Which of the following statements about the standard genetic code is NOT true? -

Question

Which of the following statements about the standard genetic code is NOT true?

- Transversions at the third codon position rarely change the amino acid specified.

- Transversions at the second codon position will usually result in a hydrophobic amino acid being replaced with a polar amino acid or vice-versa. - Base substitutions at the first codon position will often give a functionally similar (if not the same) amino acid. - Transition mutations at the third codon position rarely change the amino acid specified.

Explanation / Answer

Choice 1 is incorrect.

Reason: Base substitution of the triplet codon at any of the position results in change in identity of the amino acid provided the resultant codon does not encode for the same amino acid. This holds true for all the three positions of the codon. The most specific position is third and least specific position is first. A transversion mutation is the one in which a purine is replaced with a pyrimidine i.e. A/G are replaced with C/U. On the other hand, a transition mutation is the one in which a purine is replaced with another purine and a pyrimidine is replaced with another pyrimidine. It has been observed that changing the second position of the codon by a transversion mutation changes the chemical nature of the amino acid being encoded. For example, the codon AAU codes for asparagine whereas transition mutation at position two causes change in AAU to ACU and this ACU not encodes for threonine. These amino acids are different chemically and hence, the second position significantly brings about the change.

The same holds true for transversion mutations at position 3. There are many groups of codon sets which encode for different types of amino acids merely by transversion of the their nucleotide in the codon. For example, UUU and UUC encode for phenylalanine and transversion of these to UUA and UUG encodes for leucine.

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