Question

(a) What is a genetic code ?

(b) Why did scientists feel the need to propose a genetic code ?

(c) Genetic code is said to be "Universal". Why ?

Hint:

The genetic code is the triplet codon dictionary linking mRNA bases to amino acids. It is needed because four bases must name 20 amino acids, and it is universal across organisms.

Answer 1

Step 1: Define the genetic code (part a):
The genetic code is the set of rules that tells how the sequence of bases in mRNA is read to make a protein.
Three bases in a row, called a codon, stand for one amino acid. There are 64 codons in all.

Step 2: Why a code was needed (part b):
DNA has only four bases, A, T, G and C. Proteins are built from 20 different amino acids.
So the four letters must be arranged in groups to name all 20 amino acids. One base per amino acid would give only 4 names.
Two bases would give 16 names, which is still short. Three bases give 64 names, which is enough. George Gamow suggested the triplet code on this reasoning.

Step 3: Why it is universal (part c):
The same codon codes for the same amino acid in almost all living organisms, from bacteria to humans.
For example, UUU codes for phenylalanine in a bacterium as well as in a human cell.
This is why a human gene can work inside a bacterium in genetic engineering.

Final Answer:
(a) A set of triplet codons in mRNA that specify amino acids. (b) To link 4 bases to 20 amino acids. (c) The same codon specifies the same amino acid in nearly all organisms.