Question

1) What is the shortest possible length of an mRNA molecule that corresponds to a polypeptide 10 amino acids long? For your answer just give me the number. E.g., "12" or "120." (Ignore the 5' cap and poly-A tail.)

2) How can the deletion of one nucleotide result in a longer polypeptide being made? Write out 2 examples of mRNA sequence (before and after the mutation) and its corresponding amino acid sequence (before and after the mutation) to show how this could happen.

3)In the metabolic pathway above, paraoxon is the most toxic compound. What change would likely lead to the greatest build-up of paraoxon in the body?

Group of answer choices

a) decrease in the amount of diethylphosphate and p-nitrophenol in the body.

b) nonsense mutation near the beginning of the gene that codes for cytochrome P450.

c) silent mutation in the gene that codes for paraoxonase.  

d) severe change in the shape of the active site of paraoxonase.

Answer 1

Answer 1:

It is given that the polypeptide is 10 amino acids long. Since one amino acid is codes by a group of three nucleotides, thus for 10 amino acids, we will have,

=10× 3

= 30 nucleotides.

Taking the average distance between two bases as 1/3 nm, we can calculate the length of this mRNA as,

= 30 × (1/3)

= 10 nm

Thus, the answer should be 10 nm.

Answer 2:

Protein synthesis stops when the translation machinery encounters a stop codon. Mutation in the stop codon can generate codons that can code for amino acids. There are three stop codons: UAA, UAG, UGA.

Deletion is one such mutation which can change the reading frame of codons and result into a longer polypeptide.

Let us consider the following examples.

1) mRNA before deletion:

AAU GAG UAG AAU GGG

Amino acid sequence:

Asn Glu

Sequence of mRNA after deletion of U :

AAU GAG AGA AUG GG

Amino acid sequence:

Asn Glu Arg Met

Thus, deletion of U resulted into a 4 amino acids long polypeptide instead of two in the normal case.

(2) Let the mRNA sequence before deletion be:

UUU AAA UUA UGA CAA GGG

Sequence of amino acids before deletion:

Phe Lys Leu

Sequence of mRNA after deletion of G:

UUU AAA UUA UAC AAG GG

Sequence of amino acids after deletion:

Phe Lys Leu Tyr Lys

There are 3 amino acids before deletion but 5 after deletion.

Answer 3:

Paraoxon is an insecticide and is under the category of organophosphates. It is degraded by the enzyme paraoxonase. A change in its active site would mean that paraoxon won't be degraded and thus get accumulated. The correct choice should be:

d) severe change in the shape of the active site of paraoxonase.