Solutions For An Introduction to Genetic Analysis Chapter 9 Problem 18P

a) In the genetic code provided, 28 codons are present that do not identify a specific amino acid with the first two positions. A three letter codon is necessary to identify the amino acid. Each codon represents an amino acid. The codon sequence specifies which amino acid is going to be added in the protein synthesis. Unless the three letters in the codon are known, one can not specify which amino acid it codes. Though the first two nucleotides are same, the last nucleotide specifies the amino acid to be coded.

For example,

AGU codes for serine

AGA codes for arginine.

In both the amino acids, the first two nucleotides are same, but still the amino acid specified is different. The third nucleotide is different and it specified which amino acid is going to be coded.

b) There are three amino acids serine, leucine, and arginine. They are encoded by six different codons and if we know the amino acid no need of understanding the first two nucleotides.

In this case, though the amino acid is known, we cannot not predict the codon, as there are six different codons coding for same amino acid. Hence, we fail to know the first two nucleotides of the codon.