2425 [H2] CI2.3_Cell Division and the Cell Cycle (N)(Tr).pdf

2023 JC1 BIOLOGY LECTURE NOTES CORE IDEA 2: GENETICS AND INHERITANCE H2 TOPIC 2.1: Cell Division and the Cell Cycle Learnin...

2023 JC1 BIOLOGY LECTURE NOTES CORE IDEA 2: GENETICS AND INHERITANCE H2 TOPIC 2.1: Cell Division and the Cell Cycle Learning Outcomes: You should be able to: (n) describe the events that occur during the mitotic cell cycle and the main stages of mitosis (including the behaviour of chromosomes, nuclear envelope, cell surface membrane and centrioles) (o) (modified) explain the significance of the mitotic cell cycle (including growth, repair and asexual reproduction) (s) describe the events that occur during the meiotic cell cycle and the main stages of meiosis (including the behaviour of chromosomes, nuclear envelope, cell surface membrane and centrioles) (names of the main stages are expected, but not the sub-divisions of prophase) (t) explain the significance of the meiotic cell cycle (including how meiosis and random fertilisation can lead to variation) References: Campbell, N. A., Reece, J. B., et al. (2018). Biology A Global Approach (Eleventh Edition) Chapter 12 Mitosis, p284 – 299 Chapter 13 Sexual Life Cycles and Meiosis, p304 - 317 Note: Some of these references (including previous editions) are available in our library. You may wish to borrow them or visit the links to supplement your reading when necessary. Contents 1 INTRODUCTION..................................................................................................................... 2 2 CHROMOSOMES IN A CELL................................................................................................. 3 3 PHASES OF THE MITOTIC CELL CYCLE............................................................................. 4 3.1 INTERPHASE............................................................................................................... 5 3.2 MITOSIS........................................................................................................................ 6 3.3 Cytokinesis.................................................................................................................. 9 4 SIGNIFICANCE OF MITOSIS................................................................................................ 12 5 PHASES OF THE MEIOTIC CELL CYCLE........................................................................... 14 5.1 MEIOSIS....................................................................................................................... 14 5.1.2 MEIOSIS I............................................................................................................. 16 5.1.3 MEIOSIS II............................................................................................................ 21 5.2 SIGNIFICANCE OF MEIOSIS..................................................................................... 25  A Bridge to O Level Biology (6093 syllabus), You should be able to: (a) state the importance of mitosis in growth, repair and asexual reproduction (b) explain the need for the production of genetically identical cells (c) identify, with the aid of diagrams, the main stages of mitosis (d) state what is meant by homologous pairs of chromosomes (e) identify, with the aid of diagrams, the main stages of meiosis (names of the sub-divisions of prophase are not required) (f) define the terms haploid and diploid, and explain the need for a reduction division process prior to fertilisation in sexual reproduction (g) state how meiosis and fertilisation can lead to variation Note that there are no cell division in 5078 syllabus (Science (Biology)) 1 INTRODUCTION The cell cycle comprises interphase, nuclear division and cytokinesis. There are two types of nuclear division: mitosis and meiosis. A cell cycle that involves mitosis will give rise to genetically identical cells and this is important for growth, repair and the asexual reproduction of organisms. A cell cycle that involves meiosis occurs in the reproductive organs of organisms and is important for sexual reproduction. Meiosis results in gametes having half the amount of genetic material present in somatic cells. Fig. 1.1: Types of nuclear and cell division There are some links in the SLS lesson to help you review and revise your knowledge on the following: Cell Theory 2 2 CHROMOSOMES IN A CELL Homologous Chromosomes In a diploid