1.5 - Nucleic acids

Question 1

Name the type of bond between:
a) complementary base pairs
b) adjacent nucleotides in a DNA strand

Answer 1

• Hydrogen bond
• Phosphodiester (bonds)

Question 2

Describe the structure of DNA. (5)

Answer 2

• Polymer of nucleotides
• Each nucleotide formed from deoxyribose, a phosphate (group) and an organic/nitrogenous base
• Phosphodiester bonds (between nucleotides)
• Double helix/2 strands held by hydrogen bonds
• (Hydrogen bonds/pairing) between adenine, thymine and
  cytosine, guanine

Question 3

Describe how a phosphodiester bond is formed between two nucleotides within a DNA molecule. (2)

Answer 3

• Condensation (reaction)/loss of water
• (Between) phosphate and deoxyribose
• (Catalysed by) DNA polymerase

Question 4

In the process of semi-conservative DNA replication, the two strands within a DNA molecule are separated. Each then acts as a template for the formation of a new complementary strand. Describe how the separation of strands occurs. (2)

Answer 4

• DNA helicase
• Breaks hydrogen bonds between base pairs/ AT and GC/complementary bases
  / Breaks hydrogen bonds between polynucleotide strands

Question 5

Describe the role of DNA polymerase in the semi-conservative replication of DNA. (2)

Answer 5

• Joins (adjacent DNA) nucleotides
• (Catalyses) condensation (reactions)
• (Catalyses formation of) phosphodiester bonds (between
  adjacent nucleotides) / Joins (adjacent DNA) nucleotides

Question 6

Use your knowledge of semi-conservative replication of DNA to suggest:
a) the role of the single-stranded DNA fragments
b) the role of the DNA nucleotides (3)

Answer 6

Role of single-stranded DNA fragments:
- Template
- Determines order of nucleotides/ bases
Role of DNA nucleotides:
- Forms complementary pairs / A–T, G-C
/ Forms complementary (DNA) strand

Question 7

Give two features of DNA and explain how each one is important in the semi-conservative replication of DNA. (2)

Answer 7

• Weak / easily broken hydrogen bonds between bases allow two strands to separate / unzip
• Two strands, so both can act as templates
• Complementary base pairing allows accurate replication

Question 8

Describe the role of two named enzymes in the process of semiconservative replication of DNA. (3)

Answer 8

• (DNA) helicase causes the breaking of hydrogen/H bonds (between DNA strands)
• DNA polymerase joins the (DNA) nucleotides
• Forming phosphodiester bonds

Question 9

During replication, the two DNA strands separate and each acts as a template for the production of a new strand. As new DNA strands are produced, nucleotides can only be added in the 5’ to 3’ direction. Use the figure in part (a) and your knowledge of enzyme action and DNA replication to explain why new nucleotides can only be added in a 5’ to 3’ direction. (4)

Answer 9

• Reference to DNA polymerase;
• (Which is) specific
• Only complementary with / binds to 5’ end (of strand)
• Shapes of 5’ end and 3’ end are different / description of how different