The genetic code and mRNAAQA A-Level Biology: Subtopic test

1. 1
   Bacteria have been genetically modified to make human insulin by inserting the human insulin gene into their DNA. Mature insulin consists of two polypeptide chains: the A chain has 21 amino acids and the B chain has 30 amino acids.

   (a)

   Which feature of the genetic code allows a bacterium to make the correct amino acid sequence of human insulin from the human gene?

   [1 mark]

   • AIt is degenerate, so most amino acids are coded for by more than one triplet
   • BIt is non-overlapping, so each base is part of only one triplet
   • CIt is a doublet code, so two bases code for each amino acid
   • DIt is universal, so the same triplet codes for the same amino acid in all organisms

   (b)

   How many DNA bases are needed to code for the amino acids of the B chain, not counting any stop codon?

   [1 mark]

   • A30
   • B60
   • C90
   • D120

   (c)

   Explain why a code of two bases per amino acid could not code for all of the amino acids used to make proteins, and why a triplet code can.

   [2 marks]

   Total for question 1: 4 marks

2. 2
   A short region of mRNA has the base sequence AUG GCU UCC AAG UAA. Part of the genetic code is as follows: AUG codes for methionine (and starts translation); GCU and GCC both code for alanine; UCC and UCU both code for serine; AAG and AAA both code for lysine; UAA is a stop codon.

   (a)

   Which sequence of amino acids is coded for by this region of mRNA?

   [1 mark]

   • AMethionine, serine, alanine, lysine
   • BMethionine, alanine, serine, lysine
   • CAlanine, serine, lysine
   • DLysine, serine, alanine, methionine

   (b)

   A base substitution changes the codon GCU to GCC. What is the effect on the polypeptide?

   [1 mark]

   • AThere is no change to the amino acid sequence, because the genetic code is degenerate
   • BAlanine is replaced by serine
   • CThe polypeptide is shorter because a stop codon is made
   • DAll the following amino acids change

   (c)

   Give the base sequence of the DNA template strand that was transcribed to make the first two codons (AUG GCU), written in the same order.

   [2 marks]

   Total for question 2: 4 marks

3. 3
   A student is comparing the two types of RNA that take part in making a polypeptide from the genetic information in a gene: messenger RNA (mRNA) and transfer RNA (tRNA).

   (a)

   Describe the structure of a molecule of mRNA.

   [3 marks]

   (b)

   Describe the structure of a molecule of tRNA and explain how this structure enables it to take part in making a polypeptide.

   [4 marks]

   Total for question 3: 7 marks

4. 4
   A researcher made an artificial mRNA with the base sequence AUG UUU UUU GAA UAG and added it to a cell-free system that makes polypeptides. The polypeptide made was methionine, phenylalanine, phenylalanine, glutamic acid. She then used a second mRNA, AUG UUC UUC GAG UAG, and obtained the same polypeptide. The relevant codons are: AUG methionine (start); UUU and UUC both phenylalanine; GAA and GAG both glutamic acid; UAG stop. She is also preparing a report on how genetic information is stored and used in a cell.

   (a)

   Explain what the results of the two experiments show about the genetic code and how the base sequence of mRNA is related to the amino acid sequence of the polypeptide.

   [6 marks]

   (b)

   Describe the structure of tRNA, and deduce the mRNA codon recognised by a tRNA with the anticodon AAG and the amino acid it carries. Include in your answer what is meant by the genome and the proteome of a cell.

   [6 marks]

   Total for question 4: 12 marks

End of questions