Molecular Basis of Inheritance — BIOLOGY STD 12 Science — Question

2. A codon in mRNA is read in a non - contiguous fashion.

Explanation:

The genetic code is a triplet code called a codon. The information in DNA must reside in the sequence of four nucleotides that constitute the DNA: A, T, G and C.

Certain codons contain start and stop signals to initiate and terminate translation. The initiation codon is usually AUG, which specifies methionine. In few mRNA, GUG and UUG also act as initiation codon. Out of 64 codons three do not code for any amino acid and these are referred to as stop or termination codons (UAA, UAG and UGA). There are some properties of genetic code and they are as follows:

1. Genetic code is unambiguous, i.e., each triplet specifies only a single amino acid.
2. No internal punctuation is used in the code. Thus, the code is said to be commaless. Thus once the translation of mRNA begins, the code is read in a non-contiguous fashion i.e one after the other with no breaks between them.
3. The code is degenerate meaning that a given amino acid can be coded by more than one triplet codon. This is the case with 18 out of 21 amino acids. These different codons that code for a single amino acid are said to be synonomous. For example, UUC and UUU are synonomous codons for phenylalanine. The code is non overlapping. After translation commences, any single ribonucleotide at a specific location within the mRNA is part of only one triplet. The genetic code is nearly universal meaning thereby that same code is used throughout all life forms. Exceptions exist though. For example, UAG is a stop codon in some species while in others it may code for pyrrolysine.