Section 1 - Unit 2: Nucleic Acids

Question 1

Explain why new nucleotides can only be added in a 5’ to 3’ direction (4 marks)

Answer 1

• DNA polymerase
• Is specific
• And is only complementary to the 5’ end of the strand
• The shapes of 5’ end and 3’ end are different

Question 2

Describe the role of DNA polymerase in DNA replication (1 mark)

Answer 2

• Joins nucleotides (to form new strand)

Question 3

Other than being smaller, give two ways in which prokaryotic DNA is different from eukaryotic DNA (2 marks)

Answer 3

• Prokaryotic DNA is circular vs eukaryotic DNA is linear

- Prokaryotic DNA is not associated with proteins/histones vs eukaryotic DNA is

Question 4

Using your knowledge of DNA structure and function, explain how although humans and animals have very similar percentages of each base in their DNA, they are very different organisms (2 marks)

Answer 4

• Have different genes

- So bases are in a different sequence

Question 5

Explain how the events that take place during interphase and mitosis lead to the production of two genetically identical cells (4 marks)

Answer 5

• DNA replicated
• Due to complementary base-pairing
• Two sister chromatids are produced
• Each chromatid moves to opposite ends of the poles

Question 6

Explain how the structure of DNA is related to its functions (6 marks)

Answer 6

• Sugar-phosphate backbone provides strength/ protects hydrogen bonds
• Long, large molecule to store lots of information
• Double helix so compact
• Base sequence allows information to be stored
• Double stranded so replication can occur semi-conservatively
• Weak hydrogen bonds for strand separation
• Many hydrogen bonds for stability

Question 7

Explain why DNA helicase is important in DNA replication (2 marks)

Answer 7

• Separates the double helix

- So that nucleotides can attach

Question 8

Describe and explain how the structure of DNA results in accurate replication (4 marks)

Answer 8

Any four from:

• Two strands therefore semi-conservative replication is possible
• Hydrogen bonds holds strands together
• Hydrogen bonds are weak so strands can separate
• Base sequence acts as template
• A with T, C with G
• DNA is one parent and one new strand;

Question 9

Describe the role of the sequence of bases on one strand of DNA (1 mark)

Answer 9

• Determines sequence of amino acids

Question 10

Describe how the two strands of a DNA molecule are held together (1 mark)

Answer 10

• Hydrogen bonds

Question 11

Describe one advantage of DNA molecules having two strands (1 mark)

Answer 11

• Stability/Protects Bases/Replication

Question 12

Describe how, after the parent DNA strands separate, the second strand of DNA is formed (3 marks)

Answer 12

Any 3 from:

• Semi-conservative replication
• Complementary base pairing
• Hydrogen bonding of bases
• Condensation of nucleotides
• DNA polymerase involved

Question 13

Describe how the proportion of DNA that contained 15N changed at each division when bacteria were grown on the 14N medium (2 marks)

Answer 13

• Decreases by 50%

- At every generation

Question 14

Give two ways doctors could use base sequences to compare different types of viruses (2 marks)

Answer 14

• Compare DNA base sequence

- Compare mRNA base sequence

Question 15

Explain why the replication of DNA is described as semi-conservative (2 marks)

Answer 15

• Each strand acts as a template

- DNA is composed of one new strand and one original/parent strand

Question 16

Explain how DNA replicates (4 marks)

Answer 16

• Hydrogen bonds broken
• Semi-conservative replication
• Nucleotides line up - specific base pairing
• DNA polymerase

Question 17

Define homologous chromosomes (3 marks)

Answer 17

• Two chromosomes
• With same genes
• At same loci

Question 18

Describe how ATP is resynthesised in cells (2 marks)

Answer 18

• From ADP and phosphate
• By ATP synthase
• During respiration/photosynthesis

Question 19

Give two ways in which the hydrolysis of ATP is used in cells (2 marks)

Answer 19

• To provide energy for other reactions

- To add phosphate to other substances and make them more reactive/ change their shape

Question 20

Explain some uses of phosphate in the body (5 marks)

Answer 20

• Used to make RNA
• Used to make DNA
• Used to make ATP/ADP
• Used to make membranes
• Used for phosphorylation

Question 21

Give two ways in which ATP is a suitable energy source for cells to use (2 marks)

Answer 21

• Releases relatively small amount of energy

- Releases energy instantaneously

Question 22

Explain why ATP is useful in many biological processes (4 marks)

Answer 22

• Releases energy in small amounts
• Broken down in a single step/energy available rapidly
• Adds phosphate to substances and makes them more reactive/lowers their Ae
• Can be reformed/made again

Question 23

Explain why it is important for plants to produce ATP during respiration in addition to during photosynthesis (5 marks)

Answer 23

• In the dark no ATP production in photosynthesis
• So some tissues are unable to photosynthesise/produce ATP
• ATP cannot be moved from cell to cell/stored
• Plant uses more ATP than produced in photosynthesis
• ATP for active transport

Question 24

Write a simple equation to show how ATP is synthesised from ADP (1 mark)

Answer 24

• ADP + Pi → ATP

Question 25

Explain why it is necessary for humans to synthesise such a large amount of ATP (3 marks)

Answer 25

- ATP is unstable - ATP cannot be stored / is an immediate source of energy - Required in active transport - ATP only releases a small amount of energy at a time

Question 26

Give two advantages of ATP as an energy storage molecule within a cell (2 marks)

Answer 26

- Cannot pass out of cell - Quickly broken down - Releases small amounts of energy

Question 27

State what happens to the energy which is not incorporated into ATP when glucose is respired (1 mark)

Answer 27

- It is released as heat

Question 28

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

Answer 28

- Joins (adjacent DNA) nucleotides - Catalyses condensation reactions - Catalyses formation of phosphodiester bonds

Question 29

Describe how an enzyme can be phosphorylated

Answer 29

- Attachment of Pi to enzyme | - Which comes from hydrolysis of ATP