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

The genetic code, the set of rules by which information encoded in genetic material is translated into proteins, possesses distinct properties that ensure accurate biological function. First, it is a triplet code, where three nucleotides form a codon specifying one amino acid; this was demonstrated by Crick, Brenner, and colleagues through frame-shift mutation analysis. Second, it is degenerate, meaning multiple codons can specify the same amino acid (e.g., six codons for leucine), which buffers against mutations. Third, it is non-overlapping, with each nucleotide participating in only one codon. Fourth, it is commaless, lacking punctuation between codons. Fifth, it is non-ambiguous (specific), as each codon specifies only one amino acid or stop signal. Sixth, it is nearly universal across organisms, supporting common ancestry. Finally, it has specific start and stop codons; AUG initiates translation (coding for methionine), while UAA, UAG, and UGA signal termination.

The Wobble Hypothesis, proposed by Francis Crick, explains how a limited number of tRNAs can recognize all 61 sense codons. It posits that base-pairing rules are strict at the first two positions of the codon but flexible at the third (wobble) position. This allows a single tRNA anticodon to pair with multiple codons. For instance, an anticodon ending in inosine (I) can pair with U, C, or A at the codon’s third position, while guanine (G) in the anticodon can pair with cytosine (C) or uracil (U). This flexibility reduces the number of tRNA species required for protein synthesis.

Cytoplasmic Inheritance refers to the transmission of genetic information via organelles (chloroplasts and mitochondria) rather than nuclear chromosomes. Its characteristic features include: (1) Maternal (uniparental) transmission, where traits are inherited exclusively from the female parent because the egg contributes the bulk of the cytoplasm; (2) Absence of Mendelian segregation, as organellar genes do not follow 3:1 or 1:1 ratios; (3) Reciprocal cross differences, where the phenotype of the F1 generation depends on the sex of the parent contributing the cytoplasm; and (4) Vegetative segregation, leading to variegation or mosaic patterns rather than uniform expression, as seen in Mirabilis jalapa.

Chloroplast and mitochondrial genes play specific roles in this inheritance. Chloroplast genes encode components of the photosynthetic machinery (photosystems, cytochrome b6f) and ribosomal RNAs. Mutations in these genes can cause albino sectors or variegation, as famously observed by Correns in Mirabilis. In agriculture, chloroplast transformation is explored for traits like herbicide resistance (e.g., atrazine resistance in Solanum), offering containment because chloroplasts are rarely transmitted via pollen. Mitochondrial genes encode subunits of the respiratory chain (Complex I, II, III, IV). A critical application is Cytoplasmic Male Sterility (CMS), where mitochondrial mutations disrupt pollen development. In maize, T-cytoplasm CMS is widely used. In India, CMS lines are pivotal for hybrid seed production in crops like rice and pearl millet (using sources like Tift 23A), enabling the commercial success of hybrid varieties.

Biosafety concerns of transgenic plants are multifaceted. Gene flow to wild relatives is a primary risk; for example, Bt cotton genes have been detected in wild Gossypium species, potentially creating superweeds or disrupting wild gene pools. Pest resistance is another issue, as target pests like Helicoverpa armigera have developed resistance to Bt toxins in Indian fields, necessitating refuge strategies. Non-target effects must be assessed, as Bt proteins might impact beneficial insects like Trichogramma or Coccinella, though field studies generally show minimal impact. Allergenicity and toxicity of novel proteins require rigorous testing to ensure food safety. Horizontal gene transfer to soil microbes, while theoretically possible, remains a low-probability risk. Furthermore, transgenics may impact biodiversity by reducing genetic diversity in monocultures and affecting traditional farming systems.

Regulatory frameworks in India, such as the Environment (Protection) Act, 1986, and the Genetic Engineering Appraisal Committee (GEAC), oversee risk assessment. The moratorium on Bt brinjal highlights the need for public participation and case-by-case risk assessment. Understanding the precision of the genetic code and the mechanisms of cytoplasmic inheritance is essential for designing safer transgenic crops, such as using chloroplast transformation to prevent gene escape, thereby balancing agricultural productivity with ecological safety.