Question

The base sequences in mRNA that code for certain amino acids are

Glu: GAA, GAG

Val: GUU, GUC, GUA, GUG

Met: AUG

Trp: UGG

Phe: UUU, UUC

Asp: GAU, GAC

These sequences are complementary to the sequences in DNA.

a. Give the corresponding sequences in DNA for the amino acids listed above.

b. Give a DNA sequence that would code for the peptide trp-glu-phe-met.

c. How many different DNA sequences can code for the peptide in part b?

d. What is the peptide that is produced from the DNA sequence T-A-C-C-T-G-A-A-G?

e. What other DNA sequences would yield the same tripeptide as in part d?

Short answer

(a) CTT and CTC for glu

CAA, CAG, CAT and CAC for val

TAC for met

ACC for trp

AAA and AAG for phe

CTA and CTG for asp

(b) The whole DNA sequence is - AAA (or AAG) - TAC}ACC - CTT (or CTC) - AAA (or AAG) - TAC.

(c) Four different DNA sequences can produce this peptide.

(d) The peptide has the form Met-Asp-Phe

(e) The DNA sequence are TAC-CTG-AAA

Step-by-step solution

To give the  corresponding sequences

Corresponding sequence of DNA for the amino acids are CTT and CTC for glutamate, , CAG, CAT and CAC for valine, TAC for methionine, ACC for tryptophan, AAA and AAG for phenylalanine and CTA and CTG for aspartatic acid. These sequences are complementary with each code and uracil (using in mRNA) is replaced by thymine (using in DNA).

Hence CTT and CTC for glu

CAA, CAG, CAT and CAC for val

TAC for met

ACC for trp

AAA and AAG for phe

CTA and CTG for asp

To give a DNA sequence

DNA sequence should be complementary to the mRNA code. In DNA, instead of uracil we should use thymine.

Tryptophan mRNA code sequence is UGG. Therefore, DNA coding sequence should be ACC.

Glutamic acid has two mRNA coding sequences, GAA and GAG. Complementary DNA sequences can be CTC or CTT.

Phenylalanine has two mRNA coding sequences, UUU and UUC. Complementary sequences (DNA sequences) are AAA and AAG.

The last coding mRNA sequence is for methionine (AUG). Complementary DNA sequence is TAC.

Therefore, the whole DNA sequence is - AAA (or AAG) - TAC}ACC - CTT (or CTC) - AAA (or AAG) - TAC.

To find the Different DNA sequences

There are four different DNA sequences that can produce this peptide. DNA sequence is ACC - CTT (or CTC) - AAA (or AAG) - TAC.

The first one is ACC-CTT-AAA-TAC.

The second one is ACC-CTT-AAG-TAC.

The third one is ACC-CTC-AAA-TAC.

The last one is ACC-CTC-AAG-TAC.

Hence Four different DNA sequences can produce this peptide.

To find peptide

In this exercise, the DNA sequence is given (TAC-CTG-AAG). First, we have to create complementary mRNA sequences out of DNA sequences. The complementary sequence is AUG-GAC-UUG.

The last step is determining which code produces which peptide from the table. AUG is mRNA sequence for methionine, GAC for aspartic acid, and UUG for phenylalanine

Therefore, the peptide has the form Met-Asp-Phe.

To find other DNA sequences

Four DNA sequences can be translated, giving desirable peptides. One of them was given in the previous exercise. Therefore, three DNA sequences can give the same peptide.

The first one is TAC-CTG-AAA

The second one is TAC-CTA-AAA

The third one is TAC-CTA-AAC

The process has the following steps:

We should write down mRNA codes for each amino acid.

Methionine has one sequence: AUG

Aspartic acid has two sequences: GAU and GAC

Phenylalanine has two sequences: UUU and UUC

We should combine all of them to produce desirable peptides.

We have four options because we should multiply the number of sequences per amino acid (1x2x2 = 4).

The last step is to make a complementary sequence to each mRNA sequence. For example, mRNA sequence AUG-GAC-UUU will have complementary DNA sequence TAC-CTG-AAA

Hence the DNA sequence is TAC-CTG-AAA.