Practice Exercise (this is not an assignment)
Practice Exercise (this is not an assignment)

The table provided at the bottom of this page will help you to answer the questions below:

1. The DNA sequence below represents a fragment of a coding region of the collagen gene. What amino acid sequence is coded for by this sequence?

. . . CTTTTTTGTGGT . . .

As the result of a point mutation, the third base, thiamine, was replaced by cytosine. The new DNA sequence reads as follows:

. . . CTCTTTTGTGGT . . .

Did this mutation affect the structure of this protein? If yes, what is the new amino acid sequence?

What if a similar point mutation affected the second base instead, so that the mutated DNA sequence would be:

. . . CCTTTTTGTGGT . . .

Would this mutation have any effect on protein structure? If yes, what would the new amino acid sequence corresponding to this sequence be?

2. A small peptide has the following amino acid sequence:

Arginine . Lysine . Glutamine . Glycine

Write two possible DNA sequences that could code for the structure of this peptide.

20 Amino acids, their single-letter data-base codes (SLC), and their corresponding DNA codons


Amino Acid

 SLC

DNA codons

Isoleucine  

I

ATT, ATC, ATA
Leucine  

L

 CTT, CTC, CTA, CTG, TTA, TTG
Valine

V

 GTT, GTC, GTA, GTG
Phenylalanine  

F

 TTT, TTC
Methionine M ATG
Cysteine  C

TGT, TGC
Alanine      

A

GCT, GCC, GCA, GCG

Glycine  

G

 GGT, GGC, GGA, GGG
Proline      

P

 CCT, CCC, CCA, CCG
Threonine  

T

 ACT, ACC, ACA, ACG
Serine        S TCT, TCC, TCA, TCG, AGT, AGC

Tyrosine  

Y TAT, TAC
Tryptophan   W TGG
Glutamine   Q CAA, CAG
Asparagine   N AAT, AAC
Histidine 

H

 CAT, CAC
Glutamic acid  

E

 GAA, GAG

Aspartic acid 

D

 GAT, GAC
Lysine       

K

 AAA, AAG
Arginine  

R

 CGT, CGC, CGA, CGG, AGA, AGG
Stop codons Stop TAA, TAG, TGA
In this table, the twenty amino acids found in proteins are listed, along with the single-letter code used to represent these amino acids in protein data bases. The DNA codons representing each amino acid are also listed. All 64 possible 3-letter combinations of the DNA coding units T, C, A and G are used either to encode one of these amino acids or as one of the three stop codons that signals the end of a sequence. While DNA can be decoded unambiguously, it is not possible to predict a DNA sequence from its protein sequence. Because most amino acids have multiple codons, a number of possible DNA sequences might represent the same protein sequence.