FCH 532 Homework 2
Put your name and student ID number on the top of each page.
Your assignment will not be accepted if it is not legible.
1. The base sequence of one of the strands of a 20-bp duplex DNA is:
5’-GTACCGTTCGACGGTACATC-3’
What is the base sequence of its complementary strand?
Draw the standard Watson-Crick base pair structures showing hydrogen bonds. Where is
the minor groove? Where is the major groove? Explain how this structure is important
for the structure of DNA, Chargaff’s rules and heredity.
2. Non-Watson-Crick base pairs are of biological importance. For example: (a)
Hypoxanthine (6-oxopurine) is often one of the bases of the anticodon of tRNA . With
what base on mRNA is hypoxanthine likely to pair? Draw the structure of this base pair.
(b) The third position of the codon-anticodon interaction between tRNA and mRNA is
often a G-U base pair. Draw a plausible structure for such a base pair. (c) many of
species of tRNA contain a hydrogen bonded U-A-U assembly. Draw two plausible
structures for this assembly in which each U forms at least two hydrogen bonds with the
A. (d) Mutations may arise during DNA replication when mispairing occurs as a result of
a rare tautomeric form of a base. Draw the structure of base pair with proper WatsonCrick geometry that contains a rare tautomeric form of adenine. What base sequence
change would be caused by such mispairing?
3. (a) What is the molecular mass and contour length of a segment of B-DNA that
specifies a 40-kD protein? (b) How many helical turns does this DNA have and what is
its axial ratio (length to width ratio)?
4. What are the differences between RNA polymerase and DNA polymerase?
5. Describe continuous and discontinuous replication.
6. The following duplex DNA is transcribed from right to left as printed:
5’-TCTGACTATTCAGCTCTCTGGCACATAGCA-3’
3’-AGACTGATAAGTCGAGAGACCGTGTATCGT-5’
(a) Identify the template strand. (b) What is the amino acid sequence of the
polypeptide that this DNA sequence encodes? Assume that translation starts at the first
initiation codon. (c) Why doesn’t the UGA sequence in the mRNA transcript cause
transcription to terminate?
7. After undergoing splicing, a mature mRNA has the following sequence, where the
vertical line indicates the position of the splice junction (the nucleotides from between
which an intron had been removed).
5’-CUAGAUGGUAG|
GUACGGUUAUGGGAUAACUCUG-3’
(a) What is the sequence of the polypeptide specified by this mRNA? Assume that
translation starts at the first initiation codon. (b) What would the polypeptide sequence be
if the splicing system had erroneously deleted the GU on the 3’ side of the splice
junction? (c) What would the polypeptide sequence be if the splicing system had
erroneously failed toi excise a G at the splice junction? (d) is there any relationship
between the polypeptides specified in b and c and, if so, why?