Question

What is a codon and how do you identify them in a strand of mRNA? There are 64 potential codons for only 20 amino acids. Why does this occur? Why is it also important to know the start codon before determining what a codon would be in a given strand of mRNA? After answering this first part, explain the difference between the DNA template and coding strands and explain how each strand relates to the mRNA strand that is produced?

Answer 1

1). Every three successive nucleotides of mRNA together is considered as codon.

2). Out of 64 codons, 3 three codons are stop codons that do not represent any amino acid. The remaining 61 codons, some codons represents the same amino acid. So that 20 amino acids are represented by 61 codons.

The code is degenerate: The occurrence of more than one codon for a single amino acid is referred to as degenerate. Most amino acids have more than one codon. The degeneracy usually occurs in the third letter of the code

3). mRNA is single stranded long chain of polynucleotide transcribed from DNA template strand. But only particular codons are required for the synthesis of polypeptide. So that always there would be starting codon. The start codon is the initiation signal for translation that is found on a messenger RNA (mRNA) strand. AUG is the start codon.

4).

DNA template and coding strands are complementary to each other. Template strand runs from 3' to 5' from left side to right side respectively where coding strand runs from 5' to 3' from left side to right side respectively.

Template strand is formed by continuous process whereas coding strand is formed by discontinuous process by joining of okazaki fragments.

mRNA is produced from DNA template strand. So, that DNA is complementary to mRNA

Coding strand is similar to mRNA except at thymine position.