Test: Genetic code (NCERT) - NEET MCQ

Polypeptide synthesis is signalled by two initiation codons - AUG (methionine) and GUG (valine). Polypeptide chain termination is signalled by three termination codons UAA (ochre), UAG (amber) and UGA (opal). They do not specify any amino acid and are hence called non-sense codons. Anticodon loop is present on tRNA.

Since there are 64 triplet codons and only 20 amino acids, genetic code is non-ambiguous. Non-ambiguous code means that there is no ambiguity about a particular code. One codon specifies only one amino acid and not any other except GUG which normally codes for valine but in certain conditions it also codes for N-formyl methionine as initiation codon.

Since there are 64 triplet codons and only 20 amino acids, the incorporation of some amino acids must be influenced by more than one codon. Only tryptophan (UGG) and methionine (AUG) are specified by single codons. All other amino acids are specified by two (e.g., phenylalanine UUU, UUC) to six (e.g., arginine CGU, CGC, CGA, CGG, AGA, AGG) codons. The latter are called degenerate or redundant codons. In degenerate codons, generally, the first two nitrogen bases are similar while the third one is different. As the third nitrogen base has no effect on coding, the same is called wobble position.

Most of the gene. mutations involve a change in only a single nucleotide or nitrogen base of the cistron. These gene mutations are called point mutations e.g. sickle cell anaemia in which polypeptide chain coding for haemoglobin contains valine instead of glutamic acid due to substitution of T by A in second position of triplet codon.

In tRNA, there is a TWC loop which contains pseudouridine and ribothymidine. The loop is the site for attaching to ribosomes. Another loop, DHU loop contains dihydrouridine. It is a binding site for aminoacyl synthetase enzyme. tRNA molecules have unpaired (single-stranded) CCAOH sequence at the 3 end. This is called amino acid binding site because the amino acid becomes covalently attached to adenylic acid or A of CCA sequence during polypeptide synthesis. Anticodon loop is made up of three nitrogen bases for recognizing and attaching to the codon of mRNA.