Problem set 2, BioS 220, 2011 Name _________________________ TA ______________________
1
1. If during DNA replication, RNA bases are found in the newly synthesized DNA, which
enzyme or enzymes may be defective? (1 point)
DNA polymerase I. DNA polymerase I has 5โ to 3โ exonuclease activity and normally cleaves
RNA primers to replace with DNA.
2. In the fruit fly Drosophila melanogaster, DNA replication at a single replication fork occurs
at a rate of about 2600 nucleotide pairs per minute. The DNA molecule occurring in one of
the largest chromosomes of this species has been estimated to contain 6 x 107 nucleotide
pairs. If replication of this molecule was initiated at a single origin in the middle of the
chromosome, estimate the time, in days, required for complete replication of the
chromosome. (1 point)
6 x 107 basepairs / (2 forks per origin x 2600 basepairs per fork per minute) = 11538 minutes = 8
days
3. The following figure shows a replication fork in a segment of DNA, You are viewing the part
that is just opening. (2 points) a. Along which template strand (upper or
lower) does continuous synthesis occur? (1
point) Lower
b. Along which template strand are
numerous Okazaki fragments formed? (1
point)
Upper
4. The temperature at which half of the hydrogen bonds in a sample of duplex DNA have
denatured is called the melting point, or Tm. Listed below are the base compositions of three
different duplex DNA molecules.
I. 5โ ATTCTAACTTTGAT 3โ II. 5โ CGCACTGGCTAACC 3โ III. 5โ CGCATTATTGTCAA 3โ
3โ TAAGATTGAAACTA 5โ 3โ GCGTGACCGATTGG 5โ 3โ GCGTAATAACAGTT 5โ
Order these three molecules in terms of their melting points from highest to lowest. (1 point)
II > III > I
II has 37 hydrogen bonds; III has 33 hydrogen bonds; I has 31 hydrogen bonds
3โ
5โ
replication fork moves this way
