Question

Write the important features of genetic code.

Answer 1

Hint: A group of a combination of 3 nucleotides forms the codon that codes for amino acid and hence, is known as genetic code. These are universal, unambiguous, and redundant in features as genetic code.

Complete answer:
A codon is a triplet of nucleotide bases forming 64 different codons each coding for 1 out of the 20 known amino acids. These 64 genetic codons have the following features: (figure1)
They are triplet codons that are each codon made up of 3 nucleotides.
Genetic code is universal, that is the four basic nucleotides that form the basis of DNA in any organism also form the genetic code that codes for the same amino acid in all the organisms.
Genetic code encodes the sequence-specific for an amino acid when translated into mRNA.
Genetic code is unambiguous, that is each codon code for only one amino acid and there is no confusion during translation. In other words, genetic code is very specific for an amino acid. For example AUG codes for only methionine.
1) Degeneracy: Genetic code is redundant, that is one amino acid is represented by more than one triplet codon. For example
2) Start and stop signal: During translation, each protein starts with a start codon or chain initiation codon (AUG), and the chain is terminated at stop codon or chain termination codon (UAA, UAG, and UGA either of the three codons).
3) Polarity: genetic code has polarity and is read in 5' to 3' direction only.
4) Non-overlapping: a single nucleotide of a codon does not overlap with the other codon and is very specific for one codon at a time. That is, no nucleotide is shared by two codons at the same time.
Degeneracy in genetic code is of two types:
 Partial having the first two nucleotides are similar and only the 3rd nucleotide changes in the codon. For example CUU and CUC both codes for leucine.
Complete degeneracy is when all the 4 nucleotides (A, U, G, and C) take the third position and still codes for the same amino acid. For example UCU, UCC, UCA, and UCG all have codon codes for only one amino acid, Serine.
Of the total 64 codons (4 nucleotides forming different combination in a group of four that is 44 = 64), 61 codons are known as sense codon as they code for specific amino acid and rest 3 codons (UAA, UAG, and UGA) are known as nonsense codon as they do not code for any amino acid and amino acid chain gets terminated at this codon. Thus, mutations that make a codon any of these 3 stop codons are known as nonsense mutation.

Note: Genetic codon is very important during translation as it decides the amino acids, whose sequence decides the protein and its structure in an organism. Even a single change in the triplet codon affects the structure of the protein as it changes the amino acid it codes for. And hence to be very sure of the sequence of nucleotides we have different proofreading mechanisms that cross-check if the nucleotide is copied without any mistake and is synthesized without any errors.

Figure 1: Genetic codes and the corresponding amino acid that it codes for