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Nucleic Acids · Paper 2 — Free Biology SL Practice Questions

1MasterySAQ-SDNA replication and transcription5 marksPaper 2~8 min

A replication fork forms during DNA replication in a bacterium. - Rate of DNA polymerase III:

1000

nucleotides per second - Length of the bacterial chromosome:

4.6 \times 10^{6}

base pairs - Energy required to add one nucleotide to a growing strand:

2.0 \times 10^{-19}\,\text{J}

(a)

State the role of DNA helicase in DNA replication. [1 mark]

(b)

Calculate the minimum time, in seconds, required to replicate the entire bacterial chromosome. Assume replication begins at a single origin of replication and proceeds bidirectionally. [2 marks]

(c)

The total energy consumed during replication of the bacterial chromosome is

1.84 \times 10^{-12}\,\text{J}

. Explain why the total number of nucleotides added during replication is

9.2 \times 10^{6}

, and not

4.6 \times 10^{6}

. [2 marks]

Solutions

2MasterySAQ-SDNA replication and transcription5 marksPaper 2~8 min

A segment of double-stranded DNA has the following sequence:

5'\text{-ATGCCTAG-}3' \quad \text{(coding strand)}

3'\text{-TACGGATC-}5' \quad \text{(template strand)}

(a)

State the direction in which RNA polymerase synthesizes mRNA. [1 mark]

(b)

State one way in which the base composition of mRNA differs from that of the DNA template strand. [1 mark]

(c)

Write the complete mRNA sequence transcribed from the template strand, clearly labelling the

5'

and

3'

ends. Show your working. [3 marks]

Solutions

3ChallengeSAQ-LGene expression regulation7 marksPaper 2~11 min

A research team investigates regulation of the lac operon in E. coli under different environmental conditions. The relative rate of transcription of the lacZ gene (encoding

\beta

-galactosidase) is measured under three conditions, normalised so that Condition 1 =

0.1

. Condition — Glucose — Lactose — Relative transcription rate 1 — present — absent —

0.1

2 — absent — present —

100

3 — present —

2.0

(a)

State which condition produces the lowest transcription rate of lacZ and identify the molecule responsible for repressing transcription under that condition. [1 mark]

(b)

Calculate the fold-change in transcription rate between Condition 1 and Condition 2. [2 marks]

(c)

Explain the molecular mechanism that accounts for the difference in transcription rates between Condition 1 and Condition 2. [2 marks]

(d)

Evaluate the biological significance of the transcription rate observed in Condition 3 compared to Condition 2, with reference to catabolite repression and the efficiency of energy use in E. coli. [2 marks]

Solutions

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