(a) Protein sorting in Golgi apparatus (10 marks) (b) Structure and function of Lampbrush chromosome (10 marks) (c) Symptoms…

(a) Protein sorting in the Golgi apparatus

The Golgi apparatus is a polarised stack of flattened cisternae: the cis-Golgi network (entry face, towards the ER), cis, medial and trans cisternae, and the trans-Golgi network (TGN, exit face). Proteins made on the rough ER arrive in COPII-coated vesicles through the ER–Golgi intermediate compartment and move through the stack, largely by cisternal maturation.

• Retrieval of ER residents: soluble ER proteins carry a C-terminal KDEL signal (KKXX for membrane proteins); KDEL receptors in the cis-Golgi bind them and return them to the ER in COPI-coated vesicles, which also recycle Golgi enzymes backwards through the stack.
• Processing in sequence: each cisterna holds different enzymes. In the cis-Golgi, lysosomal hydrolases receive a mannose-6-phosphate (M6P) tag (GlcNAc-phosphotransferase recognises a signal patch on the folded enzyme); in the medial Golgi, mannosidase I and II trim mannose and N-acetylglucosamine is added; in the trans Golgi, galactose and sialic acid are added, and O-linked glycosylation and tyrosine sulphation occur.
• Sorting in the TGN: lysosomal enzymes bind M6P receptors and are packaged into clathrin-coated vesicles (with AP-1 and GGA adaptors) that go to late endosomes, where the acidic pH releases them; the receptors return. Lack of the phosphotransferase causes I-cell disease, in which the enzymes are secreted instead.
• Regulated secretion: hormones, neuropeptides and digestive zymogens aggregate in the TGN and are stored in dense secretory granules (e.g., insulin in β-cells) until a signal, often a rise in Ca²⁺, triggers exocytosis.
• Constitutive secretion: the default pathway continuously delivers membrane proteins, lipids and extracellular matrix proteins (e.g., collagen, antibodies from plasma cells) to the surface; in polarised epithelia, separate carriers go to apical and basolateral surfaces.

Specific fusion is ensured by Rab GTPases and matching SNAREs (v-SNAREs such as VAMP on vesicles, t-SNAREs such as syntaxin on targets).

(b) Structure and function of lampbrush chromosomes

Lampbrush chromosomes, first seen by Flemming (1882) and named by Rückert (1892) after the brushes used to clean lamp chimneys, are giant, extended bivalents in the diplotene stage of meiotic prophase I in growing oocytes of amphibians (especially newts such as Triturus and Notophthalmus, also Xenopus), birds (chicken), reptiles, fishes and some insects. The oocyte stays in diplotene for months, and each bivalent may reach several hundred micrometres, up to about 1 mm in newts.

Structure:

• Each bivalent consists of two homologues held together at chiasmata; each homologue has two sister chromatids.
• The axis of each homologue is a row of dense, condensed chromomeres (about 1–2 µm).
• From each chromomere a pair of lateral loops projects (one from each sister chromatid), each loop being a decondensed stretch of DNA from about 5 to 100 µm long.
• Each loop is a transcription unit: RNA polymerase II molecules move along it, and the growing RNA transcripts, coated with proteins (ribonucleoprotein matrix), make the loop thin at one end and thick at the other.
• Loops are specific in position and shape for each chromosome and can be used as landmarks in mapping.

Functions:

• They are sites of intense transcription, producing the large store of maternal mRNAs, and also histone and other RNAs, that the egg needs for early cleavage before the embryo's own genes are switched on.
• They show how chromatin can be locally unfolded for transcription and then refolded, as the loops retract when the oocyte matures.
• Their size makes them a classical model for studying chromosome organisation, transcription units and RNA processing, including by in situ hybridisation to nascent transcripts.

(c) Symptoms, causes and treatment of thalassaemia

Thalassaemias are inherited, autosomal recessive disorders in which synthesis of one type of globin chain is reduced or absent, causing an excess of the other chain.

