell cycle in rapidly proliferating human cells. Based on the graph, which of the following labelled phases corresponds to S phase of the cell cycle? A Phase A, because it is the longest phase of the cell cycle. B Phase B, because the amount of cellular DNA is doubled during this phase. C Phase C, because it occurs immediately after the amount of cellular DNA is doubled. D Phase D, because the amount of cellular DNA is halved during this phase. 4 Nuclear Division Dr M. Ellul The Cell Cycle is ____________________________________ ____________________________________ During the Cell Cycle, a cell ____________________________________ ___________________________________ ___________________________________ ___________________________________ Interphase is _____________________________________________________________ Interphase is divided into three phases: ___, ___, & ___ G1 Phase SPhase G2 Phase The G1 phase is a The S phase During the G2 period of activity in replicates phase, many of which cells _______ ________________and the organelles ____________________ synthesizes _______ and molecules __________ Cells will molecules. When required for _______________ and DNA replication is ____________ synthesize new completed, ___________________ ___________ _____________ When G2 is ____________________ ____________________ completed, the ____________________ ____________________ cell is ready to ____________________ enter the ____________________ 5 Nuclear Division Dr M. Ellul 8.2: Nuclear division 8.2.2: Mitosis Syllabus: The significance of mitosis in growth and replacement of cells, regeneration of body parts, asexual reproduction and gamete production in plants. Appreciate the fact that the nuclei of the daughter cells produced are genetically identical to the parent cell nucleus. The events occurring during prophase, metaphase, anaphase and telophase. Candidates are not expected to differentiate between early and late stages of prophase, metaphase, anaphase and telophase. The process of cytokinesis in animal and plant cells. 6 Nuclear Division Dr M. Ellul Learning outcomes Describe the different stages of mitosis. Explain the significance of mitosis. After interphase Mitosis begins. The cell cycle alternates between interphase and mitosis as diagrammed below. in Somatic cells Mitosis has four phases: Prophase Metaphase Anaphase Telophase NB: the process of mitosis is a dynamic, continuous process, not a set of discrete steps. Mitosis is considered nuclear division since the focus is upon the genetic material of the cell. Label the diagram. ~ Chromatin S chromosomes > nucleosomes - (grDof 8) # histone Proteins - * (polymerotides DNA Strand 7 Nuclear Division Dr M. Ellul ↑Chromosome = I Molecule o f DNA Prophase each chromosome : Duplicated (interphase The first part of mitosis is Prophase. Prophase is the longest stage of mitosis. During prophase, the nucleus begins to disappear. The chromatin pulls together, and forms pairs of rope-like structures called chromatid pairs. These point of constriction on the chromosome chromatid pairs are identical chromosomes that developed during the S phase containing repeated disk-like structure DNA sequences Protein in nature of interphase. The chromatid pairs are held together by at the centromere. The that bind specific proteins attachment sites for the spindle centromere is a point of constriction on the chromosome containing repeated microtubules DNA sequences that bind specific proteins. These proteins make up a disk-like structure called kinetochores. Kinetochores function as attachment sites for the microtubules required to separate the chromosomes during the later stages of cell division. Kinetochore microtubules are spindle fibres that attach to the kinetochores and move the chromosomes to the centre of the cell. The spindle is a structure formed from microtubules. These consist of tubulin dimers (alpha and beta tubulin). The spindle serves as a ‘track’ along which the chromatids/chromosomes will move during nuclear division. Before the spindle starts forming, the centrosome Draw a cross-section through a centriole. determines its orientation. The centrosome is an organelle in the cytoplasm found near the nucleus. It consists of a pair of centrioles. The centrosome is also known as the microtubule-organizing center of the cell. Spindle fibres begin to appear in Prophase and along with the centrioles they migrate to the poles of the cell. In animal cells, two centriole pairs move to the opposite poles of the cell forming between them an arrangement of microtubules called spindle fibres. When completely formed the whole assembly of microtubules is called spindle apparatus. A radial array of microtubules called an aster extend from the centrioles towards the plasma membrane. The main function of asters is to hold the two centrioles at the two opposite poles and help the spindle apparatus to position during nuclear division. Centrioles are not present in plants but the spindle apparatus forms nonetheless. The next phase (Metaphase) begins when the chromosomes become aligned in the centre of