Cell and Nuclear Division (Mitosis and Meiosis)

Question 1

(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 membrane and centrioles).

Answer 1

Mitosis
Nuclear division:
• Nucleus of parent cell divides once to produce two genetically identical daughter nuclei with same number and types of chromosomes as parent nucleus. • Four main phases: prophase, metaphase, anaphase, telophase

Prophase
• Chromatin fibre becomes more tightly coiled, condensing into chromosomes.
• Each duplicated chromosome appears as two identical sister chromatids joined at their centromeres. • Centrosome organise microtubules into spindle fibres and the radial array of short microtubules extending from each centrosome are called asters.
• Centrosomes migrate to opposite poles of the cell by lengthening of microtubules.
o In animal cells, each pair of centrioles migrates to
opposite poles of the cell.
• Nucleolus disperses and seems to disappear.
• Nuclear envelope fragments

Metaphase (longest phase in mitosis)
• Microtubules from centrosome are attached to kinetochore at the centromere of each chromatid of chromosome, becoming kinetochore microtubules.
• Centromeres of chromosomes are aligned along the metaphase plate (an imaginary plane equidistant between the two poles of the cell) by microtubules.
• Non-kinetochore microtubules interact with those from the opposite pole of the spindle.

Anaphase (shortest phase in mitosis)
• Centromere separates and two sister chromatids separate, thus becoming two daughter chromosomes. • Daughter chromosomes migrate toward opposite poles of the cell, with the centromere leading the way as kinetochore microtubules shorten.
• Non-kinetochore microtubules lengthen, leading to cell elongation.
• By the end of anaphase, the two poles of the cell have equivalent and complete set of chromosomes.

Telophase
• Nuclear envelopes reform from the fragments of the endomembrane system to form two nuclei.
• Nucleolus reappears.
• Chromosomes become less condensed to form chromatin.
• Microtubules disperse by depolymerising.

Cytokinesis
Cytokinesis usually occurs by late telophase, dividing the cytoplasm evenly between the two daughter cells that appear shortly after the end of mitosis.

In animal cells,
• Cell surface membrane invaginates towards the metaphase plate
• A ring of actin microfilaments contracts by interacting with myosin molecules.
• The cleavage furrow deepens until the parent cell is pinched into two, producing two daughter cells.

In plant cells,
• Golgi vesicles that contain cell wall material (cellulose) move along microtubules, towards the metaphase plate and fuse together to produce a cell plate.
• Cell plate enlarges as more vesicles fuse with it, until its surrounding membrane fuses with cell surface membrane along the perimeter of the cell.
• A new cell wall is formed between the two daughter cells.

Question 2

(o) Explain the significance of the mitotic cell cycle (including growth, repair and asexual reproduction) and the need to regulate it tightly. (Knowledge that dysregulation of checkpoints of cell division can result in uncontrolled cell division and cancer is required, but detail of the mechanism is not required.)

Answer 2

d) Significance of mitosis
• Mitosis confers genetic stability between generations of cells.
• Each parent cell produces two genetically identical daughter cells with same number and types of chromosomes as parent nucleus (no variation in genetic information).
• The daughter cells have identical genetic information as the parent cell, due to semi-conservative replication of parental DNA during synthesis phase of interphase. • This enables growth, repair and asexual reproduction.

(i) Organism Growth
• To grow from one cell to a multicellular organism, all daughter cells must be genetically identical to the parent cell. However, genetically identical cells are able to differentiate to different cell types due to differential gene expression.

(ii) Tissue Repair
• Cells that are lost, damaged or worn-out are replaced by genetically identical cells.

(iii) Asexual reproduction
• Involves one single parent, producing offsprings that are genetically identical to parent known as clones.
• This is common in plants - vegetative propagation, e.g. bulbs in onions

The Control of the Mitotic Cell Cycle
The rate of cell division is highly regulated at various checkpoints in the cell cycle

There are three checkpoints during the mitotic cell cycle at G1, G2 and metaphase.
• G1 checkpoint ensures that the cell size is adequate and there are sufficient nutrients available and growth factors present for cell to undergo mitosis.
o If the cell does not pass the G1 checkpoint, the cell will exit the cycle, switching into a nondividing state called the G0 phase.
• G2 checkpoint ensures that the cell size is adequate and semi-conservative DNA replication has been completed successfully.
• Metaphase checkpoint ensures that all chromosomes are attached to spindle fibres / microtubules at the metaphase plate before proceeding to anaphase.
• When all checkpoints are passed, cell cycle continues to the next stage.

Question 3

(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 membrane and centrioles). (Names of the main stages are expected, but not the sub-divisions of prophase.)