species, each type of chromosome is found in a pair and are called homologous chromosomes. Homologous chromosomes are similar to each other as they are nearly identical in size; have the same banding pattern and same centromere location; have the same gene loci i.e. a gene found on one homolog will have a corresponding gene at the same position on the next homolog. Fig. 2.1: Homologous chromosome pairs Fig. 2.2: Homologous chromosome pair Homologous chromosomes are not identical to each other as they may have different alleles for the same gene; one chromosome originates from the male parent, while the other chromosome originates from the female parent. There are some links in the SLS lesson to help you review and revise your knowledge on the following: Karyotype and Homologous chromosomes 3  Learning Outcome 2(n): Describe the events that occur during the mitotic cell cycle and the main stages of mitosis (including the behaviour of chromosomes, nuclear envelope, cell surface membrane and centrioles) 3 PHASES OF THE MITOTIC CELL CYCLE Cells do not divide continuously but undergo a regular cycle division separated by periods of cell growth. This is known as the cell cycle and has three stages. Fig. 3.1: A broad overview of the phases in the Cell Cycle The cell cycle is divided into 3 stages, namely Interphase, which is sequentially further divided into 3 phases o Gap 1 (G1) phase o Synthesis (S) phase o Gap 2 (G2) phase Mitotic phase, which is sequentially further divided into o Prophase o Metaphase o Anaphase o Telophase Cytokinesis The duration of these cell cycle phases varies considerably in different kinds of cells. 4 3.1 INTERPHASE Interphase occurs during most of the cell cycle, and is sometimes known as the resting phase, because no division takes place. However, interphase is marked by a period of intense chemical activity and sub-divided into three parts: First growth (G1) phase is when the proteins from each cell organelles are synthesised are produced. Synthesis (S) phase is when the DNA is replicated. Second growth (G2) phase is when organelles grow and divide, and energy stores are increased. In summary, G1 and G2 are important phases to allow time for cell growth, duplication of organelles, storing of energy and synthesis of proteins required for cell division. G1 and G2 phases also provide time for the cell to monitor the internal and external environment to ensure that conditions are suitable and preparations are complete before the cell commits itself to the DNA synthesis and mitosis. Fig. 3.1.1: Phases of the Cell Cycle During interphase in eukaryotic cells, the centrosome duplicates. The centrosome is the major microtubule- organising centre (MTOC) which comprised a pair of centrioles. Centrosomes are not present in plant cells because plant cells do not have centrioles. Fig. 3.1.2: Animal cell with centrosome 5 3.2 MITOSIS Definition Nuclear division followed by cytokinesis to form 2 genetically identical daughter cells with the same number of chromosomes and amount of DNA as the parent cell. Stages of Mitosis The mitotic division is divided into 4 phases – (a) Prophase, (b) Metaphase, (c) Anaphase, (d) Telophase after which cytoplasmic division known as cytokinesis will occur. Fig. 3.2.1: Stages of Mitosis a) Prophase This is the longest phase of the cell cycle. Fig. 3.2.2: Prophase 6 Behaviour of Chromosomes As DNA condenses, chromatin thread shortens and thickens to form visible chromosomes. Each chromosome appears as 2 sister chromatids held together at the centromere. Nucleus Nucleoli disappear and the nuclear envelope breaks down into small vesicles which disperse. Spindle Fibres Outside the nucleus, two centrosomes move towards opposite poles of cells and organize microtubules into spindle fibres. Some spindle fibres attach to the kinetochore (protein structure) at the centromere of the chromosome. These spindle fibres are known as kinetochore microtubules. *Note that centromere is different from centrosome! Fig. 3.2.3: Kinetochores on the chromosome 7 b) Metaphase Behaviour of chromosomes Chromosomes will randomly aligned in a single row at the metaphase plate. Nucleus Nucleus absent Spindle fibres Centrosomes are now at opposite poles of the cell. The kinetochore spindle fibres will then attach to the kinetochore at the centromere region of the chromosomes. Fig. 3.2.4: Metaphase c) Anaphase Behaviour of chromosomes Centromeres divide