(a) Draw the structure of a microtubule and explain its role in chromosomal movements during cell division. (b) What is…

(a) Microtubule: structure and role in chromosome movement

The labelled diagram should show:

• a hollow, unbranched cylinder about 25 nm wide with a lumen of about 15 nm, its wall made of 13 protofilaments (seen in cross-section);
• each protofilament as a chain of α-tubulin and β-tubulin heterodimers laid head to tail, with the β-tubulin (GTP-bound) plus (+) end and the α-tubulin minus (−) end labelled.

Minus ends are nucleated at the centrosome by γ-tubulin ring complexes; plus ends grow and shrink (dynamic instability). Spindle microtubules are of three kinds:

• kinetochore microtubules capture chromosomes at kinetochores, align them on the metaphase plate and, in anaphase A, shorten by depolymerisation, pulling the chromatids to the poles;
• polar microtubules overlap at the equator and are pushed apart by kinesin-5, lengthening the spindle (anaphase B);
• astral microtubules anchor the poles to the cortex, where dynein pulls them apart.

(b) Mutation and point mutations

A mutation is a sudden, heritable change in the base sequence of the genome. A point mutation alters a single base pair.

• Transition: a purine replaces a purine or a pyrimidine replaces a pyrimidine (A↔G, C↔T).
• Transversion: a purine replaces a pyrimidine or the reverse (for example A→C).

By their effect on the codon, substitutions are:

• Silent (synonymous): the new codon codes for the same amino acid because the code is degenerate.
• Missense: a different amino acid is inserted; GAG→GTG at codon 6 of β-globin replaces glutamic acid by valine and causes sickle-cell anaemia.
• Nonsense: a sense codon becomes a stop codon (UAA, UAG or UGA), giving a truncated protein.

Frameshift: insertion or deletion of one or two bases (not a multiple of three) shifts the reading frame, so every codon downstream is changed.

(c) Cosmid: schematic organisation and advantage over plasmid

A cosmid is a plasmid carrying the cos site of phage λ. The labelled diagram should show a small circular DNA of about 5–7 kb with:

• the plasmid origin of replication (ori);
• a selectable marker, for example ampicillin resistance (ampᴿ);
• the λ cos site, the recognition sequence for packaging;
• a unique restriction site or multiple cloning site, where foreign DNA of about 35–45 kb is inserted.

Recombinant concatemers with cos sites 37–52 kb apart are packaged in vitro into λ heads; the DNA is injected into E. coli by infection, circularises at its cohesive ends and replicates as a plasmid.

Preferred over a plasmid because:

• it carries much larger inserts (about 35–45 kb against about 10 kb or less), so fewer clones make a genomic library;
• packaging and infection are more efficient than transformation;
• it carries no phage genes, so the host is not lysed.

(d) Hardy–Weinberg law and factors upsetting equilibrium

Hardy and Weinberg (1908) showed that in a large, randomly mating population, in the absence of mutation, migration, selection and drift, allele and genotype frequencies stay constant from generation to generation.

For one autosomal locus with alleles A and a at frequencies p and q, p + q = 1. After one generation of random mating the genotypes are AA = p², Aa = 2pq and aa = q², so p² + 2pq + q² = 1. The new frequency of A is p′ = p² + ½(2pq) = p(p + q) = p, so nothing changes.

Factors that upset the equilibrium:

• Mutation: adds new alleles and shifts frequencies.
• Gene flow (migration): brings alleles in or out.
• Genetic drift: chance change in small populations, as in the founder effect and bottleneck.
• Non-random mating: inbreeding raises homozygotes without altering allele frequency.
• Natural selection: unequal survival and fertility of genotypes.

(e) Theory of natural selection

Charles Darwin and Alfred Russel Wallace proposed it jointly in 1858, and Darwin elaborated it in On the Origin of Species (1859). Its postulates are:

• Overproduction: organisms produce more offspring than the resources can support.
• Struggle for existence: the surplus competes for food, space and mates, and faces predators, disease and climate.
• Variation: individuals differ, and many differences are heritable.
• Survival of the fittest: those with favourable variations survive and leave more offspring; fitness means reproductive success.
• Accumulation: over generations favourable variations spread, giving adaptation and, eventually, new species.

Example: in industrial Britain soot darkened tree bark, and the dark form of the peppered moth (Biston betularia), first recorded at Manchester in 1848, reached 98% there by 1895 because pale moths were more easily eaten by birds; the pale form recovered as the air cleared.

In modern terms, selection acts on phenotypes and changes allele frequencies; mutation supplies the variation.