Question

1.     How does each of your mutations affect the amino acid sequences? Are the mutations missense mutations, silent mutations or nonsense mutations?

a.     Point mutation?

b.     Frameshift-insertion?

c.     Frameshift-deletion?

2.     What differences did you notice between the point mutation and the frameshift mutations?

3.     Is it possible to determine the DNA sequence from the following amino acid sequence?: Leu Pro Arg. Why or Why not?

References:

Answer 1

Ans. #a. Change in nucleotide sequence by 1 nucleotide is called a point mutation. The change can be in form of substitution, addition, or deletion of 1 nucleotide. Addition or deletion of 1 nucleotide always leads to frameshift mutation.

# Substitution mutation can be following types-

#I. Silent mutation: The point mutation that does not change amino acid sequence is called a Silent Mutation. Silent point mutation arise due to substitution of one base with another base AND the mutated codon codes the same amino acid as wild type. It does not affect the amino acid sequence of resultant proteins. For example-

            CCU, CCC, CCA and CCG all encode for the same amino acid.

So, a mutation that changes the last letter of the codon does not change amino acid sequence.

#II. Missense mutation: The point mutation that changes amino acid sequence is called a Missense Mutation. Silent point mutation arise due to substitution of one base with another base AND the mutated codon encodes a different amino acid from the wild type. It does not affect the amino acid sequence of resultant proteins. For example-

            Point mutation (substitution) in Hb^A allele at second letter of 6^th codon results Hb^S allele. Hb^S allele has 6^th codon GTG encoding valine (instead of normal codon GAG that encodes glutamate) in the ?- chain of hemoglobin.

#III. Nonsense mutation: The point mutation that creates a premature STOP codon is called a nonsense mutation. For example. The change in UGG to UGA (UGG = Tyr, UGA = STOP) is a non-sense mutation.

#b. Frameshift insertion: Insertion of 1 nucleotide causes frameshift-insertion mutation.

Frameshift mutation is the insertion or deletion of on ‘n’ residues in the reading frame of mRNA, where n is NOT divisible by 3. It thus changes the reading frame of mRNA because the reading frame is shifted by a number not equal to 3 or its multiple – which is the basic unit (triplet codon) of translation.

It may result largest extent of changes in the amino acid sequence depending on the position of mutation (closer is the mutation to 5’end of mRNA, greater would be change in amino acids sequence).

It leads to loss and structural and functional integrity of the protein.  

#c. Frameshift deletion: deletion of 1 nucleotide causes frameshift-deletion mutation.

#d. Frameshift mutation almost always lead to change in amino acid sequence at large scale, and the resultant protein in non-functional.

Point mutations may lead to-

            No change in amino acid sequence (silent mutation).

            Change in one amino acid sequence (Missense mutation)

            Truncated protein (Nonsense mutation).

#e. Leucine is encoded by UUA, UUG, CUU, CUC, CUA, CUG codons.

Proline is encoded by CCU, CCC, CCA, CCG codons.

Arginine is encoded by CGU, CGC, CGA, CGG.

Tracing back to mRNA would lead to numerous combination of nucleotide sequence. Therefore, predicting the exact nucleotide sequence form these amino acids is NOT possible.