Question

How many nucleotides of DNA are required to code a polypeptide if in an E.coli, a finished polypeptide has $162$ amino acids of which the first amino acid is not a methionine compound?

• A. $486$
• B. $54$
• C. $489$
• D. $492$

Answer 1

Answer: (D)

Proteins are formed on the ribosomes, with the help of mRNA, which binds to the ribosomes, and tRNA which attaches the amino acids. This process is conversion of nucleotide sequence of the mRNA into the amino acid sequence of the protein. That is the reason, it is called translation. Some important points about the process are like:

• codons are present on the mRNA which is specific for a given amino acid. They are recognised by the anticodons present on the tRNA.
• codons are triplet (a bunch of three nucleotides together).
• for most of the proteins, the first codon is AUG, which codes for the amino acid methionine.
• there are three termination codons, UAA, UAG, and UGA, which do not have amino acids for them and stop the translation process

Many proteins (as in this case, as well) undergo post-translational modifications in which the first amino acid (Met) is removed. Also, the last codon does not have any amino acid.
Therefore, we can think of a functional polypeptide whose first amino acid is removed during modification and the termination codons stopped the process.
Therefore, $if,thereare162aminoacidsinthefinishedpolypeptide,therewere162\times 3=486codonsbut,sincemethioninewasremoved,later,whichwascoded,wehave486+3=489codonsalsoaddingtheterminationcodon,wehave489+3=492codons$
Thus the DNA which have the gene for this hypothetical protein, will have 492 nucleotides.