In the genetic code, a.some codons specify more than one amino acid. b.some codons consist of two nucleotides. c.some amino acids are not specified by any codons. d.many amino acids are specified by more than one codon.

D

Explanation:

In the central dogma of genetic expression, a codon is a triplet nucleotide sequence in a mRNA that specifies a particular amino acid. Each group of codon contains information for one amino acid.

There are only four nucleotides in an mRNA; Adenine, Guanine, Cytosine and Uracil. If these nucleotides are sequenced three at a time, there will be 64 combinations (where 61 of these combinations code for an amino acid while the remaining three represent stop codons; UAA, UAG, UGA) of the 4 nucleotides i.e. 4^3 or 4×4×4.

There are only 20 different amino acids, meaning that there are more possible codons than the amino acid they code for. Due to this reason, most amino acids are indicated by more than one codon i.e more than one codon can code for a particular amino acid in the genetic code. This phenomenon is referred to as DEGENERACY or REDUNDANCY in genetic coding.

For example; Phenylalanine, an amino acid, is coded for by UUU and UUC codons. Also, leucine is coded for by CUU, CUC, CUA and CUG codons.

Note that, all amino acids except tryptophan (coded for by only one amino acid, UGG) is coded for by 2 or more codons.

These degenerate pattern is important to the genetic code because it helps minimize the effect incorrect placement of nucleotides can have on protein synthesis i.e. a nucleotide can be incorrectly placed due to mutation and still not affect the synthesis of protein. e.g. a nucleotide sequence of UUU can be incorrectly placed as UUC, but because the codons both code for one amino acid, no harm is done.