(a) Semiconservative replication of DNA (10 marks) (b) Difference between pinocytosis and phagocytosis (10 marks) (c) Principle…

(a) Semiconservative replication of DNA (10 marks)

Meaning. Each daughter DNA duplex has one old (parental) strand and one newly made strand, as Watson and Crick predicted in 1953 from complementary base pairing.

Proof: Meselson and Stahl (1958). E. coli grown for many generations in ¹⁵N (heavy) medium was shifted to ¹⁴N (light) medium, and DNA was analysed by CsCl density-gradient centrifugation.

• Parental DNA formed one heavy band.
• After one generation there was one band of intermediate density, which ruled out conservative replication (separate heavy and light bands).
• After two generations there were intermediate and light bands, which ruled out dispersive replication (a single band of falling density).

Machinery. Helicase unwinds the helix at oriC, single-strand binding proteins stabilise the strands and topoisomerase relieves supercoiling. Primase makes an RNA primer, and DNA polymerase III extends it 5′→3′, continuously on the leading strand and as Okazaki fragments on the lagging strand. Polymerase I replaces the primers and ligase seals the nicks.

(b) Pinocytosis and phagocytosis (10 marks)

Both are forms of endocytosis.

• Material: pinocytosis takes in fluid and dissolved solutes; phagocytosis takes in large solid particles such as bacteria and debris.
• Vesicle size: pinocytic vesicles are about 0.1–0.2 μm; phagosomes are above 0.5 μm.
• Cells: pinocytosis occurs in almost all cells; phagocytosis in specialised cells such as macrophages and neutrophils, and in Amoeba for feeding.
• Specificity: pinocytosis is usually non-specific (fluid phase), though clathrin-coated pits can be receptor-mediated; phagocytosis is triggered by receptors (Fc, complement) binding the particle.
• Machinery: pinocytosis uses clathrin or caveolin with dynamin; phagocytosis uses actin-driven pseudopodia.
• Fate: the pinocytic vesicle joins endosomes and lysosomes or is recycled; the phagosome fuses with a lysosome and digests its contents, leaving a residual body.

(c) AFLP: principle and applications (10 marks)

Principle. Amplified Fragment Length Polymorphism (Vos et al., 1995) is a PCR-based fingerprinting method that needs no prior sequence information.

• DNA is cut with a rare cutter (EcoRI) and a frequent cutter (MseI).
• Adapters of known sequence are ligated to the fragment ends.
• Pre-selective PCR uses primers matching the adapter plus one extra base; selective PCR uses primers with two or three extra bases, so only a subset is amplified, and the primers are labelled.
• The products are separated on a denaturing gel or by capillary electrophoresis, giving 50–100 bands. Band presence or absence differs between individuals because of mutations in restriction sites or selective bases, or insertions and deletions.

Applications. Genetic diversity and cultivar identification, linkage and QTL mapping, marker-assisted selection, phylogeny, and DNA fingerprinting. AFLP markers are dominant.

(d) Symptoms, causes and treatment of haemophilia (10 marks)

Symptoms. Prolonged bleeding after injury, easy bruising, nosebleeds, blood in urine, and spontaneous bleeding into joints (haemarthrosis) and muscles. Repeated joint bleeds cause arthropathy, and intracranial bleeding is dangerous.

Causes. Haemophilia is an X-linked recessive disease seen mainly in males. Mutation of the F8 gene (haemophilia A, factor VIII deficiency) or the F9 gene (haemophilia B, factor IX deficiency) blocks the intrinsic clotting pathway, so fibrin forms poorly. Carrier mothers pass it to half of their sons, and about one-third of cases are new mutations.

Treatment. Replacement of the missing factor (plasma-derived or recombinant) on demand or as prophylaxis; desmopressin for mild type A; tranexamic acid; emicizumab for type A; and gene therapy. Joint bleeds need rest, ice and compression, and aspirin and NSAIDs are avoided.

(e) Role of variation in evolutionary change (10 marks)

Heritable variation is the raw material of evolution: selection, drift and gene flow can only change the frequency of alleles that already exist, so a population with no genetic variance cannot evolve.

Kinds. Continuous (quantitative, polygenic, such as height) and discontinuous (such as blood groups); heritable (genetic) and non-heritable (environmental); somatic and germinal.

Sources. Mutation (the ultimate source), recombination by crossing over and independent assortment, random fertilisation, and gene flow.

Action on variation.

• Selection: dark forms of the peppered moth Biston betularia rose in polluted areas; beak size in Darwin's finches changed in the 1977 drought.
• Drift: the founder effect (Ellis–van Creveld syndrome in the Amish) and bottlenecks (northern elephant seals) change frequencies by chance and reduce variation.
• Gene flow introduces new alleles or homogenises populations.
• Balanced polymorphism: the sickle-cell allele persists in malarial regions through heterozygote advantage.