(a) Describe properties of genetic code and briefly explain Wobble hypothesis. 15+5=20 (b) What are the characteristic features…

(a) Properties of the genetic code and the Wobble hypothesis The genetic code is the set of triplet mRNA codons specifying amino acids. Its properties: it is triplet (64 codons); degenerate (61 sense codons for 20 amino acids, e.g. leucine has six); unambiguous (a codon specifies one amino acid only); non-overlapping and commaless, read in a fixed frame from AUG; nearly universal (exceptions in mitochondria and some protists); and has start (AUG) and three stop codons (UAA, UAG, UGA). Synonymous codons usually differ in the third base. Crick's Wobble hypothesis (1966) explains how fewer than 61 tRNAs read all sense codons. The first two codon–anticodon pairs obey strict Watson–Crick rules, but the third codon base (5′ end of the anticodon, position 34) pairs loosely. G in the anticodon reads C or U; U reads A or G; inosine (a modified adenine) reads U, C or A. Hence about 31–32 tRNA types suffice, and a mutation in the third base is often silent.

(b) Cytoplasmic inheritance Cytoplasmic (extranuclear) inheritance is transmission of traits through DNA of plastids and mitochondria. Features: reciprocal crosses differ, with the trait following the female parent because the egg contributes nearly all the cytoplasm; there is no Mendelian segregation ratio; it is not linked to nuclear genes, and backcrossing to the male parent never restores the nuclear-dominant expectation; organelles divide independently, so mixed cells can sort out (heteroplasmy). Chloroplast genes: variegation in Mirabilis jalapa (four-o'clock), where green, white and variegated branches give progeny matching the branch bearing the ovule, and streptomycin resistance in Chlamydomonas (chloroplast, uniparental). Mitochondrial genes: cytoplasmic male sterility (CMS), as in maize T-cytoplasm, sorghum and rice, caused by chimeric mitochondrial open reading frames; it is exploited for hybrid seed production without emasculation.

(c) Biosafety of transgenic plants Biosafety means safeguards against harm to health and environment from genetically modified organisms. Concerns: gene flow to cross-compatible wild relatives or weedy cultivars, creating herbicide-tolerant “superweeds”; effects on non-target organisms and biodiversity (Bt protein on pollinators, soil microbes, beneficial insects) and resistance build-up in pests; allergenicity, toxicity and antibiotic-resistance marker genes in food; and socio-economic issues like seed monopoly. Safeguards: case-by-case risk assessment, contained and confined field trials, buffer and refuge crops, marker-free vectors, and terminator-gene avoidance. In India the Environment (Protection) Act 1986 Rules 1989 and the Genetic Engineering Appraisal Committee (GEAC) regulate it; internationally the Cartagena Protocol on Biosafety (2000) governs transboundary movement.