Q17-2TYU - Campbell Biology

Question

Which of the following is not true of a codon?

1. It may code for the same amino acid as another codon.

2. It never codes for more than one amino acid.

3. It extends from one end of a tRNA molecule

4. It is the basic unit of the genetic code.

Step-by-Step Solution

Answer

1. The statement “It may code for the same amino acid as another codon” is false.

2. The statement “It never codes for more than one amino acid” is false.

3. The statement “It extends from one end of a tRNA molecule” is true.

4. The statement “It is the basic unit of the genetic code” is false.

Step 1: Codon

An mRNA (messenger ribonucleic acid) molecule is a sequence of nucleotides transcribed from the nucleotides present in the DNA (deoxyribonucleic acid). A series of three-nucleotide sequences of mRNA that code for specific amino acids are called codons.

Step 2: Explanation of option (A)

Many codons can code for the same amino acid. This is called the degeneracy of the genetic code. For example, GAG and GAA are two codons that code for glutamic acid.

Thus, a codon that codes a specific amino acid can also be coded by another codon. 

Therefore, the given statement is false.

Step 3: Explanation of option (B)

A codon codes for a single amino acid, but an amino acid can be coded by more than one codon. For example, GAG only codes for glutamic acid, but glutamic acid is coded by two codons (GAG and GAA).

Thus, codon codes for a single amino acid.

Therefore, the given statement is false.

Step 4: Explanation of option (C)

Codon is the triplet nucleotide sequence on mRNA. However, one end of the tRNA contains the amino acid coded by the mRNA codon.

Thus, the tRNA molecule does not contain a codon.

Therefore, the given statement is true.

Step 5: Explanation of option (D)

The sequence of nucleotides on the RNA molecule that specifies the amino acids of the polypeptide chain is referred to as the genetic code. The mRNA codon codes for the amino acid.

Thus, the codon is the basic unit of the genetic code.

Therefore, the given statement is false.