Write on the following in about 150 words each : 10×5=50 (a) Block to polyspermy (10 marks) (b) Fate map of frog embryo (10…

Block to polyspermy. Polyspermy is prevented by two sequential blocks. The fast electrical block begins when sperm entry changes the oocyte membrane potential. In sea urchins this is a large, transient depolarization, often described from about −70 mV to positive values, due to Na+ influx; in Xenopus the resting potential is about −30 mV and the fertilization potential is smaller, so the electrical block is weaker. The slow permanent block follows cortical granule exocytosis. Granule contents modify and harden the zona pellucida (or vitelline layer in amphibians), forming a fertilization envelope that prevents further sperm binding or penetration. In mammals, the zona reaction is the principal block, with little or no fast electrical block; in amphibians the slow cortical reaction is decisive. Thus the fast block gives immediate protection, while the slow block provides a durable, species-specific barrier in most eggs, ensuring monospermy and normal embryonic development.

Fate map of frog embryo. The frog blastula fate map shows which surface regions give rise to which tissues. The animal pole, especially the animal cap, is presumptive ectoderm; the dorsal-animal hemisphere forms neural ectoderm and the neural tube. The sub-equatorial marginal zone is presumptive mesoderm: the dorsal and dorsal-lateral marginal zone gives paraxial mesoderm, including somites, and axial structures; the lateral marginal zone gives lateral plate mesoderm; the ventral marginal zone gives ventral mesoderm. The vegetal pole is presumptive endoderm, forming gut and associated organs. Vogt’s vital staining and transplantation experiments established these lineages by following labelled cells. The dorsal lip of the blastopore, within the dorsal marginal zone, acts as the organizer, inducing neural tissue and establishing the dorsal-ventral and anteroposterior axes. Thus the fate map links surface position to germ-layer identity and provides the basis for developmental patterning and organ specification in the embryo.

Activation energy based mechanism of enzyme action. Enzymes increase reaction rates by lowering the activation energy required to reach the transition state. In transition-state theory, the rate depends on the free-energy barrier, ΔG‡; an enzyme binds the transition state more favourably than the substrate, stabilizing it and reducing Ea,catalyzed << Ea,uncatalyzed. In an energy profile, substrate and product free energies remain unchanged, but the catalysed pathway has a lower peak than the uncatalysed pathway. This explains why enzymes accelerate both forward and reverse reactions without changing equilibrium. Michaelis-Menten kinetics captures the functional consequence: V = Vmax[S]/(Km+[S]), where kcat reflects the conversion of enzyme-substrate complex to product. Catalysis mainly increases kcat by lowering the activation barrier, while Km reflects apparent affinity, influenced by binding and catalytic steps. Thus the activation-energy mechanism links molecular binding to measurable enzyme kinetics. It does not consume enzyme and does not alter the reaction’s thermodynamic endpoint.

Structure of dipeptide unit of a protein. A dipeptide, H2N–CHR–CO–NH–CHR–COOH, is formed by condensation between the α-carboxyl group of one amino acid and the α-amino group of another, releasing water and creating a peptide bond. The peptide bond has partial double-bond character because of resonance between the carbonyl oxygen and the amide nitrogen; this makes the C–N bond planar and restricts rotation about it. The ω angle is usually trans, about 180°, placing the two α-carbons on opposite sides, although cis peptide bonds can occur, especially with proline. Rotation is possible mainly about the N–Cα bond, called φ, and the Cα–C bond, called ψ. Steric hindrance among side chains and backbone atoms restricts allowed φ and ψ combinations, as shown in a Ramachandran plot; glycine has greater freedom, while proline is highly restricted. The dipeptide retains a free N-terminus and C-terminus, and its planar peptide backbone is the repeating structural unit of proteins.

Transmission of nerve impulse through synapse. Nerve impulses cross synapses either electrically or chemically. Electrical synapses use gap junctions, allowing direct ion flow; transmission is fast and usually bidirectional. Chemical synapses are one-way because neurotransmitter vesicles are presynaptic and receptors are postsynaptic. When an action potential reaches the presynaptic terminal, voltage-gated Ca2+ channels open, Ca2+ enters, and Ca2+-dependent exocytosis releases neurotransmitter into the synaptic cleft. The transmitter binds postsynaptic receptors; ionotropic receptors open ion channels and produce an EPSP or IPSP depending on the ion conductance, while metabotropic receptors act through second messengers. If depolarization reaches threshold, a postsynaptic action potential is generated. The synaptic delay, about 0.5–1 ms, is the total time from presynaptic action potential to postsynaptic response, arising from Ca2+ entry, vesicle fusion, diffusion, and receptor activation. Termination occurs by reuptake, enzymatic degradation, diffusion, and glial uptake or diffusion. This ensures precise timing and directionality of neural signalling.