into two and the kinetochore microtubules shortens to pull sister chromatids apart. Separated sister chromatids is each now known as a chromosome. Each daughter chromosome will be led by its centromere to opposite spindle poles. Nucleus Nucleus absent Fig. 3.2.5: Anaphase Spindle fibres Kinetochore microtubules shorten, pulling the daughter chromosomes to opposite poles, centromeres first. Non-kinetochore spindle fibres from each pole lengthen to elongate the cell. 8 d) Telophase Behaviour of chromosomes The two sets of daughter chromosomes reach the respective spindle poles. Chromosomes decondense to return to the original chromatin. Nucleus Nuclear envelope re-forms around the chromosomes at each pole. Nucleoli reappear due to reorganization of the chromosomes within the nucleus. Fig. 3.2.6: Telophase Two genetically identical daughter nuclei are formed. Spindle Fibres Spindle fibres disintegrate. 3.3 Cytokinesis ▪ Cytokinesis is the final step of the cell cycle and involves the division of the cytoplasm. ▪ It typically overlaps with the later stages of mitosis. ▪ This process is different in animal and plant cells. a) Cytokinesis in animal cells ▪ Cell membrane invaginates during end of telophase towards the centre of the elongated cell. ▪ A contraction by ring of microfilaments appears around the centre form a cleavage furrow. ▪ Furrow rapidly deepens until it completely divides the cell into two. Fig. 3.3.1: Cytokinesis in animal cell 9 b) Cytokinesis in plant cells ▪ Golgi vesicles move to the middle of the cell, fuse to form the cell plate. ▪ Vesicular contents contribute to the cell wall and its membranes form the cell surface membranes of the daughter cells. ▪ Cell plate enlarges until its surrounding membrane fuses with the plasma membrane along the perimeter of the cell, forming two daughter cells. Fig. 3.3.2: Cytokinesis in plant cell Fig. 3.3.3 Stages of mitosis in a plant cell 10  2023 JC1 BIOLOGY LECTURE NOTES CHECKPOINT 1: Summarise the key points of mitosis and cytokinesis in the table below. Stage + Sketch Behaviour of chromosomes Nucleus Spindle Fibres Prophase Metaphase Anaphase Telophase Cytokinesis in animal cells: Cytokinesis in plant cells:  2023 JC1 BIOLOGY LECTURE NOTES Learning Outcome 2(o) modified: explain the significance of the mitotic cell cycle (including growth, repair and asexual reproduction) 4 SIGNIFICANCE OF MITOSIS Mitosis results in 2 daughter cells containing the same number and type of chromosomes as the parental cell, hence daughter cells are genetically identical to the parental cell. Mitosis hence allows for: 1. Genetic Stability: Daughter cells were directly derived from parent cell, similar to clones as there are no alternations in the genetic material, hence maintaining the genetic stability of an organism. 2. Asexual Reproduction: Cells of a parent organism can divide by mitosis to produce genetically identical daughter cells that can form new offspring. Asexual reproduction is a relatively rapid form of reproduction because there is no delay as there is no need to search for a mate. Hence, a large number can be quickly established to colonise the local area. Examples include fungus, potato, onion, and ginger. 3. Growth When two gametes fuse together to form a zygote, this cell has all the genetic information needed to for the new organism. The cell first divides to give a group of genetically identical cells. This is growth by increase in cell numbers. 4. Cell Repairment Mitosis supplies new cells required to repair worn out or damaged tissue. Damaged and worn cells must be replaced by genetically identical cells to return a tissue to its former condition. 5. Regeneration Some animals can regenerate whole parts of body (e.g. tail of lizard) Regeneration involves the production of genetically identical cells to replace missing tissue. CHECKPOINT 2 (a) State the stages of cell division that the cell is in for A – E. Ans: interphase prophase metaphase anaphase telophase A B C D E (b) State the stages of cell division that the cell is in for V – Z. V: Telophase & Cytokinesis W: Anaphase X: Interphase Y: Prophase Z: Metaphase 13  Learning Outcome 2(s): describe the events that occur during the meiotic cell cycle and the main stages of meiosis (including the behaviour of chromosomes, nuclear envelope, cell surface membrane and centrioles) (names of the main stages are expected, but not the sub-divisions of prophase) 5 