the cell. - aster Spindle apparatus 8 Nuclear Division Dr M. Ellul Prophase: The chromosomes coil and are distinct The nuclear envelop disintegrates The centrosomes begin moving apart and the spindle apparatus forms Distinguish between centrosome and centromere. The centrosome is an organelle which is responsible for the formation of the mitotic spindle. It consists of a pair of centrioles. ___________________________________________________________________________________________________ The centromere is a region on the chromosome where sister chromatids are ___________________________________________________________________________________________________ joined. ___________________________________________________________________________________________________ ___________________________________________________________________________________________________ 9 Nuclear Division Dr M. Ellul Metaphase Metaphase begins when the chromatid pairs line up along the centre of the cell. This makes it possible for the chromatids to position > - Tubulin themselves so that they can migrate to the protein opposite poles of the cell. The metaphase plate is an imaginary plate perpendicular to the axis of the spindle where the chromosomes align. Metaphase: The chromosomes become aligned 10 Nuclear Division Dr M. Ellul Anaphase Anaphase is the stage where the daughter chromosomes each consisting of one chromatid move to the poles of the spindle. Up to this stage of mitosis, sister chromatids have been held together at the centromere by proteins known as cohesin proteins. In anaphase there is simultaneous removal of these proteins from all the chromosomes. As a result, the chromatid pairs split and the spindle fibres contract pulling each chromosome toward their pole. This process continues until the chromosomes arrive at each pole. Sister Cohesion proteins - chromatids 3 splitting due to the removal of Cohesion Proteins. Kinetocore > - fibers pulling Anaphase: The chromatids separate What would happen to a chromosome that lost cohesin protein between sister chromatids before metaphase? sister chromatids separate before metaphase, it can lead to ________________________________________________________________________________________ incorrect chromosome segregation during metaphase and ________________________________________________________________________________________ anaphase. ________________________________________________________________________________________ This can result in aneuploidy, a condition where daughter cells have ________________________________________________________________________________________ an abnormal number of chromosomes. ________________________________________________________________________________________ 11 Nuclear Division Dr M. Ellul Telophase When Telophase begins the spindle fibres disappear and the nucleus reappears and the cell splits into two. This split is called cytokinesis. We now have 2 daughter cells, each identical in the number and type of chromosomes. They are smaller than the mother cell and will begin to develop starting interphase again. Telophase: The nuclear envelope reappears. The chromosomes uncoil. The spindle apparatus breaks down. The cell divides into two (cytokinesis) 12 Nuclear Division Dr M. Ellul In which cells does mitosis occur? Somatic cells (cells which are not gametes ( The significance of mitosis The process of mitosis is of great biological significance and is fundamental to many biological processes: Growth of multicellular organisms The two daughter cells produced are genetically identical to one another (clones) and have the same number of chromosomes as the parent cell. This enables unicellular zygotes (as the zygote divides by mitosis) to grow into multicellular organisms. Growth may occur across the whole body of the organism or be confined to certain regions, such as in the meristems (growing points) of plants. Replacement of cells & repair of tissues & regeneration of body parts Damaged tissues can be repaired by mitosis followed by cell division. As cells are constantly dying they need to be continually replaced by genetically identical cells. In humans, for example, cell replacement occurs particularly rapidly in the skin and the lining of the gut. Some animals can regenerate body parts, for example, zebrafish can regenerate fins. Asexual reproduction Asexual reproduction is the production of new individuals of a species by a single parent organism – the offspring are genetically identical to the parent. For unicellular organisms such as Amoeba, cell division results in the reproduction of a genetically identical offspring. For multicellular organisms (as seen with many plant species) new individuals grow from the parent organism (by cell division) and then detach (‘bud off’) from the parent in different ways. Some examples of these are budding in Hydra and yeast and runners from strawberries. Plant life cycles In plants, the life cycle alternates between a diploid sporophyte generation and a haploid gametophyte generation. Mitosis is essential in the gametophyte generation, where it allows the haploid spores (produced by meiosis) to grow into multicellular gametophytes, which will produce gametes. 