Nucleic Acids, ATP, Water & Inorganic Ions

Question 1

Describe the structure of DNA.

Answer 1

1. Sugar-phosphate backbone/double stranded/helix provides strength/stability/protects bases/protects hydrogen bonds;
2. Long/large molecule so can store lots of information
3. Helix/coiled so compact
4. Base sequence allows information to be stored/base sequence codes for amino acids/protein
5. Double stranded so replication can occur semi-conservatively/strands can act as templates
6. Complementary base pairing/A-T and C-G so accurate replication/identical copies can be made
7. Weak hydrogen bonds for replication/unzipping/strand separation
8. Many weak hydrogen bonds so stable/strong molecule

Question 2

Describe semi-conservative replication.

Answer 2

1. Strands separate/H-bonds break
2. DNA helicase (involved)
3. Both strands/each strand acts as a template
4. Free nucleotides attach
5. Complementary/specific base pairing due to H bonds forming between bases Adenine to Thymine and Cytosine to Guanine
6. DNA polymerase joins adjacent nucleotides on new strand forming phosphodiester bonds by condensation
7. Semi-conservative replication/new DNA molecules contain one old strand and one new strand

Question 3

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

Answer 3

1. Joins adjacent DNA nucleotides
2. Catalyses condensation reactions
3. Catalyses formation of phosphodiester bonds between adjacent nucleotides

Question 4

Name the two scientists who proposed models of the chemical structure of DNA and of DNA replication.

Answer 4

Crick and Watson

Question 5

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

Answer 5

1. Weak hydrogen bonds between bases allow two strands to separate
2. Two strands, so both can act as templates
3. Complementary base pairing allows accurate replication

Question 6

ATP is an energy source used in many cell processes. Give two ways in which ATP is a suitable energy source for cells to use.

Answer 6

1. Releases relatively small amount of energy/ little energy is lost as heat;
2. Releases energy instantaneously
3. Phosphorylates other compounds, making them more reactive
4. Can be rapidly re-synthesised
5. Does not leave cells

Question 7

Describe how an enzyme can be phosphorylated.

Answer 7

1. Attachment/association of (inorganic) phosphate (to the enzyme)
2. (Released from) hydrolysis of ATP
   OR
   (Released from) ATP to ADP + Pi

Question 8

Explain five properties that make water important for organisms.

Answer 8

1. A metabolite in condensation/hydrolysis/ photosynthesis/respiration
2. A solvent so (metabolic) reactions can occur
   OR
   A solvent so allowing transport of substances
3. High specific heat capacity so buffers changes in temperature
4. Large latent heat of vaporisation so provides a cooling effect (through evaporation)
5. Cohesion (between water molecules) so supports columns of water (in plants)
6. Cohesion (between water molecules) so produces surface tension supporting (small) organisms

Question 9

Describe the roles of iron ions, sodium ions, and phosphate ions in cells.

Answer 9

Iron ions:
1. Haemoglobin binds/associates with oxygen OR
Haemoglobin transports/loads oxygen
Sodium ions:
2. Co-transport of glucose/amino acids (into cells)
3. (Because) sodium moved out by active transport/Na–K pump
4. Creates a sodium concentration/diffusion gradient
5. Affects osmosis/water potential
Phosphate ions:
6. Affects osmosis/water potential
7. Joins nucleotides/in phosphodiester bond/in backbone of DNA/RNA/in nucleotides
8. Used to produce ATP
9. Phosphorylates other compounds making them more reactive
10. Hydrophilic/water soluble part of phospholipid bilayer/membrane