PHASES OF THE MEIOTIC CELL CYCLE 5.1 MEIOSIS Definition Meiosis (meio, to reduce) is the reductive division of a nucleus to produce 4 daughter nuclei, each containing half the number of chromosomes found in the original nucleus. A single diploid cell would give rise to four haploid cells. This occurs when one DNA replication is followed by two nuclear and cell divisions. Daughter cells may be genetically different from the parental cell. Fig. 5.1.1: Stages of meiosis in a human germ cell 14 Stages of Meiosis Before undergoing nuclear division, the cell undergoes Interphase. The interphase stage is the same for all cells and involves G1, S and G2 phases. In S phase of these interphase, DNA replicates so that the parent cell contains twice the amount of DNA. In G2 phase, the centrosomes replicates. The nuclear division is divided into 2 main phases with each phase further subdivided into another 4 sub-phases: Meiosis I Meiosis II Prophase I Prophase II Metaphase I Metaphase II Anaphase I Anaphase II Telophase I & cytokinesis Telophase II & cytokinesis Fig. 5.1.2: Phases of meiosis illustrating changes in DNA content 15 5.1.2 MEIOSIS I Fig. 5.1.2.1: Phases of Meiosis I There are four phases in Meiosis I Prophase I Metaphase I Anaphse I Telophase I and Cytokinesis a) Prophase I Behaviour of Chromosomes Chromatin threads becomes more tightly coiled, condensing into visible chromosomes. Each chromosome is made up of 2 sister chromatids held together by the centromere. Synapsis: Homologous chromosomes pair up to form bivalents in a process known as synapsis. Crossing Over: Non-sister chromatids may cross over at one or more chiasmata (singular: chiasma), allowing reciprocal exchange of genetic material, giving rise to genetic variation. o E.g. of Non-sister chromatids: 1 chromatid from paternal chromosome and 1 chromatid from Fig. 5.1.2.2: Bivalent formation maternal chromosome undergo crossing over 16  Fig. 5.1.2.3: Crossing over between homologous chromosomes. Nucleus The nuclear envelope disintegrates, and the nucleolus disappear. Spindle Fibres The two centrosomes move towards opposite ends of the cell, and organise microtubules into spindle fibres. b) Metaphase I Behaviour of Chromosomes Homologous pairs of chromosomes are aligned on metaphase plate, creating 2 rows. Independent assortment of homologous chromosomes occurs. o Orientation of each bivalent along the metaphase plate is random and independent of the orientation of other bivalents. Nucleus Nucleus absent. Fig. 5.1.2.4: Metaphase I Spindle Fibres Kinetochore spindle fibres attach to centromere of each chromosome at the kinetochore. Non-kinetochore microtubules lengthen to elongate the cell. 17  Fig. 5.1.2.5: Possible arrangements of bivalents on the metaphase plate (Independent assortment of homologous chromosomes) c) Anaphase I Behaviour of chromosomes Homologous chromosomes move towards opposite poles of the spindle. o Sister chromatids do not separate during anaphase I o Each chromosome still = pair of sister chromatids. o Sister chromatids will remain attached at the centromeric region until anaphase II This separates the chromosome into two haploid sets, one set at each end of the spindle. o Each daughter cell now has half the number of chromosomes as compared to parent cell. o For example, if parent cell has 46 chromosomes, each daughter cell now has 23 chromosomes. o This is known as reductive division. Fig. 5.1.2.6: Anaphase I Nucleus Nucleus absent. Spindle Fibres Kinetochore spindle fibres shorten and pull one homologous chromosome from each pair towards spindle poles. Non-kinetochore microtubules lengthen to elongate the cell. 18  d) Telophase I & Cytokinesis Behaviour of chromosomes Chromosomes uncoil and decondense to form chromatin threads. Nucleus Nucleolus and nuclear envelope reforms around chromosomes at each pole. Spindle Fibres Spindle fibres disintegrate. e) Cytokinesis In animal cells, cytokinesis takes place by invagination of cell membrane and furrowing of cytoplasm at the equator of the cell. In most plant cells, there is no telophase I and the cell goes directly into metaphase II. Fig. 5.1.2.7: Telophase I & Cytokinesis At the end of meiosis I, from a parent cell which was diploid (2n), two haploid (n) daughter cells are formed, each with only 1 homologous chromosome from each pair. Each homologous chromosome still exists as two sister chromatids connected by a centromere. 