dna application

Question 1

Sometimes, a mutagenic agent causes DNA to break. A different enzyme called ATM binds to the broken DNA. This leads to the activation of a protein coded for by a tumour suppressor gene. The effect of ATM binding is to stop cell division until DNA is repaired.

A mutation could result in a person having non-functional forms of the gene that produces ATM.

What can you predict about the possible effects of having a non-functional form of ATM?

Answer 1

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;
5. May have no effect in diploid / heterozygous (organism);
6. (Which) still has a functional ATM / ATM gene;

Question 1

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.

Answer 1

(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.

Question 2

Scientists determined that a sample of DNA contained 18% adenine.

What were the percentages of thymine and guanine in this sample of DNA?

Percentage of thymine

Percentage of guanine

Answer 2

Thymine 18 (%);

2. Guanine 32 (%).

Question 3

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

Answer 3

Decreases by 50%;

Per generation / per division;

Only accessible if linked to first marking point

OR

15N makes up ½ after 1 division;

Makes up ¼ after 2nd division;

Question 4

The change in the proportion of DNA containing 15N is due to the way in which DNA replicates. Explain how

Answer 4

In DNA replication strands separate;
Each acts as template (for formation of new strand);
One strand in each new molecule / semi-conservative replication;
New strands made using 14N.

2 max

Question 5

Cyclin D stimulates the phosphorylation of DNA polymerase, which activates the DNA polymerase.

Describe how an enzyme can be phosphorylated.

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Answer 5

Attachment/association of (inorganic) phosphate (to the enzyme);

For ‘phosphate/Pi’ accept PO43– and P in a circle.

Accept ‘phosphate goes to the enzyme’.

Ignore named bonding or position of phosphate attaching to enzyme but reject formation of E-S complex.

2. (Released from) hydrolysis of ATP

OR

(Released from) ATP to ADP + Pi;

Question 6

Some tumour cells contain higher than normal concentrations of cyclin D.

Use the graph above to suggest why higher than normal concentrations of cyclin D could result in a tumour.

Answer 6

Shortens interphase

OR

Cells begin DNA replication earlier OR

DNA replication (starts) faster;

Accept ‘starts mitosis earlier’.

2. Fast(er) cell cycle/division/multiplication/mitosis

OR

Uncontrolled cell division/mitosis;

Accept ‘(May result in) mutation in a tumour suppressor gene’ OR ‘(May result in) mutation in an oncogene’.

Ignore uncontrolled growth;

3. (Resulting in) a mass/group of abnormal/excessive cells;

Question 7

The nucleus and a chloroplast of a plant cell both contain DNA.

Give three ways in which the DNA in a chloroplast is different from DNA in the nucleus.

Answer 7

In chloroplasts

1.   DNA shorter;

Accept smaller

2.   Fewer genes;

3.   DNA circular not linear;

Accept DNA in a loop not linear

Accept no chromosomes (in chloroplast) unlike nucleus

4.   Not associated with protein/histones, unlike nuclear DNA;

Question 8

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

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

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2. the role of the DNA nucleotides. _

Answer 8

1.   Template;

2.   Determines order of nucleotides/bases;

Role of DNA nucleotides

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