Video Transcript
Which of these statements is not
true about the standard amino acids that belong to the genetic code? (A) Each amino acid contains a
codon of three nucleotides in its structure. (B) Amino acids are sometimes coded
by several codons in the genetic code. (C) A sequence of amino acids forms
a polypeptide, which forms a protein. (D) There are 20 amino acids in the
standard genetic code.
In order for a gene in DNA to be
converted into a protein, it needs to go through two steps. First, the DNA sequence is
converted into mRNA during a process called transcription. Then, the sequence of mRNA is
converted into a sequence of amino acids to form a polypeptide during
translation. This polypeptide can then fold to
form a protein. So, answer choice (C) is a correct
statement about amino acids and therefore is incorrect.
Let’s look at how a sequence of
mRNA can be translated into amino acids. A group of three nucleotides in
mRNA is called a codon. In this nine-nucleotide sequence of
mRNA, there’s three codons. The specific sequence in a codon
corresponds to a specific amino acid. The sequence AGG corresponds to the
amino acid arginine.
To determine what codon codes for
which amino acid, we can use a codon wheel as shown here. Here, we can see that codon AGG
corresponds to arginine. However, the codon AGA also
corresponds to arginine. This is because the genetic code is
redundant, or degenerate, and amino acids are sometimes coded by several codons. So, answer choice (B) is a correct
statement and is, therefore, incorrect.
Also, there are 20 amino acids in
the standard genetic code. So, answer choice (D) is a correct
statement and is, therefore, incorrect.
The incorrect statement is given by
answer choice (A). Amino acids do not contain
nucleotides in their structure. mRNA contains codons which are made up of three
nucleotides. Therefore, the correct answer is
(A). Each amino acid contains a codon of
three nucleotides in its structure.
