Question

How many stop codons are there?

Answer 1

Hint: The genetic code is a set of genetic information present in the nucleotide sequence of DNA that directs the translation of RNA into 20 amino acids- the basic units of proteins in the living cells. The genetic code is made up of codons in a non-overlapping fashion, which is a three-letter chain of nucleotides.

Complete answer:
During protein synthesis, the amino acids have to be arranged in order to synthesize a polypeptide chain, which is decided by the genetic code. In 1961, the idea of codon was introduced by Francis Crick and colleagues. But the genetic code was deciphered by Marshall Nirenberg and co-workers.
They showed that four nucleotide bases A (adenine), U (uracil), G (guanine), and C (cytosine) form codons of different base combinations. These combinations code for all 20 amino acids during protein synthesis.
In the genetic code, each codon consists of three bases arranged in a specific order, where each combination corresponds to one particular amino acid. RNA has four bases, so there are 64 potential combinations of nucleotide triplets in the genetic code.
Each codon can code for only one amino acid, but the same amino acid can be coded by multiple codons. This coding of one amino acid by more than one codon is called the redundancy of the genetic code. For example, the amino acid lysine can be coded by both AAG and AAA codons.
Genetic code is non-overlapping, which means one nucleotide can only be a part of one codon, not two codons that are next to each other.
Genetic code is commaless- without punctuation.
Genetic codes show universality- Same codon codes for the same amino acids from bacteria to human beings.
AUG is the most common start codon which begins the translation. In eukaryotes, AUG codes for methionine and prokaryotes, it codes for formyl methionine.
Polypeptide chain synthesis halts by the signal received by a stop codon. Stop codons, also called nonsense or termination codons are three in numbers- UAG, UGA, and UAA named amber, opal, and ochre respectively.

Note:
Since no tRNA anticodons compliment the stop codons, the stop codons trigger the ribosome to release the new polypeptide chain. Methionine, coded by AUG and Tryptophan, coded by UGG- are the only two amino acids that are coded by just a single codon. The other 18 out of 20 amino acids are coded by two to six codons.