Q.Write the important features of Genetic code.

Properties of the Genetic Code

The genetic code is the set of rules by which the nucleotide sequence in mRNA is

translated into the amino-acid sequence of a protein — in effect, a dictionary that

tells the ribosome which amino acid to add for each triplet of bases it reads.

Why Scientists Felt the Need to Propose a Code

A protein is built from just 20 different amino acids, but a nucleic acid is built

from only 4 different nitrogenous bases. If one base coded for one amino acid, only

4 amino acids could ever be specified — far too few. George Gamow was among the first

to point out this mismatch and to argue mathematically that the code had to involve

more than one base per amino acid: even reading bases two at a time gives only

4×4 = 16 combinations, still short of 20, so scientists reasoned the code had to be

read in groups of at least three bases. This was later experimentally confirmed and the

code fully deciphered — largely through the work of Har Gobind Khorana and Marshall

Nirenberg, using synthetic RNA templates in cell-free translation systems.

Key Properties

• Triplet — each "word" of the code (a codon) is exactly three consecutive nucleotides, and 3 bases from a 4-letter alphabet give $4^3 = 64$ possible codons — comfortably enough to specify 20 amino acids.
• Degenerate (redundant) — most amino acids are specified by more than one codon (of the 64 codons, 61 code for amino acids and 3 are stop signals, so several codons map to the same amino acid). This redundancy is thought to protect against the effect of some point mutations.
• Unambiguous — despite being degenerate, a given codon always codes for the same, single amino acid — it never specifies more than one.
• Commaless and non-overlapping — the code is read continuously from a fixed starting point, three bases at a time, with no punctuation between codons and no base shared between two consecutive codons. …