19  2n 2n n Fig. 5.1.2.8: Diploid cell (2n = 2) to haploid cells (n = 1) after Meiosis I CHECKPOINT 3 Q1: Using information from the figure above, fill in the blanks below. At the end of Meiosis I, two __________ daughter cells would be produced from a __________ germ cell. The original diploid chromosome number, n = __________ has been halved to n = __________ in each of the daughter cell. Both the cells would be genetically __________. 20  2023 JC1 BIOLOGY LECTURE NOTES CHECKPOINT 4: Summarise the key points of meiosis I in the table below. Stage + Sketch Behaviour of chromosomes Nucleus Spindle Fibres Prophase I Metaphase I Anaphase I Telophase I  2023 JC1 BIOLOGY LECTURE NOTES 5.1.3 MEIOSIS II Fig. 5.1.3.1: Phase of Meiosis II The events of the stages are similar to those occurring in mitosis. a) Prophase II Behaviour of Chromosomes As DNA condenses, chromatin thread shortens and thickens to form visible chromosomes. Each chromosome appears as 2 sister chromatids held together at the centromere. Nucleus Nucleoli disappear and the nuclear envelope breaks down into small vesicles which disperse. Spindle Fibres Outside the nucleus, two centrosomes move towards opposite poles of cells and organize microtubules into spindle fibres. Some spindle fibres attach to the kinetochore (protein structure) at the centromere of the chromosome. These spindle fibres are known as kinetochore microtubules.  b) Metaphase II Behaviour of chromosomes Chromosomes will randomly align in a single row at the metaphase plate. Nucleus Nucleus absent Spindle fibres Centrosomes are now at opposite poles of the cell. The kinetochore spindle fibres will then attach to the kinetochore at the centromere region of the chromosomes. c) Anaphase II Behaviour of chromosomes Centromeres divide into two and the kinetochore microtubules shortens to pull sister chromatids apart. Separated sister chromatids is each now known as a chromosome. Each daughter chromosome will be led by its centromere to opposite spindle poles. Nucleus Nucleus absent Spindle fibres Kinetochore microtubules shorten, pulling the daughter chromosomes to opposite poles, centromeres first. Non-kinetochore spindle fibres from each pole lengthen to elongate the cell. d) Telophase II and Cytokinesis Behaviour of chromosomes Chromosomes uncoil and decondense to form chromatin threads. Nucleus Nucleolus and nuclear envelope reforms around chromosomes at each pole. Spindle Fibres Spindle fibres disintegrate. The meiotic division of one parent cell produces 4 daughter cells, each with only 1 set of chromosomes, haploid (n) and half the amount of DNA as compared to the parent cell. 23 CHECKPOINT 5 A diploid germ cell with 2n = 6 and undergoes meiosis to form haploid daughter cells. Assuming the germ cell has 10g of DNA before interphase, in the figure below, (a) draw the number of chromosomes each of the cells below. (b) state the ploidy of each cell (c) state the amount of DNA in each cell Ploidy DNA amount Diagrammatic Representaion of Process of each in each cell / g cell 24  Learning Outcome 2(t): explain the significance of the meiotic cell cycle (including how meiosis and random fertilisation can lead to variation) 5.2 SIGNIFICANCE OF MEIOSIS Meiosis results in the reduction of the number of chromosomes sets from diploid to haploid, allowing the production of gametes that are genetically different from the parent cell, contributing to genetic variation among offsprings. Meiosis contributes to genetic variation by a) Crossing over between homologous chromosomes during Prophase I to produce recombinant chromosomes. During crossing over, there is a reciprocal exchange of DNA between the non-sister chromatids of the same homologous pair. The resultant recombinant chromosomes carry a different genetic combination of alleles as compared to the parent cell. Fig. 5.2.1: Crossing over between homologous chromosomes 25 b) Independent assortment of chromosomes during Metaphase I At metaphase I, the homolgous pairs of chromosomes, each consisting of one maternal and one paternal chromosome are positioned independently of the other pairs during metaphase I. Both pairs of paternal copy are on the left and maternal copy on the right OR One of the pair has the paternal copy on the left and maternal copy right, while the next pair has the maternal copy on the left and paternal copy on the right Fig. 5.2.2: Independent assortment of homologous chromosomes Fig. 5.2.3: Independent assortment of homologous chromosomes The homolog of each pair will be separated into daughter cells independently of the other pair. Each daughter cell represents one outcome of all possible combination of maternal and paternal chromosomes. The number of possible combinations when chromosomes sort independently during meiosis is 2n, where n is the haplod number of the organism. 