13 Nuclear Division Dr M. Ellul Cytokinesis in plant and animal cells It is the process by which the cell cytoplasm is divided into two halves, giving rise to two daughter cells. Cytokinesis is a part of the M phase and takes place immediately after the division of the nucleus i.e. mitosis. The process of cytokinesis is different in animal and plant cells. Cytokinesis in Animal Cells In animal cells, cytokinesis takes place by a cleavage furrow that develops in the centre of the cell at the same place where the metaphase plate was formed during mitosis. The furrow is formed on the opposite poles of the cell and start growing inwards, pinching the cell into two halves. A constricting belt of actin filaments cause cytokinesis. As the filaments slide past each other, the diameter of the belt decreases, pinching the cell and create a cleavage furrow around the cell’s circumference. Thus, two daughter cells are formed. Cytokinesis in Plant Cells Plants cells are surrounded by a thick cell wall. A simple furrow cannot divide the cell into two halves as in the case of animal cells. In plant cells, vesicles derived from the Golgi apparatus start aligning in the center of the cell and forming a cell plate. A new cell wall is formed along this cell plate dividing the cell into two daughter cells. The space between the daughter cells becomes impregnated with pectins and is called a middle lamella. 14 Nuclear Division Dr M. Ellul 15 Nuclear Division Dr M. Ellul Identify the stages of mitosis in the image below: telophase j · anaphase metaphase ↳ prophase For following set of questions refer to the images below representing the phases of mitosis F a.. ErArC Put the images in the correct order from prophase to cytokinesis. B D -. b. In which phase would the nuclear envelope fragment? Prophase.B c. In which phase would the chromosomes line up at the metaphase plate? metaphase A d. In which phase would two daughter nuclei form? Telophase D e. In which phase would chromatin condense and the mitotic spindle begin to form? Anaphase ( 16 Nuclear Division Dr M. Ellul Self-assessment: Review Questions 1. List and describe the stages of mitosis. 2. Give the significance of mitosis. 3. Does mitosis promote genetic variation? 4. How does cytokinesis differ in plants and animals? 5. In an experiment, onion root tip cells were observed and the total number of cells together with the number of cells in each stage of mitosis were recorded. The following results were tabulated: Total number of nuclei 380 Number of nuclei in prophase 17 Number of nuclei in metaphase 3 Number of nuclei in anaphase 1 Number of nuclei in telophase 3 a. Why is the number of cell in prophase higher than in any other stage of mitosis? b. The mitotic index is a measure of the proliferation status of a cell population (i.e. the proportion of dividing cells) and is the ratio between the number of cells in mitosis and the total number of cells. 𝑐𝑒𝑙𝑙𝑠 𝑖𝑛 𝑚𝑖𝑡𝑜𝑠𝑖𝑠 𝑀𝑖𝑡𝑜𝑡𝑖𝑐 𝑖𝑛𝑑𝑒𝑥 = 𝑡𝑜𝑡𝑎𝑙 𝑛𝑢𝑚𝑏𝑒𝑟 𝑜𝑓 𝑐𝑒𝑙𝑙𝑠 Use the above data to calculate the mitotic index. c. How would you expect the mitotic index to vary if we were to record it at increasing distances away from the root tip cap? Why? Reflect on what you learnt: Draw an annotated flow chart summarising the different stages of mitosis. 17 Nuclear Division Dr M. Ellul 8.2: Nuclear 7. division 8.2.3:8.Meiosis Syllabus: The significance of meiosis in production of gametes in animals and spores in plants. Meiosis as a reduction division to produce haploid cells. The events occurring during the first and second meiotic divisions. Candidates are not expected to differentiate between early and late stages of prophase I/II, metaphase I/II, anaphase I/II an d telophase I/II. Differences between I and II for each stage (prophase, metaphase, anaphase and telophase) is expected. The significance of meiosis in generating genetic diversity through: (1) synapsis and crossing over at chiasmata during prophase I; (2) random alignment of maternal and paternal chromosomes at the equator during metaphase I and independent assortment of chromosomes during anaphase I; (3) random alignment of chromosomes at the equator during metaphase II and independent assortment of chromatids during anapha se II. The significance of random fertilization in generating diversity. Candidates should be able to compare and contrast mitosis and meiosis. 