CfE Higher Sht1_2
HBiol Sht 1_2
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1. In 1957, Meselson and Stahl conducted experiments to investigate the process of DNA replication. Their experiment attempted to distinguish three hypotheses regarding the way DNA replication proceeds:
i.conservative (one of the daughter molecules is the parent one),
ii.semi-conservative (both daughter molecules contain a strand belonging to the parent molecule and a newly synthesised strand),
iii.or dispersive (both strands of both daughter molecules contain fragments of the parent molecule and fragments which are newly synthesised).
In their experiment, Meselson and Stahl followed the protocol shown below.
Following a period of growth in flask 2, the DNA of the bacteria was analysed. To determine the density of the DNA strands in the bacteria.
a) What method could used to separate the DNA strands of differing densities?
b) Into what part of a DNA nucleotide was the nitrogen incorporated?
c) The results of the experiment indicated that DNA replication was semi-conservative. In your jotter draw a test tube with two labelled bands to represent the positions of DNA made with only 15N and a second band for DNA containing only 14N. Add a third band to show where the DNA formed by semi-conservative replication would be positioned.
d) Redraw a second test tube in your jotter and show the results that would have been expected if DNA replication was conservative.
e) Describe the expected results of the CsCl density gradient centrifugation if DNA replication was dispersive.
f) What property of DNA lead Watson and Crick in 1953 to predict semi-conservative replication as the most likely copying mechanism?
2. A single strand of a DNA molecule has 6000 nucleotides of which 24% are adenine and 18% are cytosine.
a) Calculate the combined percentage of thymine and guanine bases on the same DNA strand.
b) How many guanine bases would be present on the complementary strand of this DNA molecule?
3. The diagram below shows part of a DNA molecule during replication and other molecules associated with it.
a) Name molecule X which is associated with the tightly coiled DNA.
b) Name the type of bond shown at Y.
c) Name base Z.
d) What feature of an organism is determined by the sequence of its bases?
e) In nucleic acids, the phosphate group is attached to which carbon of deoxyribose sugar?
f) Copy and complete the following, choosing the appropriate word from each bracketed pair.
4. A DNA strand has the sequence A-C-A-G-C-C-G-T-A. What would the sequence of its complementary strand be?