26 c) Random fertilisation Meiosis produces haploid daughter cells (gametes) with only half the number of chromosomes (n) as the parent cell (2n). This is important for sexual reproduction because the chromosome number must be halved in gametes to ensure the restoration of the diploid state upon gametes fusion to form a zygote (diploid). Fig. 5.2.4: Fusion of gametes The fusion of a haploid male gamete with a haploid female gamete during fertilisation will produce a diploid zygote, restoring the diploid condition of the organism. As each gamete represents one out of the 2n possible combinations of chromosomes, random fertlisation will increase the possible diploid combnations for a zygote by 2n x 2n. 27 CHECKPOINT 6 Compare the mitotic and meiotic processes. Property MITOSIS MEIOSIS Number of divisions 1 nuclear division 2 successive nuclear divisions Behaviour of Homologous chromosomes do Homologous chromosomes pair up homologous not pair up. to form a bivalent in prophase I. chromosomes in prophase There is no crossing over between There maybe crossing over which homologous chromosomes results in a reciprocal exchange of genetic material between non- sister chromatids to produce recombinant chromosomes. Behaviour of Chromosomes form a single row Chromosomes form two rows chromosomes at the during metaphase during metaphase I. metaphase plate Behaviour of Division of centromere and No division of centromere. chromosomes during chromatids separate and move to Homologous chromosomes anaphase opposite spindle poles. separates during anaphase I and move to opposite spindle poles Division of centromere and chromatids separate and move to opposite spindle poles in anaphase II Ploidy of daughter cells Diploid Haploid Amount of DNA in each Same as the parent cell Half of the parent cell daughter cell Genetic composition of Genetically identical to the parent Genetically different from the daughter cells cell and to each other parent cell and from each other Number of daughter 2 4 cells formed from each parent cell Cell types involved In all somatic cells Only in germ cells Role Asexual reproduction, growth and Increase genetic variation for repair sexual reproduction by producing gametes Phases Share same types of phases i.e. start cell cycle with interphase, followed by Prophase Metaphase, Anaphase, Telophase and end with cytokinesis Product Produce new daughter cells Starting point Both begin with a single diploid parent cell 28  Answers CHECKPOINT 1: Summarise the key points of mitosis and cytokinesis in the table below. Stage + Behaviour of Nucleus Spindle Fibres Sketch chromosomes Prophase Chromatin fibres Nuclear envelope Duplicated centrosomes tightly coiled disintegrates move to opposite poles of Condense into (To allow cell discrete attachment of Kinetochore microtubules chromosomes spindle fibres) invade nuclear area Two identical Nucleolus Attach to chromosome via sister chromatids disappears kinetochore protein on Joined at centromere centromere Metaphase Aligned on Absent Centrosomes at opposite metaphase plate poles of cell Anaphase Centromere Absent Kinetochore microtubules separate to two shorten Sister chromatids Pull sister chromatids to separate. opposite poles of cell Each sister With centromere leading chromatid becomes first chromosome Non-kinetochore microtubules lengthen to elongate cell Telophase Daughter Nucleoli reappear Microtubule disassembles chromosomes (To protect arrived at poles. chromosomes which Chromosome are decondensing) decondense to form Nuclear envelope chromatin reassembles Cytokinesis in animal cells: Cytokinesis in plant cells: Starts at late anaphase Golgi vesicles move to middle of cell and Cleavage furrow fuse to form cell plate. Contractile ring of filaments Cell plate enlarges until surrounding Contract to pinch cell into two membrane fuses with plasma membrane. Forming two daughter cells with identical Forming two daughter cells nuclei 29 CHECKPOINT 2 (a) State the stages of cell division that the cell is in for A – E. Ans: interphase prophase metaphase anaphase telophase A B C D E (b) State the stages of cell division that the cell is in for V – Z. V: Telophase & Cytokinesis W: Anaphase X: Interphase Y: Prophase Z: Metaphase 30 CHECKPOINT 3 Q1: Using information from the figure above, fill in the blanks below. At the end of Meiosis I, two haploid daughter cells would be produced from a diploid germ cell. The original diploid chromosome number, n = 4 has been halved to n = 2 in each of the daughter cell. Both the cells would be genetically non-identical. 