Write on the following in about 150 words each : 10×5=50 (a) Protein sorting in Golgi apparatus (10 marks) (b) Structure and…

(a) Protein sorting in Golgi apparatus. Protein sorting in the Golgi apparatus is a compartmentalised, receptor-mediated process that directs cargo from the cis-Golgi network through medial and trans cisternae to cellular destinations. The cis, medial, and trans cisternae form sequential sorting compartments. Newly synthesized proteins enter via COPII vesicles from the endoplasmic reticulum, are modified by glycosylation and phosphorylation, and are sorted by cargo receptors. Lysosomal hydrolases acquire mannose-6-phosphate tags in the cis-Golgi; M6P receptors bind them in the trans-Golgi network and are internalised into clathrin-coated vesicles, delivering enzymes to endosomes and lysosomes. Other cargo follows constitutive secretion, continuously delivering membrane and extracellular proteins, or regulated secretion, where hormones and enzymes are stored in secretory granules until a signal triggers exocytosis. Clathrin-coated vesicles, adaptor proteins and SNAREs ensure specificity. This system maintains organelle identity, controls secretion, and prevents mislocalisation, making it central to cell homeostasis.

(b) Structure and function of Lampbrush chromosome. Lampbrush chromosomes are highly extended diplotene bivalents seen in growing oocytes of amphibians, especially Rana and Bufo, where meiosis is arrested for months or years. Each chromosome consists of a central axis with numerous lateral loops projecting from chromomeres; the loops are decondensed chromatin rich in RNA polymerase activity and represent sites of active transcription. The loops transcribe maternal mRNAs and structural RNAs, while proteins are later translated from these transcripts to provision the oocyte. The morphology is highly conserved, allowing loops to be mapped as gene activity markers and used in in situ hybridisation and gene expression studies. They also provide a visible record of transcriptional timing during oogenesis. Their large size makes them a classical model for chromosome structure, transcription, and oocyte development. In Indian zoology, they are important for understanding amphibian reproductive biology and for demonstrating the relationship between chromatin form and RNA synthesis.

(c) Symptoms, causes and treatment of thalassemia. Thalassemia is a group of inherited haemoglobinopathies caused by reduced or absent synthesis of α or β globin chains due to point mutations, deletions, or regulatory mutations. The imbalance causes ineffective erythropoiesis, haemolysis, and microcytic hypochromic anaemia. Symptoms include pallor, fatigue, splenomegaly, bone deformities, and iron overload in severe forms. β-thalassemia trait usually shows elevated HbA2 on haemoglobin electrophoresis, while α-thalassemia trait may be normal or show reduced HbA. Clinical severity ranges from trait to transfusion-dependent disease. Diagnosis includes complete blood count, haemoglobin electrophoresis, HPLC, and DNA testing; prenatal diagnosis by chorionic villus sampling or amniocentesis is important for carrier couples. Treatment includes transfusion, chelation, folic acid, and curative haematopoietic stem cell transplantation. India has high prevalence, especially in Gujarat, Maharashtra, Punjab, and other parts of the Indian subcontinent, making carrier screening and genetic counselling public-health priorities.

(d) Coupling and repulsion phases of linkage. Linkage phase describes how two alleles are arranged on homologous chromosomes. In coupling, or cis, phase a heterozygote has AB/ab, so the dominant alleles are on one chromosome and recessive alleles on the other; parental gametes are AB and ab, while recombinant gametes are Ab and aB. In repulsion, or trans, phase the genotype is Ab/aB; parental gametes are Ab and aB, and recombinant gametes are AB and ab. A test cross with a homozygous recessive parent reveals these gametes directly. If genes are unlinked, the test cross gives a 1:1:1:1 ratio; linkage makes parental classes exceed recombinant classes. Recombination frequency is calculated as recombinant progeny divided by total progeny, multiplied by 100. For short intervals, recombination frequency approximates physical distance, but at longer distances double crossovers reduce observed recombination and underestimate map distance. Thus phase determines which phenotypic classes appear parental or recombinant in mapping experiments.

(e) Sympatric and allopatric speciation. Speciation is the formation of new species. Allopatric speciation occurs when populations are separated by a geographic barrier, such as a mountain range, river, sea, or island, through vicariance or dispersal. Reduced gene flow allows genetic drift, mutation, and natural selection to diverge populations until reproductive isolation evolves. The Nicobar megapode, isolated on the Nicobar Islands, is an Indian example of allopatric divergence. Sympatric speciation occurs without a physical barrier, often within the same habitat, through polyploidy, host-race formation, or disruptive selection. Polyploidy is common in plants and can instantly create reproductive isolation; host races arise when populations specialise on different host plants or resources; disruptive selection favours extreme phenotypes. In both modes, reinforcement can strengthen prezygotic isolation when hybrids have reduced fitness, for example by mate preference, temporal isolation, or behavioural differences. Thus geographic separation and ecological selection are two major routes to biodiversity in Indian biogeography. Collectively, they connect molecular and chromosomal events to disease, inheritance, and species diversity.