Download Bio/CS 251 Bioinformatics Homework 4 20 points

Bio/CS 251 Bioinformatics
Homework 4
20 points
Due date: Friday, 2/10/06 in class
Use the Genetic Code to assist with the following questions:
1. The following represents a tetrapeptide: NH2 – Met – Glu – Ile – Trp – COOH
Below each amino acid, list all of the possible codons that specify that particular amino acid.
NH2 – MET – GLU – ILE – TRP – COOH
AUG
GAA
AUA
GAG
AUU
UGG
AUC
2. After translation has started, if AUA occupies the peptidyl site of the ribosome, what codon will be
in the aminoacyl site?
The peptidyl site would be occupied by a peptidyl-tRNA that carries the MET-GLU-ILE
tripeptide, and the aminoacyl site would contain the next aa-tRNA to be added to the
growing peptide. In this case the aminoacyl site would contain the UGG codon to pair with
the anticodon of Trp-tRNA.
3. A mutational event inserts an extra nucleotide pair into the DNA, for example between the GLU
and ILE codons. Which of the following outcomes do you expect?
a.
b.
c.
d.
e.
No protein at all
A protein in which one amino acid is changed.
A protein in which three amino acids are changed.
A protein in which two amino acids are changed.
A protein in which most of the amino acids after the site of the insertion are changed.
Either of these choices are correct, depending upon how you interpret the question. (If this
were a long peptide, instead of a tetrapeptide, answer e. would be the correct choice)
4. For each of the possible GLU codons, write out the mRNA codon from 5’  3’, then above it write
out the sequence of the anticodon with which it would pair, and be sure to indicate the polarity of
the tRNA sequence. Could all of the GLU codons be recognized by a single tRNA? If so, show
the single anticodon sequence that could serve both codons.
Anticodon
3’
C U U
5’
3’
C U C
5’
Codon
5’
G A A
3’
5’
G A G
3’
- OR Each of the above codons could be recognized by a single anticodon with
the sequence 5’ UUC 3’
This is because, according to wobble rules for base pairing, U in the first position
of the anticodon can legitimately pair with either A or G in the 3rd codon position, as
shown below.
Anticodon
3’
C U U
5’
3’
C U U
5’
Codon
5’
G A A
3’
5’
G A G
3’
5. a. How is ILE specified? (How many codons are used to specify ILE, and what are they?)
From Question #1 (above), there are three ILE codons, AUA, AUC, and AUU
b. Can a single tRNA serve to recognize all ILE codons? Draw the anticodon-codon pairings,
and explain.
A single anticodon with Inosine in the 5’ position can serve all three ILE codons:
Anticodon
3’
U A
I
Codon
5’
A U
A
C
U
5’
3’