(a) (i) Write in detail the chemical composition of microtubules. (10 marks) (ii) Elaborate the functions of cytoplasmic…

(a)(i) Chemical composition of microtubules (10 marks)

Microtubules are hollow cylindrical tubes about 25 nm in outer diameter, with a lumen of about 15 nm, and are made mainly (over 95%) of the protein tubulin, with associated proteins and only traces of other components.

Tubulin. The building unit is a heterodimer of alpha-tubulin and beta-tubulin, each of about 55 kDa (dimer about 110 kDa), joined non-covalently. Each subunit binds one molecule of GTP. GTP bound to alpha-tubulin is permanently held, but the GTP of beta-tubulin is exchangeable and is hydrolysed to GDP after the dimer is added to the tube; the GTP cap stabilises the end. Gamma-tubulin, in a ring complex (gamma-TuRC) at the microtubule-organising centre, nucleates assembly at the minus end.

Assembly. The dimers join head to tail into protofilaments, and 13 protofilaments arranged side by side form the wall. The tube is polar, with a fast-growing plus end (beta-tubulin exposed) and a slow-growing minus end (alpha-tubulin exposed), and shows dynamic instability, alternating growth and shrinkage. Assembly needs GTP and Mg²⁺; low temperature, high Ca²⁺ and high pressure cause disassembly.

Associated proteins. Microtubule-associated proteins (MAP2, tau, MAP4, and MAP65 in plants) stabilise and bundle the tubes, katanin severs them, and the motor proteins kinesin (towards the plus end) and dynein (towards the minus end) move along them.

Drugs. Colchicine, vinblastine and nocodazole bind tubulin and block assembly; taxol stabilises the tubes and blocks their disassembly.

(a)(ii) Functions of cytoplasmic microtubules (10 marks)

• Cytoskeleton: give shape and support, and resist compression, and organise the cytoplasm.
• Intracellular transport: kinesin and dynein carry vesicles, mitochondria, Golgi and ER elements and mRNA along the tracks, as in axonal transport, and position organelles.
• Cell division: the mitotic and meiotic spindle (kinetochore, polar and astral microtubules) moves chromosomes; in plants, the preprophase band marks the plane of division and the phragmoplast guides the cell plate.
• Cell wall formation in plants: cortical microtubules lie beneath the plasma membrane and guide cellulose synthase complexes, and so the orientation of cellulose microfibrils, which decides the direction of cell elongation and patterns of secondary wall thickening (as in xylem).
• Motility: form the axoneme (9 + 2) of cilia and flagella, and the centrioles and basal bodies.
• Cell polarity and morphogenesis, growth of pollen tubes and root hairs, and organisation of the ER and Golgi apparatus.
• They are targets of drugs: colchicine arrests mitosis and induces polyploidy, and taxol and vinca alkaloids are anticancer drugs.

(b) Passive transport across the plasma membrane (15 marks)

Passive transport moves substances down their concentration or electrochemical gradient, without expenditure of metabolic energy. Its means are:

• Simple diffusion: small non-polar or lipid-soluble molecules (O₂, CO₂, N₂, steroids, ethanol, and some small uncharged polar molecules like water and urea slowly) dissolve in the lipid bilayer and pass through it. The rate follows Fick's law and increases with the gradient, with lipid solubility (partition coefficient) and with small size, and it is not saturable and needs no protein.
• Osmosis: net diffusion of water across the selectively permeable membrane from lower to higher solute concentration (higher to lower water potential). Water crosses through the bilayer and, faster, through aquaporins (PIPs and TIPs in plants).
• Facilitated diffusion through channel proteins: ions and polar molecules cross still down the gradient, through water-filled selective pores that are often gated (voltage-gated, ligand-gated); K⁺, Na⁺, Ca²⁺ and Cl⁻ channels carry 10⁶ to 10⁸ ions per second, and the K⁺ channels of guard cells are an example in plants.
• Facilitated diffusion through carrier (uniporter) proteins: the solute binds the carrier, which changes conformation and releases it on the other side, as with the GLUT glucose transporter; carriers are specific, saturable, show Vmax and Km, and are subject to competitive inhibition.

Compared with active transport, passive transport is not against a gradient, needs no ATP, and stops at equilibrium (for ions, at the electrochemical equilibrium).

(c)(i) Significance of molecular markers (8 marks)

Molecular markers are DNA sequences (RFLP, RAPD, AFLP, SSR or microsatellites, ISSR, SNP) or proteins (isozymes) that are detectable and mark a position in the genome. Unlike morphological markers they are abundant, neutral, not affected by the environment and detectable at any stage of growth. Their significance:

• Assessment of genetic diversity, and characterisation and conservation of germplasm.
• Identification of varieties, DNA fingerprinting and protection of breeders' rights, and testing of the purity of hybrid seed.
• Construction of genetic linkage maps, and mapping of genes and QTLs.
• Marker-assisted selection and backcrossing, and pyramiding of genes, as in Improved Samba Mahsuri rice with three bacterial blight resistance genes (xa5, xa13, Xa21) and Swarna-Sub1 with flooding tolerance.
• Study of phylogeny and taxonomy, and detection of transgenes.

(c)(ii) Significance of polyploidy in agriculture (7 marks)

Polyploidy is the presence of more than two sets of chromosomes, either autopolyploidy (from one species) or allopolyploidy (from hybrids between species). Its uses:

• Larger organs (gigantism), as in tetraploid varieties of rye, berseem and ornamentals.
• Seedless fruits, as in triploid banana and triploid watermelon (from a 4n by 2n cross).
• New crops, such as bread wheat (AABBDD, hexaploid), cotton (allotetraploid), Brassica napus (from B. rapa and B. oleracea), tobacco, and the man-made cereal triticale (wheat by rye).
• Restoration of fertility in sterile interspecific hybrids by doubling of chromosomes, and fixation of hybrid vigour, since duplicated genomes keep heterozygosity.
• Higher stress tolerance and adaptability, and higher sugar content in triploid sugar beet.
• Induction by colchicine (0.1 to 0.5%), oryzalin or temperature shock.