31 CHECKPOINT 4: Summarise the key points of meiosis I in the table below. Stage + Behaviour of Nucleus Spindle Fibres Sketch chromosomes Prophase I Chromatin fibres tightly Nuclear Duplicated centrosomes coiled & condense into envelope move to opposite poles discrete chromosomes. disintegrates. of cell. Homologous Nucleolus Kinetochore chromosomes pair up to disappears microtubules attach to form bivalents via chromosome via synapsis. kinetochore protein on Formation of chiasmata centromere between non-sister Kinetochore from one chromatids pole attach to Crossing over at centromere of a homologous region homologous between non sister chromosome from each chromatids bivalent Chromatids carrying Kinetochore from different combination opposite pole attach to of alleles. centromere of the Genetic variation in other homologous gametes chromosome from each bivalent Metaphase I Aligned on metaphase Absent Centrosomes at opposite plate poles of cell One chromosome of Centromeres each bivalent facing equidistance each pole Independent assortment of homologous chromosomes Orientation of each bivalent is random and independent of orientation of other bivalents 32 Anaphase I Homologous Absent Kinetochore chromosomes move to microtubules shorten. opposite ends of poles Pull sister chromatids to opposite poles of cell With centromere first Non-kinetochore microtubules lengthen to elongate cell Telophase I Homologous Nucleoli Microtubule chromosomes arrived reappear disassembles at poles (To protect Chromosome chromosomes decondense to form which are chromatin decondensing) Nuclear envelope reassembles 33 CHECKPOINT 5 A diploid germ cell with 2n = 6 and undergoes meiosis to form haploid daughter cells. Assuming the germ cell has 10g of DNA before interphase, in the figure below, (a) draw the number of chromosomes each of the cells below. (b) state the ploidy of each cell (c) state the amount of DNA in each cell Ploidy DNA amount Diagrammatic Representation of Process of each in each cell / g cell 6 10 6 20 6 20 3 10 3 5 Alternatively, you can express the ploidy of each cell and DNA amount in each cell in a graphical form. 34 CHECKPOINT 6 Compare the mitotic and meiotic processes. Property MITOSIS MEIOSIS Number of divisions 1 nuclear division 2 successive nuclear divisions Behaviour of Homologous chromosomes do Homologous chromosomes pair up homologous not pair up. to form a bivalent in prophase I. chromosomes in prophase There is no crossing over between There maybe crossing over which homologous chromosomes results in a reciprocal exchange of genetic material between non- sister chromatids to produce recombinant chromosomes. Behaviour of Chromosomes form a single row Chromosomes form two rows chromosomes at the during metaphase during metaphase I. metaphase plate Behaviour of Division of centromere and No division of centromere. chromosomes during chromatids separate and move to Homologous chromosomes anaphase opposite spindle poles. separates during anaphase I and move to opposite spindle poles Division of centromere and chromatids separate and move to opposite spindle poles in anaphase II Ploidy of daughter cells Diploid Haploid Amount of DNA in each Same as the parent cell Half of the parent cell daughter cell Genetic composition of Genetically identical to the parent Genetically different from the daughter cells cell and to each other parent cell and from each other Number of daughter 2 4 cells formed from each parent cell Cell types involved In all somatic cells Only in germ cells Role Asexual reproduction, growth and Increase genetic variation for repair sexual reproduction by producing gametes Phases Share same types of phases i.e. start cell cycle with interphase, followed by Prophase Metaphase, Anaphase, Telophase and end with cytokinesis Product Produce new daughter cells Starting point Both begin with a single diploid parent cell 35