Question 9.
a. The diagram below represents part of the DNA double helix.
A B
O H O
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Worked Solution & Example Answer:Question 9 - VCE - SSCE Chemistry - Question 9 - 2009 - Paper 1
Step 1
i. Write down the formula of the atom or groups of atoms represented by A and B.
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Answer
A: C5H5O (for a sugar unit like ribose)
B: C5H5N (for a nitrogenous base, adenine or thymine, for instance)
Step 2
ii. Name the base that is complementary to the base shown.
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Answer
If A is adenine, the complementary base is thymine.
Step 3
b. Explain why the human viral DNA has a lower melting temperature than the monkey viral DNA.
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Answer
The melting temperature of DNA is influenced by its composition of nucleotides, particularly the ratio of adenine-thymine (A-T) pairs to guanine-cytosine (G-C) pairs. Human viral DNA, containing a higher percentage of adenine, will have more A-T pairs compared to G-C pairs. Since A-T pairs are held together by two hydrogen bonds, while G-C pairs are held together by three, the presence of more A-T pairs leads to a lower melting temperature. Thus, the human viral DNA being composed of a greater percentage of adenine than the monkey viral DNA results in it having a lower melting temperature.