18  Nuclear Division Dr M. Ellul piidnumb Competepinumbochromosomea : of one of each chromosome. copy Cells. Learning outcomes Describe the different stages of meiosis. Explain the significance of meiosis. Distinguish between the prophase I and II, metaphase I and II, anaphase I and II, telophase I and II. Compare and contrast mitosis and meiosis. Meiosis is a form of nuclear division involving the reduction from a diploid number (2n) of chromosomes to the haploid number. Meiosis occurs during the formation of sperm and ova in animals and during spore formation in most plants. It involves a single duplication of chromosomes in the parent cell followed by two cycles of nuclear divisions and cell divisions. Meiosis involves two nuclear divisions named meiosis I and meiosis II. The overall process of meiosis is as follows: a single duplication of chromosomes in the parent cell · & * two cycles of nuclear divisions and = cell divisions In a diploid cell, the chromosomes occur in pairs. The members of each pair are called homologous chromosomes (or homologues). Homologous chromosomes look alike – they have the same length and centromere position. All the chromosomes are replicated during interphase before meiosis I and consist of two identical pairs called sister chromatids attached at the centromere of each chromosome. 19 Nuclear Division Dr M. Ellul During meiosis I, homologous chromosomes are separated; each daughter nucleus receives one homologue of each pair of chromosomes. The sister chromatids do not separate during meiosis I, so at the completion of meiosis I each chromosome still consists of two sister chromatids. In meiosis II, the sister chromatids split into two independent daughter chromosomes, with one going into each daughter nucleus II. Four 1. How many chromosomes did the cell start with? ___________ 2. How many chromosomes are present at the end of meiosis I? (Hint: count the centromeres) _________________ two two 3. How many chromosomes are present in each cell at the end of meiosis II? ______________ Reduction in the number of chromosomes from diploid to haploid occurs during meiosis I. Meiosis II is essentially the same as mitosis in a haploid cell, with the number of chromosomes remaining the same. Each nuclear division is usually accompanied by cytokinesis. Therefore, a cell that undergoes both meiosis I and meiosis II produces a total of four haploid cells. Interphase This is of variable length depending on the species. There is replication of cell organelles and increase in size of the cell. DNA replicates and each chromosome exists as a pair of chromatids. 20  Nuclear Division Dr M. Ellul of Bivalents > - pair homologous chromosomes. Meiosis I with 4 Tetrad Structure which is formed chromatids during pairing of homologous Prophase I chromosomes synapsis This is the longest phase and is divided in 5 stages: I Leptotene needed Zygotene names not Pachytene Diplotene Diakinesis The chromosomes shorten and become visible as single structures. Homologous chromosomes, that is chromosomes from the maternal and paternal nuclei, come together and pair up. Each pair has the same length and their centromeres are in the same position and they usually have the same number of genes arranged in the same linear order. The chromosomes get connected by a protein-RNA complex called the synaptonemal complex. The synaptonemal complex holds the bivalents together in such a way that the DNA of the non-sister chromatids is aligned. The process of pairing is called synapsis and the paired homologous chromosomes are called bivalents. The bivalents shorten and thicken. ↑ 1. Homologous chromosomes pair up. 2. the paired homologous chromosomes - bivalents. Process called synapsis The homologous chromosomes of the bivalents now fall apart and appear to repel each other partially. Each chromosome is now composed of two sister chromatids. The two chromosomes are joined at several points. These points are called chiasmata. A chiasma is a site of exchange of genetic material between non-sister chromatids. Genes from one chromosome become attached to genes from other chromosome. This is called genetic crossing over (genetic recombination). Crossing-over is a way to increase genetic variation in the gametes. Prophase I Nuclear membrane breaks down DNA coils into chromosomes Centrioles migrate to opposite poles and start spindle formation Homologous chromosomes pair up (synapsis) Crossing over may occur 21 Nuclear Division Dr M. Ellul Metaphase I The bivalents independently align across the equatorial (metaphase) plate of the spindle. The orientation of the maternal homologue of each bivalent may be aligned toward either pole. Also, the parental homologue of each bivalent can be orientated at either pole. Therefore, all possible combinations of chromosomes can occur in the daughter cells. Metaphase 1 Independent alignment of homologous chromosomes Anaphase I The two centromeres of each bivalent do not divide. Spindle fibres pull the whole centromeres. Each attached to two chromatids towards opposite poles of the spindle. The chromosomes are separated into two haploid sets of chromosomes in daughter cells. Anaphase I Separation of homologous chromosomes 22 Nuclear Division Dr M. Ellul Telophase I The first meiotic division ends when homologous centromeres and their pairs of chromatids arrive at opposite poles. There has been reduction of chromosome number, but each pole possesses chromosomes composed of two chromatids. Telophase I and Cytokinesis Nuclear membrane forms around each cluster of chromosomes Cytokinesis occurs forming two cells. In animals and some plants the chromatids usually uncoil and a nuclear membrane forms