3.1.4: DNA structure and replication

Question 1

Describe DNA replication (6)

Answer 1

1. DNA helicase unwinds DNA/double
   helix
   OR
   DNA helicase breaks hydrogen bonds;
2. Both strands act as templates;
3. (Free DNA) nucleotides line up in
   complementary pairs/Adenine-Thymine
   and Guanine-Cytosine;
4. DNA polymerase joins nucleotides
   (of new strand);
   Reject forms hydrogen bonds/joins bases
5. Forming phosphodiester bonds;
6. Each new DNA molecule consists of
   one old/original/template strand and one
   new strand;

Question 2

Describe the structure of DNA (5)

Answer 2

1. Polymer of nucleotides;
   Accept ‘Polynucleotide’
   Accept for ‘phosphate’.
   phosphoric acid
2. Each nucleotide formed from
   deoxyribose, a phosphate (group) and an
   organic/nitrogenous base;
3. Phosphodiester bonds (between
   nucleotides);
4. Double helix/2 strands held by
   hydrogen bonds;
5. (Hydrogen
   bonds/pairing) between
   adenine,
   thymine and cytosine,
   guanine;

Question 3

Describe and explain how the structure of DNA
allows accurate replication (4)

Answer 3

1 two strands therefore semi-
conservative replication (possible);
2 base pairing / hydrogen bonds holds
strands together
3 hydrogen bonds weak / easily
broken, allow strands to separate;
4 bases (sequence) (exposed so) act
as template / can be copied;
5 A with T, C with G / complementary
copy;
6 DNA one parent and one new
strand;

Question 4

Describe how a phosphodiester bond is
formed between two nucleotides in a DNA
molecule. (3)

Answer 4

1. condensation reaction/loss of water
2. (between) phosphate and deoxyribose
3. (catalysed by) DNA Polymerase