The template strand of a gene includes this sequence:

3’-TACTTGTCCGATATC-5’. It is mutated to

3’-TACTTGTCCAATATC-5’. For both wild-type and mutant sequences, draw the double-stranded DNA, the resulting mRNA, and the amino acid sequence each encodes. What is the effect of the mutation on the amino acid sequence?

DNA is made up of four nitrogen bases; adenine (A), thymine (T), cytosine (C), and uracil (C).Adenine in one strand base pairs with the thymine of the other strand, and cytosine of one strand base pairs with the guanineof the other through hydrogen bonding.

The double-stranded DNA sequence is:

3’-TACTTGTCCGATATC-5’

5’-ATGAACAGGCTATAG-3’

RNA is made of four bases; A, C, G, and uracil (U). Theadenine of DNA is complementary to the uracil of the mRNA, and the cytosine of DNA is complementary to guanine.Thus, the mRNA sequence of the given template strand is 5’-AUGAACAGGCUAUAG-3’.

A triplet nucleotide sequence of mRNA codes for a specific amino acid. AUG is an initiation codon that codes for methionine (Met), AAC codes for asparagine (Asn), AGG codes for arginine (Arg), CUA codes for leucine (Leu), and UAG is a stop codon.

Thus, the amino acid sequence is Met-Asn-Arg-Leu-Stop.

The mutation causes a substitution in the nucleotide sequence of the DNA.The mutation substitutes the 10th nucleotide guanine with adenine.As a result, the nucleotide sequence of the DNA changes.

Thus, the mutated double stranded sequence would be3’-TACTTGTCCAATATC-5’

5’-ATGAACAGGTTATAG-3’.

The altered DNA sequence would change the mRNA sequence. The cytosine at the 10th position would be replaced by uracil. As a result, the mRNA sequence would be 5’-AUGAACAGGUUAUAG-3’.

The changed mRNA codon UUA also codes for leucine. As a result, the amino acid sequence remains the same Met-Asn-Arg-Leu-Stop.

The mutation changes the last codon of the mRNA sequence that codes for an amino acid. In the mRNA, the codon CUA is replaced by UUA. However,both UUA and CUA code for the same amino acid, leucine.

As a result, the mutated mRNA sequence codes for the same sequence of amino acids as that coded by the wild-type mRNA. This means mutation does not change the amino acid sequence of the polypeptide. This type of mutation is called a silent mutation.