• Causes: β-thalassaemia results mainly from point mutations in the HBB gene on chromosome 11 (over 200 known; in India, IVS1-5 G>C is the commonest), giving β⁰ (no chain) or β⁺ (reduced chain) alleles. α-Thalassaemia results mainly from deletions of one to four of the α-globin genes (HBA1, HBA2) on chromosome 16. Excess unpaired α chains precipitate in red-cell precursors, causing ineffective erythropoiesis and haemolysis.
• Forms: thalassaemia trait (minor) is mild microcytic anaemia with raised HbA₂ (>3.5%) in β-trait; thalassaemia intermedia needs occasional transfusion; β-thalassaemia major (Cooley's anaemia) appears at 6–24 months. In α-thalassaemia, loss of three genes causes HbH disease, and loss of all four causes hydrops fetalis (Hb Bart's).
• Symptoms: pallor, weakness, jaundice, failure to thrive, hepatosplenomegaly, expansion of marrow causing frontal bossing and prominent maxilla ("chipmunk facies") and a "hair-on-end" skull X-ray, and fractures. Repeated transfusion and increased iron absorption cause iron overload, damaging the heart, liver and endocrine glands.
• Diagnosis: complete blood count (low MCV and MCH), peripheral smear with target cells, haemoglobin HPLC or electrophoresis (raised HbF and HbA₂), and DNA analysis; prenatal diagnosis by chorionic villus sampling for carrier couples.
• Treatment: regular transfusions to keep haemoglobin at about 9–10.5 g/dL; iron chelation with desferrioxamine, deferasirox or deferiprone; folic acid; splenectomy for hypersplenism; luspatercept to reduce transfusion need. Haematopoietic stem cell transplantation from a matched sibling is curative. Gene therapies are now approved: betibeglogene autotemcel (adds a working β-globin gene) and exagamglogene autotemcel (Casgevy, CRISPR editing of BCL11A to reactivate HbF).
• Prevention: India has an estimated 3–4% carrier frequency, higher in some communities of Gujarat, Punjab, Sindh-origin and West Bengal populations, with about 10,000–15,000 affected children born each year. Carrier screening, premarital and antenatal counselling and prenatal diagnosis are key; thalassaemia is a specified disability under the Rights of Persons with Disabilities Act, 2016.

(d) Coupling and repulsion phases of linkage

When two linked genes are heterozygous in the same individual, their alleles can be arranged on the homologous chromosomes in two ways, first described by Bateson and Punnett (1905–06) in sweet pea and explained by Morgan (1911) as linkage.

• Coupling (cis) phase: the two dominant alleles lie on one chromosome and the two recessive alleles on the other (AB/ab). Parental gametes are AB and ab; crossing over gives the recombinant gametes Ab and aB.
• Repulsion (trans) phase: each chromosome carries one dominant and one recessive allele (Ab/aB). Parental gametes are Ab and aB, and recombinants are AB and ab.

Test crosses (to aabb) reveal the gametes directly. Unlinked genes give a 1:1:1:1 ratio, whereas linked genes give an excess of parental classes. For example, if genes are 20 map units apart, AB/ab gives 40% AB, 40% ab, 10% Ab and 10% aB, while Ab/aB gives 40% Ab, 40% aB, 10% AB and 10% ab. The same recombination frequency is thus seen, but the classes counted as recombinant are reversed, so the phase must be known before calculating distance.

Recombination frequency = (recombinant progeny / total progeny) × 100, where 1% equals one map unit (centiMorgan); Morgan found white and miniature in Drosophila about 34 units apart. Frequencies never exceed 50%, and over long distances double crossovers hide some recombination, so three-point crosses, interference and the coefficient of coincidence (observed/expected double crossovers) are used to correct map distances.

(e) Sympatric and allopatric speciation

Speciation is the origin of new species through the evolution of reproductive isolation. The two main modes differ in whether a geographic barrier interrupts gene flow.

Allopatric (geographic) speciation:

• A population is split by a barrier, such as a mountain range, river, sea or glacier (vicariance), or a few individuals colonise a distant area such as an island (dispersal).
• Without gene flow, mutation, drift and different selection pressures make the isolated populations diverge; reproductive isolation evolves as a by-product, so the populations cannot interbreed if they meet again.
• Examples: Darwin's finches of the Galápagos; the Isthmus of Panama separating snapping shrimps (Alpheus) into Pacific and Caribbean species; endemic species of the Western Ghats and Sri Lanka and of the Andaman and Nicobar Islands (e.g., the Nicobar megapode). Peripatric speciation, in a small peripheral founder population, is a special case where drift is important.

Sympatric speciation:

• New species arise within the same geographic area, without a physical barrier, so reproductive isolation must evolve despite potential gene flow.
• Mechanisms: polyploidy, which creates instant isolation and is common in plants (e.g., bread wheat, a hexaploid) and seen in some animals; host-race formation, as in the apple maggot fly Rhagoletis pomonella, which shifted from hawthorn to apple in North America and now mates on its own host; and disruptive selection with assortative mating, as in cichlid fishes of the Nicaraguan crater lake Apoyo and of Lake Victoria, where differences in feeding and male colour led to separate species.

Comparison: allopatric speciation is regarded as the commonest mode, especially in animals, and is slow; sympatric speciation is rarer, but can be sudden (polyploidy). In both, prezygotic barriers (habitat, temporal, behavioural, mechanical, gametic) and postzygotic barriers (hybrid inviability or sterility, as in the mule) maintain isolation, and reinforcement can strengthen prezygotic barriers when hybrids are unfit.