Question

Write an essay on genetic code.

Answer 1

Amino acid sequences of the protein are governed by the nucleotide sequences of the mRNA. This specific relationship between the nucleotide sequence and amino acid sequence is known as genetic code, which was deciphered by Marshall Nirenberg and his colleagues in early 1960s.

DNA transfers information to mRNA in the form of a code defined by a sequence of nucleotides bases. During protein synthesis, ribosomes move along the mRNA molecule and “read” its sequence three nucleotides at a time (codon) from the 5′ end to the 3′ end. Each amino acid is specified by the mRNA’s codon and then pairs with a sequence of three com­plementary nucleotides carried by a particular tRNA (anticodon). The genetic code consists of 64 triplets of nucleotides (Fig. 8.2). These triplets are called codons.

Conecept of Codon, Anticodon and Genetic Code

Since RNA is constructed from four types of nucleotides, there are 64 possible triplet sequences or codons (4x4x4=64). With three exceptions, each codon encodes for one of the 20 amino acids used in the synthesis of proteins that produces some redundancy in the code- most of the amino acids being encoded by more than one codon. Three of these possible codons specify the termination of the polypeptide chain, called “stop codons”, that leaves 61 codons to specify only 20 different amino acids. Therefore, most of the amino acids are rep­resented by more than one codon. The genetic code is said to be degenerate.

The genetic code can be expressed as either RNA codons or DNA codons. RNA codons occur in messenger RNA (mRNA) and are the codons that are actually “read” during the synthesis of polypeptides (the process called translation). But each mRNA molecule acquires its sequence of nucleotides by transcription from the corresponding gene