As is well known, the genetic code
is the representation of the amino acid sequence of proteins by means of DNA
strands. Now, the proteins of living beings are made of 20 different amino
acids. However, there are only four different nucleotides in DNA. How can just
four bases represent 20 amino acids?
With codons made of two nucleotides just
16 amino acids could be represented. As they are 20, two nucleotides are not
enough: three must be used. Indeed, that is what life has done: each amino acid
is represented by codons made of three nucleotides. The problem is, three
nucleotides could represent 64 different amino acids, rather than 20 (21,
considering that there codons mark the end of the string). What is the
solution? Obviously, some amino acids must be
represented by several codons (this is what is called degeneracy of the genetic code).
The four DNA nucleotides are made of
a skeleton of sugar and phosphoric acid, combined with a nucleobase. In DNA
there are four different bases:
- Two purines
(P): adenine (A) and guanine (G).
- Two pyrimidines
(Q): cytosine (C) y thymine (T).
