Question

What is the termination codon?

Answer 1

In molecular biology, a termination codon is also referred to as a stop codon. They are triplets in nature. The codons are codes for amino acids present in a template of the gene strand. In total, there are approx 64 codes for these amino acids.

Complete answer:
The universal genetic codes are made up of several codons or triplet bases. The standard code is made to minimize coding errors. It was George Gamow, a physicist, who argued that since there are only 4 bases and if they have to code for 20 amino acids, the code should constitute a combination of bases. He suggested that in order to code for all the 20 amino acids, the code should be made up of three nucleotides.

There are two punctuation marks in the genetic code that are called the START and STOP CODON that signals the end of protein synthesis in all organisms.

The genetic code can be read in multiple ways depending upon from where the reading is started. There are three ways of reading the code of every single strand of genetic material. These different ways of reading a nucleotide sequence are called reading frames. Each reading frame will help in the production of sequences of amino acids and hence proteins. Whereas, the double codon have 6 possible reading frames.

There are 3 STOP codons in the genetic code. They are UAG,UAA and UGA which signals the end of the polypeptide chain during translation. These codons are called as non- sense codons or termination codons because they do not code for amino acid. These codons have been named as amber (UAG), opal (UGA) and ochre (UAA). Amber was discovered by Charles Steinberg and Richard Epstein and they named it as amber after the German meaning of the last name of Harris Barnstein, their friend. The other two codons were named so, to maintain the "color names”theme.

During the synthesis of protein, STOP codon causes the release of the new polypeptide chain arising
from the ribosomes. This occurs because there are no tRNAs with these anticodons which were complementary to the STOP codons.

Note: Proof that codon was a triplet was discovered by Har Gobind Khorana who instructed the synthesis of RNA molecules with defined combinations of bases. Marshall Nirengberg cell-free system for protein synthesis, finally helped the code to be deciphered. The AUG or methionine is the start codon.