[Maximum number: 1]

Organisms often release chemicals when attacked as part of their defence system. Scientists studied lima bean plants (Phaseolus lunatus) infested with either an armyworm, Spodoptera exigua, or a herbivorous mite, Tetranychus urticae. Both organisms feed on lima bean leaves, causing the leaves to release chemicals.

The study was conducted to see which defence chemicals were produced by lima bean leaves when infested by armyworms or herbivorous mites. The scientists identified a mixture of compounds ( C ) released by the plant when attacked. Two of the chemicals in this mixture were identified (C1 and C2).

The scientists hypothesized that the defence chemicals in C act as signals to produce other chemicals ( X, Y and Z ) that are also involved in the defence of the plant.

The graphs show the amounts of chemicals X, Y and Z produced when the plants were infested by either one of the two herbivores or treated with the different chemicals C1 or C2.

(a)

Identify the process that was used to amplify the targeted genes.

[ 1 ]

[Maximum number: 5]

A polymerase chain reaction (PCR) was performed to amplify a small amount of DNA. Eight tubes were prepared as shown in the table.

The tubes were placed in a thermal cycler with the temperatures shown in the diagram and run for 25 cycles.

The image shows the result of gel electrophoresis on the eight samples.

(a)

State the number of the tube used as a control without DNA.

[ 1 ]

(b)

Deduce, with a reason, whether the primers are smaller or larger than the amplified DNA.

[ 1 ]

(c)

Explain the reason for changing the temperature during each cycle.

[ 2 ]

(d)

Predict the result that would be obtained if fewer cycles were used in this PCR process.

[ 1 ]

[Maximum number: 1]

What is the function of the primers in the polymerase chain reaction (PCR)?

To separate the two strands of DNA

To bind to the complementary strands of DNA

To determine the region of DNA that will be copied

To activate the Taq DNA polymerase

[Maximum number: 1]

How is the sequence of DNA conserved?

Unwinding of the double helix by helicase during DNA replication

Separation of sister chromatids to opposite poles during mitosis

Transcription into complementary RNA for protein synthesis

Complementary base pair matching during DNA replication

[Maximum number: 1]

Which diagram(s) illustrate(s) semi-conservative replication of DNA?

X only

Y only

Z only

X and Y only

[Maximum number: 7]

Over 50 years ago, Meselson and Stahl investigated the mechanism of DNA replication. They transferred a rapidly growing population of Escherichia coli from a growth medium containing only ${ }^{15} \mathrm{~N}$ to a growth medium with only ${ }^{14} \mathrm{~N}$ . DNA samples were centrifuged at high speed in a salt density gradient. In the original published research, DNA molecules of the same density appear as a band in the UV absorption photographs as shown.

Generations

(a)

The density of the DNA band at generation 0 is 1.724 and the density of the dark band of DNA at generation 4.1 is 1.710. Estimate the density of the DNA band at generation 1.0.

[ 1 ]

(b)

Describe the nitrogen composition of the DNA band in the E. coli at generation 1.0.

[ 1 ]

(c)

Explain the pattern shown in generation 3.0.

[ 3 ]

(d)

This experiment was designed to demonstrate whether replication was semi-conservative or conservative. Distinguish between semi-conservative replication and conservative replication.

[ 2 ]

[Maximum number: 1]

Which sequence of DNA would be suitable in DNA profiling?

---ATTCGTGAATCAGCC---

---ATTCGTGAATTTGCC---

---ATTCGTGATTGCAGC---

---ATTCGTGATTCGTGA---

[Maximum number: 1]

What is the reason for Taq DNA polymerase being used in the polymerase chain reaction (PCR)?

It does not denature at high temperatures.

It produces Okazaki fragments more rapidly.

It allows translation to proceed rapidly.

It works efficiently with helicase in PCR.

[Maximum number: 1]

The diagram shows a gel electrophoresis apparatus.

What can be deduced from the diagram?

X is the smallest fragment.

Y is positively charged.

X is moving towards the cathode.

Y travels at the greatest rate.

[Maximum number: 1]

The DNA profile from a paternity test shows six profiles. Who is the child's father?
mother