DNA, the genetic code and gene expression: exam questions

Answer: B – Hydrogen bonds link complementary bases across the two strands; phosphodiester bonds link the sugar and phosphate of adjacent nucleotides along each strand.

• two polynucleotide strands wound in a double helix (1)
• each strand has a sugar–phosphate backbone, of deoxyribose and phosphate linked by phosphodiester bonds (1)
• bases A, T, C, G; A pairs with T, C pairs with G (complementary base pairing) (1)
• held by hydrogen bonds between the bases (1)
• strands are antiparallel (1)
• Max 4

• DNA unwinds / helicase breaks the hydrogen bonds between the strands (1)
• each strand acts as a template (1)
• free nucleotides pair with complementary bases on the template (A–T, C–G) (1)
• DNA polymerase joins the nucleotides (1)
• forming phosphodiester bonds (between adjacent nucleotides) (1)
• semi-conservative: each new molecule has one old strand and one new strand (1)
• Max 5

• after 1 generation, all DNA is intermediate density, so each molecule has one ¹⁵N strand and one ¹⁴N strand (1)
• conservative model predicts one heavy band and one light band after 1 generation, which was not seen, so it is refuted (1)
• after 2 generations, light DNA (both strands ¹⁴N) and intermediate DNA (one strand ¹⁴N and one ¹⁵N) are present (1)
• semi-conservative model: the old strands (¹⁵N) each pair with new ¹⁴N strands, and the new strands then act as templates producing ¹⁴N/¹⁴N molecules (1)
• each original ¹⁵N strand is conserved in a new molecule (1)
• Max 5

• mRNA is complementary to the template strand: first two codons AUG GAA (1)
• last two codons AAA UGA (1)
• methionine – glutamic acid – lysine (1)
• UGA is a stop codon, so there is no further amino acid (1)
• If mRNA is wrong, allow one mark for the correct sequence of amino acids from a wrong mRNA sequence (error carried forward).

Answer: B – Degenerate means more than one codon can code for the same amino acid.

• genetic code is degenerate / the new codon may code for the same amino acid (1)
• so a substitution changes at most one codon / one amino acid (1)
• deletion shifts the reading frame / frameshift (1)
• all the codons after the deletion change, so many amino acids differ / the protein's primary and tertiary structure change (1)
• and so the protein may not function (1)
• Max 4

• both are single-stranded polynucleotides / contain ribose and uracil (1)
• mRNA is a long linear strand with codons; tRNA is a short folded molecule with an anticodon (1)
• mRNA carries the code from DNA to the ribosome; tRNA brings amino acids to the ribosome (1)
• tRNA has an amino acid binding site / has hydrogen-bonded base pairs within the molecule; mRNA does not (1)
• Max 3