Gene Expression Problem

Transcription

The DNA sequence provided is:

5’ TACCGAATCCATGGCTCTTAGAGGTAGTCC 3’

Transcription of the complementary strand (3’ to 5’) will produce the mRNA strand (5’ to 3’):

DNA: 3’ GGACTACCTCTAAGAGCCATGGATTCGGTA 5’

mRNA: 5’ CCUGAUGGAUCUCUCGGUACCUAAGCCAU 3’

Translation

The mRNA sequence is translated into a polypeptide sequence using the codon table.

mRNA: 5’ CCUGAUGGAUCUCUCGGUACCUAAGCCAU 3’

Codons: CCU GAU GGA UCU CUC GGU ACC UAA GCC AU

Polypeptide: Proline (Pro) - Aspartic acid (Asp) - Glycine (Gly) - Serine (Ser) - Serine (Ser) - Glycine (Gly) - Threonine (Thr) - STOP (UAA) -

Mutations and their effects

a) Nucleotide number 8 changes from a C to an A:

Original: 3’ GGACTACCTCTAAGAGCCATGGATTCGGTA 5’

Mutated: 3’ GGACTAACCTCTAAGAGCCATGGATTCGGTA 5’

mRNA: 5’ CCUGAUUGGAUCUCUCGGUACCUAAGCCAU 3’

Codons: CCU GAU UGG AUC UCU CGG UAC CUA AGC CAU

Polypeptide: Proline (Pro) - Aspartic acid (Asp) - Tryptophan (Trp) - Isoleucine (Ile) - Leucine (Leu) - Arginine (Arg) - Tyrosine (Tyr) - Leucine (Leu) - Serine (Ser) - Histidine (His)

Effect: The mutation changes the third codon from GGA (Gly) to UGG (Trp). This is a missense mutation, which results in a single amino acid change in the polypeptide.

b) Nucleotide number 16 changes from a G to an A:

Original: 3’ GGACTACCTCTAAGAGCCATGGATTCGGTA 5’

Mutated: 3’ GGACTACCTCTAAGAA CCATGGATTCGGTA 5’

mRNA: 5’ CCUGAUGGAUCUCUUUGGUACCUAAGCCAU 3’

Codons: CCU GAU GGA UCU CUU GGU ACC UAA GCC AU

Polypeptide: Proline (Pro) - Aspartic acid (Asp) - Glycine (Gly) - Serine (Ser) - Leucine (Leu) - Glycine (Gly) - Threonine (Thr) - STOP (UAA) -

Effect: The mutation changes the fifth codon from CGG (Arg) to CUG (Leu). This is another missense one where a single amino acid change occurs in the polypeptide chain of the protein.

c) Nucleotide number 12 changes from an A to a T:

Original: 3’ GGACTACCTCTAAGAGCCATGGATTCGGTA 5’

Mutated: 3’ GGACTACCTCTTAGAGCCATGGATTCGGTA 5’

mRNA: 5’ CCUGAUGGAUCUAAUCGGUACCUAAGCCAU 3’

Codons: CCU GAU GGA UCU AAU CGG UAC CUA AGC CAU

Polypeptide: Proline (Pro) - Aspartic acid (Asp) - Glycine (Gly) - Serine (Ser) - Asparagine (Asn) - Glycine (Gly) - Tyrosine (Tyr) - STOP (UAA) -

Effect: The mutation alters the fourth codon from Glu by changing its code to Lys due to AAG code. This is another missense mutation, which causes a change of one amino acid in the polypeptide

d) An insertion of a T between nucleotides 14 and 15:

Original: 3’ GGACTACCTCTAAGAGCCATGGATTCGGTA 5’

Mutated: 3’ GGACTACCTCTT AAGAGCCATGGATTCGGTA 5’

mRNA: 5’ CCUGAUGGAUCUAUCUCGGUACCUAAGCCAU 3’

Codons: CCU GAU GGA UCU AUC UCG GUA CCU AAG CCA U

Polypeptide: Proline (Pro) - Aspartic acid (Asp) - Glycine (Gly) - Isoleucine (Ile) - Serine (Ser) - Glycine (Gly) - Threonine (Thr) - STOP (UAA) -

Effect: This type of insertion results in the frameshift mutation since the reading frame changes from the point of insertion right to the end. This leads to the formation of an off-string of amino acids, starting from the point of mutation up to the next stop codon.