Cell Division

The cell cycle consists of:

Interphase (~90% of the cycle)

• G₁ phase: Cell grows; organelles replicate; proteins are synthesised
• S phase: DNA replication — each chromosome becomes two identical sister chromatids joined at the centromere; cell now has twice the DNA content
• G₂ phase: Cell continues to grow; final preparations for division (e.g., centrioles replicate)

Mitotic Phase

• Mitosis: Division of the nucleus (PMAT)
• Cytokinesis: Division of the cytoplasm

Cell Cycle Control

• Checkpoints regulate the cell cycle (G₁, G₂, and metaphase checkpoints)
• Controlled by proteins: cyclins and cyclin-dependent kinases (CDKs)
• Tumour suppressor genes (e.g., p53) halt the cycle if DNA is damaged
• Proto-oncogenes stimulate division; mutations can create oncogenes (promote uncontrolled division → cancer)

Mitosis produces two genetically identical diploid daughter cells from one parent cell.

Prophase

• Chromosomes condense (supercoil) and become visible
• Each chromosome consists of two sister chromatids joined at the centromere
• Centrioles move to opposite poles and form the spindle apparatus (microtubules)
• The nuclear envelope breaks down
• The nucleolus disappears

Metaphase

• Chromosomes align along the metaphase plate (equator)
• Spindle fibres attach to kinetochores on the centromeres
• The metaphase checkpoint ensures all chromosomes are properly attached before proceeding

Anaphase

• Centromeres split
• Sister chromatids are pulled to opposite poles by shortening spindle fibres
• Chromatids move centromere-first
• The cell elongates

Telophase

• Chromatids (now individual chromosomes) arrive at the poles
• Nuclear envelope reforms around each set
• Chromosomes decondense
• Nucleolus reappears
• Spindle fibres disassemble

Cytokinesis

• Animal cells: Cleavage furrow pinches inward (contractile ring of actin filaments)
• Plant cells: Cell plate forms in the middle (vesicles from Golgi fuse to create a new cell wall)

Meiosis produces four genetically different haploid cells (gametes) from one diploid parent cell.

Meiosis I — Reduction Division

Prophase I:

• Chromosomes condense; homologous chromosomes pair up (synapsis) forming bivalents
• Crossing over occurs: non-sister chromatids of homologous pairs exchange segments of DNA at points called chiasmata (singular: chiasma)
• This creates new combinations of alleles on chromosomes (genetic recombination)
• Nuclear envelope breaks down; spindle forms

Metaphase I:

• Bivalents line up along the metaphase plate
• Independent assortment: the orientation of each bivalent is random — each homologous pair can face either pole independently of other pairs
• With 23 pairs in humans: $2^{23} = 8{,}388{,}608$ possible combinations

Anaphase I:

• Homologous chromosomes are pulled to opposite poles (NOT sister chromatids)
• Each pole receives one chromosome from each homologous pair
• This is the reduction division — chromosome number is halved ($2n \rightarrow n$)

Telophase I:

• Nuclear envelopes may reform
• Cytokinesis produces two haploid cells
• Each cell has half the chromosome number but chromosomes still consist of two chromatids

Meiosis II — Similar to Mitosis

Prophase II → Metaphase II → Anaphase II → Telophase II

• Sister chromatids are separated (centromeres split)
• Result: four genetically different haploid cells
• In males: four sperm cells
• In females: one egg cell and three polar bodies (unequal cytokinesis)

Total possible genetic combinations from independent assortment alone in humans: $2^{23} \times 2^{23} = 2^{46} \approx 7 \times 10^{13} \text{ combinations}$

Non-disjunction is the failure of chromosomes to separate properly during meiosis.

• Can occur in meiosis I (homologous chromosomes fail to separate) or meiosis II (sister chromatids fail to separate)
• Results in gametes with an abnormal number of chromosomes (aneuploidy)
• If such a gamete is fertilised, the resulting organism has an abnormal chromosome number
• Example: Down syndrome (trisomy 21) — three copies of chromosome 21 instead of two ($2n = 47$)

Question: Explain how crossing over during meiosis leads to genetic variation. (4 marks)

Solution:

During prophase I of meiosis, homologous chromosomes pair up (synapsis) to form bivalents. The non-sister chromatids of homologous chromosomes may break at corresponding points and exchange equivalent sections of DNA. These exchange points are called chiasmata. As a result, each chromatid now contains a new combination of alleles — some from the paternal chromosome and some from the maternal chromosome. When the chromatids separate in anaphase I and anaphase II, the resulting gametes have recombinant chromosomes with unique combinations of alleles. This means each gamete is genetically different, increasing genetic variation in the offspring.

• The cell cycle consists of interphase (G₁, S, G₂) and the mitotic phase (mitosis + cytokinesis).
• Mitosis produces 2 genetically identical diploid cells (growth/repair); meiosis produces 4 genetically different haploid cells (gametes).
• Genetic variation in meiosis comes from crossing over (prophase I), independent assortment (metaphase I), and random fertilisation.
• Cell cycle checkpoints prevent uncontrolled division; failures lead to cancer.
• Non-disjunction causes aneuploidy (abnormal chromosome numbers).