DNA, the genetic code and protein synthesis

Sample questions

Original practice questions written in the style of the exam.

1. Question 15 marks

   Describe the process of transcription.

   Mark scheme

   • Hydrogen bonds between bases break and the DNA unwinds / separates at the gene (1)
   • One strand is used as the template (antisense strand) (1)
   • Free RNA nucleotides pair with complementary bases on the template, with uracil pairing with adenine (1)
   • RNA polymerase joins the nucleotides with phosphodiester bonds (1)
   • The mRNA leaves the nucleus through a nuclear pore (1)
2. Question 23 marks

   Bacteria were grown for many generations in a medium containing only heavy nitrogen ($^{15}\text{N}$). They were then transferred to a medium containing only light nitrogen ($^{14}\text{N}$) and allowed to divide once. Their DNA was extracted and centrifuged.

   Explain how the result after one division supports semi-conservative replication and not conservative replication.

   Mark scheme

   • All the DNA formed a single band at an intermediate density (1)
   • Because each molecule contains one original ($^{15}\text{N}$) strand and one new ($^{14}\text{N}$) strand (1)
   • Conservative replication would give two bands, one heavy and one light (1)