Question 1

Paper 2Medium16 marks

A short region of a gene is transcribed in a cell. The DNA template strand is shown in the direction in which it is read. Spaces separate groups of three bases.

DNA template: 3′–TAC GGA CTT ACG–5′

(a)

Compare DNA and RNA with respect to their number of strands, nitrogenous bases and pentose sugar. Include the structural difference between the sugars.

[5]
(b)

Determine the mRNA sequence produced, writing it in the 5′ to 3′ direction.

[4]
(c)

During translation, the second codon of the mRNA is read at a ribosome. For parts (i) and (ii), use the codon 5′–CCU–3′, even if your answer to part (b) differs.

(i)

Write the tRNA anticodon that pairs with this codon, in the 3′ to 5′ direction.

[1]
(ii)

Explain how this pairing contributes to the formation of a polypeptide.

[2]
(d)

Before a cell divides, its DNA is copied. Explain how complementary base pairing enables this copying.

[4]

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