at each pole and the nucleus enters interphase. Interkinesis (Interphase II) This stage is usually present only in animal cells and varies in length. There is no DNA replication at this stage. Meiosis II Prophase II The chromatids shorten and thicken and the cells have one chromosome from each homologous pair. Prophase 2 Nuclear membrane breaks down DNA coils into chromosomes Centrioles migrate to opposite poles and start spindle formation 23 Nuclear Division Dr M. Ellul Metaphase II At this division, the centromeres now behave as structurally double. They organise spindle fibres on each side to both poles and hence become aligned on the equator of the spindle. Metaphase 2 Chromosomes equator line up at the Anaphase II The centromeres divide and the spindle fibres pull the centromeres to opposite poles. The separated chromosomes are pulled along behind the centromere. Anaphase 2 Paired chromatids separate 24 Nuclear Division Dr M. Ellul Telophase II The chromosomes uncoil, lengthen and become indistinct. Nuclear membranes reform and there is subsequent cleavage (cytokinesis) to produce 4 daughter cells. Telophase 2 and cytokinesis Chromosomes arrive at poles Nuclear envelope reforms Cytokinesis occurs forming four cells. 25 Nuclear Division Dr M. Ellul Differences between meiosis I and Meiosis II for each stage Meiosis I Meiosis II Interphase Interkinesis may occur but Replication of DNA there is no replication of DNA Prophase Homologous chromosomes No pairing of homologous pair up. (synapsis). pairing chromosomes Crossing over occurs between non-sister No crossing over chromatids Metaphase Homologous pairs align Haploid number of at the metaphase plate chromosomes align at the plate Anaphase Homologous chromosomes Sister chromaids of each seperate and move to poles chromosome are seperated Telophase Two haploid daughter cells are Four haploid daughter cellsare formed, each withone chromosome formed, each with one chromatid from each homologous pair from each chromosome If sister chromatids separated at the first meiotic division, would meiosis still work? ________________________________________________________________________________________ If sister chromatids separated at the first meiotic division instead of ________________________________________________________________________________________ the second, the chromosome number would not be reduced by half, ________________________________________________________________________________________ and the ________________________________________________________________________________________ final daughter cells would still be diploid. ________________________________________________________________________________________ Meiosis promotes genetic variation. Meiosis generates genetic diversity through: synapsis and crossing over at chiasmata during prophase I; random alignment of maternal and paternal chromosomes at the equator during metaphase I and independent assortment of chromosomes during anaphase I; random alignment of chromosomes at the equator during metaphase II and independent assortment of chromatids during anaphase II. 26 Nuclear Division Dr M. Ellul The significance of random fertilization in generating diversity. The fusion of two haploid gametes results in the formation of a diploid zygote. This zygote divides by mitosis and differentiate to form a developing embryo. As meiosis results in genetically distinct gametes, random fertilisation by egg and sperm will always generate different zygotes. (Identical twins are formed after fertilisation, by the complete fission of the zygote into two separate cell masses.) The significance of meiosis Meiosis has several significant biological and evolutionary implications: 1. Genetic Diversity: Meiosis generates genetic diversity (see note above). This genetic diversity is essential for the survival of species because it provides variability that can be subjected to natural selection and adaptation to changing environments. 2. Reduction of Chromosome Number: Meiosis results in the reduction of chromosome number by half. This is important because when the gametes fuse during fertilization, the resulting zygote will have the full complement of chromosomes again. If meiosis did not occur and gametes had the same chromosome number as somatic cells, the chromosome number would double with each generation, leading to rapid increases in chromosome number and genetic complexity. 3. Maintaining Species Identity: Meiosis helps maintain the stability of a species' chromosome number over generations. Since the chromosome number is halved during meiosis, the fusion of gametes during fertilization restores the species-specific chromosome number. This prevents the accumulation of additional chromosomes over time. 4. Generation of Haploid Cells: The haploid gametes produced by meiosis are essential for sexual reproduction. When two haploid gametes fuse during fertilization, a diploid zygote is formed, which contains a full set of chromosomes. This diploid zygote then develops into a new organism. 