Cell Division

Question 1 (10)

(a) Circle divided: Interphase (G1 → S → G2) then M phase (Prophase → Metaphase → Anaphase → Telophase). (4: 1 for correct interphase order, 1 for M after G2, 1 for PMAT order, 1 for clear circular layout)

(b) (1 each)

• G1: cell grows, synthesises proteins/organelles; passes restriction/G1 checkpoint.
• S: DNA replication — each chromosome copied to form two sister chromatids.
• G2: further growth, DNA proofread/repaired, proteins for mitosis (e.g. tubulin) made.

(c) (1 each)

• End of S: 24 pg — DNA has been fully replicated (doubled). ✔
• End of G2: 24 pg — no replication in G2, content unchanged. ✔
• Each daughter after mitosis: 12 pg — replicated DNA divided equally between two cells. ✔

Question 2 (12)

(a) (4) Cyclin concentration rises and falls cyclically; CDK levels are constant but inactive alone (1). Cyclin binds CDK to form an active cyclin–CDK complex (1). Rising mitotic (M) cyclin activates M-CDK (1); this phosphorylates target proteins triggering entry into mitosis once G2/M checkpoint conditions are met (1).

(b) (1 each)

• G1 checkpoint: cell size, nutrients, growth signals, DNA damage.
• G2/M checkpoint: DNA fully and correctly replicated, no DNA damage.
• Spindle/M (metaphase) checkpoint: all chromosomes attached to spindle at metaphase plate.

(c) (5) Cyclin never degraded → CDK stays permanently active (1) → checkpoints continually driven past/overridden (1) → cell divides continuously without pausing for repair (1) → damaged/mutated DNA passed on and uncontrolled proliferation occurs (1) → forms a tumour/mass of cells = cancer (1).

Question 3 (10)

(a) (6: ~1 per stage description + 2 for anaphase count reasoning)

• Prophase: chromatin condenses into visible chromosomes (each two chromatids), nuclear envelope breaks down, spindle forms.
• Metaphase: chromosomes align single-file at the metaphase (equatorial) plate; spindle fibres attach to centromeres.
• Anaphase: centromeres split, sister chromatids pulled to opposite poles. Since $2n=8$, 8 chromatids move to each pole → 8 chromosomes per pole. ✔
• Telophase: chromosomes decondense, nuclear envelopes reform, spindle disappears.

(b) (2 per difference)

• Plant: cell plate forms in the middle and grows outward → new cell wall — because plants have a rigid cell wall that cannot pinch in.
• Animal: cleavage furrow — contractile ring of actin pinches membrane inward — possible because the flexible membrane has no cell wall.

Question 4 (12)

(a) (4)

• Anaphase I: homologous chromosomes separate; each chromosome (still 2 chromatids) moves to opposite poles → reduction division, ploidy halves. (2)
• Anaphase II: sister chromatids separate (like mitosis); centromeres split → single chromatids to poles. (2)

(b) (4) Homologous chromosomes pair up (synapsis) in prophase I (1). Non-sister chromatids exchange segments at points called chiasmata (1). This is crossing over, producing recombinant chromosomes (1). New allele combinations not present in either parent → genetic variation (1).

(c) (4) Formula: number of combinations $= 2^{n}$ where $n$ = number of homologous pairs (1). Here $2n=6 \Rightarrow n=3$ (1). So $2^{3}=8$ (1). Reasoning: each of 3 pairs orients independently at metaphase I, giving $2\times2\times2$ arrangements (1). Answer: 8. ✔

Question 5 (10)

(a) (1 per correct row, max 4)

(b) (3) Fertilisation fuses two gametes (1). If gametes were diploid, the zygote would double chromosome number each generation (1). Meiosis halves the number so that fusion restores the correct diploid number and keeps it constant across generations (1).

(c) (3) Nondisjunction = failure of homologous chromosomes (meiosis I) or sister chromatids (meiosis II) to separate, giving gametes with an abnormal chromosome number (1). Abnormal gamete has $n=24$ (one extra) (1). Fertilised by normal $n=23$: zygote = $24+23 = \textbf{47}$ chromosomes (trisomy) (1). ✔

Question 6 (6)

(a) (3) Duplicated X-shaped chromosome: two identical sister chromatids joined at the centromere (constricted region). 1 mark shape, 1 chromatids labelled, 1 centromere labelled.

(b) (3) Centromere is where the kinetochore forms (1); spindle fibres attach to the kinetochore (1); allows chromatids to be pulled apart accurately to opposite poles ensuring equal distribution (1).

[
  {"claim":"DNA doubles from 12pg to 24pg after S phase","code":"g1=12; after_S=g1*2; result = (after_S==24)"},
  {"claim":"Daughter cell DNA after mitosis returns to 12pg","code":"after_G2=24; daughter=after_G2/2; result = (daughter==12)"},
  {"claim":"Gamete combinations for 2n=6 is 8","code":"n=6//2; combos=2**n; result = (combos==8)"},
  {"claim":"Nondisjunction zygote has 47 chromosomes","code":"normal=23; abnormal=24; zygote=normal+abnormal; result = (zygote==47)"},
  {"claim":"Anaphase pole chromosome count for 2n=8 mitosis is 8","code":"twoN=8; per_pole=twoN; result = (per_pole==8)"}
]