3 buring questions: 1. What is the minimum number of single-nucleotide substitut

Question

3 buring questions:

1. What is the minimum number of single-nucleotide substitutions that would be necessary for each of the following amino acid replacements:

a) PheVal

b) TyrGly

c) MetPro

d) ArgCys?

2: A nucleotide in a codon is said to be a fourfold degenerate site if its mutation to any other nucleotide results in a synonymous mutation. How many codons in the standard genetic code have a fourfold degenerate site?

3.

A strand of DNA in the Original sequence below undergoes a mutation, such that the second C from the 5' end (underlined) is changed to a T. The mutant DNA sequence is shown below the Original. What would you expect to see in the mutant Original DNA sequence : 3' A A A T A C A A G A C C A A G T C T 5' Mutant DNA sequence : 3' A A A T A C A A G A C T A A G T C T 5' polypeptide? The mutant polypeptide would have only two amino acids The mutant polypeptide would be one amino acid longer than the original One amino acid would substitute for another The mutant polypeptide would not be synthesized There would be no change. The mutant polypeptide would be the same as the original. The mutant polypeptide would be one amino acid shorter than the original

Explanation / Answer

1. Phe - UUC to Val - GUC. First nucleotide has to be change
Tyr - UAU to Gly - GGU. First Two nucleotide should be change
Met - AUG to Pro - CCG. FirstTwo nucleotide should be change
Arg - CGU to Cys - UGU. First nucleotide has to be change

A codon is said to be a fourfold degenerate site if any nucleotide at this position specifies the same amino acid. Eg,
the third position of the glycine (GGA, GGG, GGC, GGU) is a fourfold degenerate site because all nucleotide substitutions at this site do not change the amino acid.

So, Leu, Val, Ser, Pro, Thr, Ala, Arg and Gly are fourfold degenerate.

8 Amino acids X 4 = 32 codons are a fourfold degenerate site.

3. The mRNA sequence for mutant strand

5' uuu AUG UUC UGA UUC AGA 3'

AUG - Met
UUC - Phe
UGA - Stop codon

           N- Met - Phe - 3'

So, The mutant polypeptide would have only two amino acids.

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