5. Generation of Spores: In the plant life cycle, meiosis ensures the continuation of the alternation of generations. The diploid sporophyte undergoes meiosis to produce haploid spores, which then develop into the gametophyte generation. This alternation between diploid and haploid phases is central to the plant life cycle. 6. Variation and Evolution: Meiosis contributes to genetic variation, which is a driving force behind evolution. The variability introduced by meiosis allows populations to adapt to changing environments through the process of natural selection. Traits that confer advantages in a particular environment can become more prevalent in a population over time. 27 Nuclear Division Dr M. Ellul 7. Speciation: Meiosis plays a role in speciation, the process by which new species arise. As populations become isolated from each other and undergo different selective pressures, the genetic diversity generated by meiosis can lead to the accumulation of unique traits in each population. Over time, these differences can become significant enough that the populations can no longer interbreed, resulting in the formation of new species. Main effects of meiosis: Reflect on what you learnt: Write six (or more) key words to show the biological significance of meiosis. You can also jot down a few notes here. 28  Nuclear Division Dr M. Ellul Compare and contrast between stages of mitosis and meiosis. · Mitosis Meiosis Interphase Both are preceded by interphase with includes DNA replication Common pathway Prophase → metaphase → anaphase → telophase Cytokinesis They split their cells by cytokinesis Prophase Chromosomes not visible Chromosomes visible Homologous chromosomes Homologous chromosomes remain separate pair up No chiasmata formation Chiasmata formation No crossing over Crossing over (prophase I) Metaphase Pairs of chromatids align on Pairs of chromatids line up on the equator the equator only in metaphase Centromeres line up in the II same plane on the equator Centromeres lie equidistant above and below the equator in metaphase I Anaphase Centromeres divide Chromatids only in second Chromatids separate meiotic division Separating chromatids are In anaphase I whole identical chromosomes separate Separating chromosomes are not identical due to crossing over Telophase Same number of chromosomes Half the number of present in daughter cell as chromosomes present in parent cells daughter cells. Both homologous Only one of each pair of chromosomes present in homologous chromosomes daughter cells if diploid present in daughter cell Occurrence May occur in haploid, diploid or Only occurs in diploid or polypoid cells. polypoid cells. Occurs during formation of In formation of gamete cells or somatic cells and some spores. spores. Also occurs during the formation of gametes in plants with alternation of generations. Number of divisions One Two End products Two genetically identical Four genetically different diploid cells haploid cells 29 Nuclear Division Dr M. Ellul If the chromosomes of a mitotic cell behaved in the same way as chromosomes in meiosis I, would the resulting cells have the proper chromosomal make up? _________________________________________________________________________________________ If mitotic cells behaved like meiosis 1, they would produce haploid _________________________________________________________________________________________ daughter cells with half the chromosome number of the parent cell. This would lead to cells with an incorrect chromosomal _________________________________________________________________________________________ makeup for normal cell function and _________________________________________________________________________________________ development. _________________________________________________________________________________________ 30 Nuclear Division Dr M. Ellul Matsec May 2017 This question concerns cell division. Complete the following table using the terms TRUE or FALSE. True or False Chiasmata can form during Prophase II of meiosis. False Centromeres divide only in meiosis. False Mitotic division involves the pairing-up of homologous chromosomes. False In Meiosis II, pairs of chromatids line up on the equator of the spindle. Trwe Crossing-over brings about variation. T rwe In eukaryotic cells, DNA replication occurs during the S phase of interphase. True 31 Nuclear Division Dr M. Ellul 32 Nuclear Division Dr M. Ellul Self-assessment: Review Questions 1. List and describe the stages of meiosis. 2. Give the significance of meiosis. 3. How does meiosis promote genetic variation? 4. If an egg cell has 20 chromosomes, then how many chromosomes would the somatic cells have? 5. Mules are the offspring of the mating of a horse and a donkey. Mules are sterile that is they do not have the ability to reproduce. A horse has 64 chromosomes and a donkey has 62 chromosomes. From your knowledge of meiosis deduce the diploid chromosome number of a mule. Propose a possible explanation for the fact that mules are sterile. Reflect on what you learnt: Draw an annotated flow chart summarising the different stages of meiosis. 33