DNA

Question 1

Sometimes, damage occurs during DNA replication. One enzyme involved in repairing damage to DNA is called ATR.

ATR works as follows.

*   ATR phosphorylates other enzymes involved in repairing DNA.

*   ATR also phosphorylates substrates required to repair DNA.

When ATR phosphorylates other enzymes, these enzymes become able to bind to their substrates.

Use your knowledge of enzyme structure to suggest why.

Answer 1

Changes tertiary structure of the enzyme;

2. (Enzyme) active site formed / able to be formed / active site becomes complementary;

Question 2

The enzyme-catalysed reactions activated by ATR only occur if the substrates have been phosphorylated.

Use your knowledge of energy changes in enzyme-catalysed reactions to suggest why

Answer 2

ATM will not bind to (broken) DNA;

2. DNA not repaired / cell still has broken DNA;
3. Cell division continues / tumour forms;
4. Tumour suppressor (gene) not effective / not activated;

Question 3

The enzyme-catalysed reactions activated by ATR only occur if the substrates have been phosphorylated.

Use your knowledge of energy changes in enzyme-catalysed reactions to suggest why

Answer 3

Phosphorylation / phosphate) makes substrates more reactive / raises their energy level(s) / lowers activation energy for the reaction;

Question 4

DNA is a polymer of nucleotides. Each nucleotide contains an organic base.

Explain how the organic bases help to stabilise the structure of DNA

Answer 4

Hydrogen bonds between the base pairs holds two strands together

2. Many hydrogen bonds provides strength

Question 5

The arrows in Figure 2 show the directions in which each new DNA strand is being produced.

(d) Use Figure 1, Figure 2 and your knowledge of enzyme action to explain why the arrows point in opposite directions.

for dna polymerase

Answer 5

Figure 1 shows) DNA has antiparallel strands / described;

2. (Figure 1 shows) shape of the nucleotides is different / nucleotides aligned differently;
3. Enzymes have active sites with specific shape;
4. Only substrates with complementary shape / only the 3’ end can bind with active site of enzyme / active site of DNA polymerase.

4

Question 6

A sample of epithelial tissue from the small intestine of an animal was analysed.
Some of the cells had 8.4 units of DNA, others had only 4.2 units.

(i) Use your knowledge of the cell cycle to explain why some cells had 8.4 units of DNA and others had only 4.2 units.

______________________________________________________________

______________________________________________________________

______________________________________________________________

______________________________________________________________

______________________________________________________________

(2)

(ii) How many units of DNA would you expect to be present in a gamete formed in this animal as a result of meiosis?

Answer 6

8.4 / cells with twice DNA content = replicated DNA / late interphase / prophase / metaphase / anaphase;

Any reference to interphase must suggest towards end of interphase.

‘Chromosomes replicate’ is not enough for DNA replicates.

2. 4.2 = DNA not replicated / (early) interphase / telophase / cell just divided / finished mitosis;

2

(ii) 2.1;

Question 7

Explain how a change in a sequence of DNA bases could result in a non-functional enzyme.

Answer 7

Change in (sequence of) amino acids / primary structure;

Change in hydrogen / ionic / disulfide bonds leads to change in tertiary structure / active site (of enzyme);

Substrate cannot bind / no enzyme-substrate complexes form;

Question 8

The bases in DNA nucleotides contain nitrogen.

Researchers grew bacteria on a medium containing 15N (‘heavy’ nitrogen) for several generations. They then transferred the bacteria to a medium containing 14N (‘ordinary’ nitrogen). They analysed DNA from the bacteria at three stages:

1. whilst the bacteria were growing on the 15N medium
2. after one division of the bacteria on the 14N medium
3. after two divisions of the bacteria on the 14N medium

The diagram shows their results.

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

Answer 8

Decreases by 50%;

Per generation / per division;

Question 9

The virus has single-stranded DNA as its genetic material. Explain the evidence from the table which suggests that the DNA is single-stranded.

Answer 9

amounts of A and T / C and G / complementary bases different;
therefore no base-pairing;

Question 10

A scientist replicated DNA in a test tube. To do this, he mixed an enzyme with identical single-stranded DNA fragments and a solution containing DNA nucleotides.

(b)  Name the enzyme used in this DNA replication.

___________________________________________________________________

(1)

(c)  Use your knowledge of semi-conservative replication of DNA to suggest:

1. the role of the single-stranded DNA fragments ____________________________

___________________________________________________________________

___________________________________________________________________

___________________________________________________________________

___________________________________________________________________

2. the role of the DNA nucleotides. _______________________________________

______________________________________________________

Answer 10

Role of single-stranded DNA fragments

1.   Template;

2.   Determines order of nucleotides/bases;

Role of DNA nucleotides

3.   Forms complementary pairs / A – T, G - C

OR

Forms complementary (DNA) strand;