Answer 3

Meiosis
Nuclear division:
• Nucleus of a parent cell divides twice to produce four genetically non-identical haploid daughter nuclei. • Each daughter nuclei contains half the number of chromosomes in the parent nucleus by reducing two sets of chromosomes to 1 set of chromosomes.
• It is vital for sexual reproduction and takes place in the reproductive organs of plants and animals.
• Meiosis occurs after interphase, followed by cytokinesis

Prophase I
• Chromatin fibre becomes more tightly coiled, condensing into chromosomes.
• Each duplicated chromosome appears as two identical sister chromatids joined at their centromeres. • Homologous chromosomes pair to form bivalents in a state called synapsis.
• Crossing over occurs between non-sister chromatids of homologous chromosomes resulting in chiasmata formation.
o the exchange of corresponding DNA segment between non-sister chromatids.
• Centrosome organise microtubules into spindle fibres and the radial array of short microtubules extending from each centrosome are called asters.
• Centrosomes migrate to opposite poles of the cell by lengthening of microtubules.
o In animal cells, each pair of centrioles migrates to opposite poles of the cell.
• Nucleolus disperses and seems to disappear.
• Nuclear envelope fragments.

Metaphase I
• The bivalents (pairs of homologous chromosomes) align themselves at the metaphase plate, with one chromosome in each pair facing each pole.
• The arrangement of chromosome of each bivalent is independent of the arrangement of the other bivalents (i.e. independent assortment).
• Both chromatids of one homolog are attached to kinetochore microtubules from one pole; those of the other homolog are attached to microtubules from the opposite pole.
• Non-kinetochore microtubules interact with those from the opposite pole of the spindle.

Anaphase I
• Each homolog of a bivalent separates / Homologous chromosomes separate but not sister chromatids. Centromere of each chromosome does not separate.
• Homologous chromosomes migrate toward opposite poles of the cell, with the centromere leading the way as kinetochore microtubules shorten.
• Non-kinetochore microtubules lengthen, leading to cell elongation.

Telophase I
• In some species, nuclear envelopes reforms from the endomembrane system, forming two nuclei.
• Each nucleus has a haploid set of chromosomes (duplicated chromosomes).
• Thus meiosis I is known as the reductional division because it halves the number of chromosome sets per cell.
• Nucleolus reappears.
• Chromosomes become less condensed to form chromatin.
• Microtubules disperse by depolymerising

Prophase II
• Chromatin fibre becomes more tightly coiled, condensing into chromosomes.
• Nucleolus disperses and seems to disappear.
• Nuclear envelope fragments.

Metaphase II
• Microtubules from centrosome attach to kinetochore at the centromere of each chromatid of chromosome, becoming kinetochore microtubules. (Sister chromatids may no longer be identical as crossing over may have occurred in Prophase I).
• Centromeres of chromosomes are aligned along the metaphase plate, perpendicular to the metaphase plate in metaphase I by microtubules.
• Non-kinetochore microtubules interact with those from the opposite pole of the spindle.

Anaphase II
• Centromere separates and two non-identical sister chromatids separate, thus becoming two daughter chromosomes.
• Daughter chromosomes migrate toward opposite poles of the cell, with the centromere leading the way as kinetochore microtubules shorten.
• Non-kinetochore microtubules lengthen, leading to cell elongation.

Telophase II
• Nuclear envelopes reform from the fragments of the endomembrane system to form two nuclei.
• Nucleolus reappears.
• Chromosomes become less condensed to form chromatin.
• Microtubules disperse by depolymerising.
• Four haploid daughter nuclei will arise from one diploid parent nuclei.
• Cytokinesis then occurs as in mitosis.

Question 4

(t) Explain the significance of the meiotic cell cycle (including how meiosis and random fertilisation can lead to variation).

Answer 4

Significance of meiosis
• The production of haploid gametes by meiosis is important prior to fertilisation in sexual reproduction, where fusion of gametes from different parents occurs to form a zygote.
• Meiosis ensures maintenance of chromosomal number in offspring and prevent doubling of chromosomal numbers during fertilisation
• Meiosis generates genetic variation in offspring by producing recombinant gametes through:
(i) Crossing over between non-sister chromatids of homologous chromosomes
(ii) Independent assortment of homologous chromosomes

There are three processes that contribute to the genetic variation arising from sexual reproduction:
(i) Crossing over between non-sister chromatids of homologous chromosomes
• Crossing over occurs between non-sister chromatids of homologous chromosomes resulting in chiasmata formation, during prophase I of meiosis.
• Crossing-over result in the exchange corresponding DNA segment of chromatids, thus separating alleles of linked genes and creating new allelic combinations in the chromatid.

(ii) Independent assortment of homologous chromosomes
• Independent assortment is the random orientation of pairs of homologous chromosomes along the metaphase plate independent of other bivalents during metaphase I
• Independent assortment of homologous chromosomes occurs during metaphase I leading to independent segregation during anaphase I.
• The number of possible chromosomal combinations in a gamete is 2^n, where n = haploid number of the organism.
o In humans, the number of possible chromosomal combination is 2^23, thus 2^23 different combinations of gametes can be produced by an individual

(iii) Random fertilisation

• During fertilisation, genetic material from two different individuals is combined by random fusion of gametes.
o In humans, the number of possible combinations of zygote is 223 x 223.

• Note: The source of new genetic variation is due to spontaneous mutations of DNA (e.g. errors that occur during semi-conservative replication), which results in different nucleotide sequences of a gene (alleles).
• Genetic variation in a population (genetically different individuals of the same species) is the basis for natural selection.