1. 1
   A biology textbook explains that chromosomes are made of DNA, that a gene is a length of DNA coding for a specific protein, and that different versions of the same gene, called alleles, can lead to different characteristics.

   (a)

   A biology textbook states that chromosomes are made of DNA, which contains genetic information organised into functional units. What is a gene?

   [1 mark]

   • AA length of DNA that codes for a protein
   • BAn alternative form of a chromosome
   • CA junction between two neurones
   • DA type of protein found only in the nucleus

   (b)

   Two students both have a gene for eye colour, but one has brown eyes and the other has blue eyes because they carry different versions of that gene. What term describes an alternative form of a gene?

   [1 mark]

   • AA chromosome
   • BAn allele
   • CA ribosome
   • DAn enzyme

   (c)

   Explain how different alleles of the same gene can lead to different observable characteristics in an organism.

   [2 marks]

   Total for question 1: 4 marks

2. 2
   A scientist studies how a gene for a particular enzyme is used to make that enzyme, tracing the process from the gene remaining in the nucleus to the assembly of the finished protein by ribosomes in the cytoplasm.

   (a)

   A scientist studies how the gene for an enzyme is used to make that enzyme inside a cell. The gene itself remains in the nucleus throughout this process. Which molecule carries a copy of the gene's information from the nucleus to the cytoplasm?

   [1 mark]

   • AAn amino acid
   • BA ribosome
   • CA chromosome
   • DMessenger RNA (mRNA)

   (b)

   Once mRNA has moved into the cytoplasm, it passes through a structure that assembles amino acids into the correct sequence to form the protein. What is this structure called?

   [1 mark]

   • AThe nucleus
   • BThe vacuole
   • CThe ribosome
   • DThe cell wall

   (c)

   Describe how the sequence of bases in mRNA determines the sequence of amino acids in the protein that is made.

   [2 marks]

   Total for question 2: 4 marks

3. 3
   A student is investigating how a mutation in a gene can lead to a non-functional protein being produced, and a scientist explains that not every gene in a body cell is actually used to make a protein at any given time, even though every cell contains the same genes.

   (a)

   A student is confused about why a mutation, a random change in the base sequence of a gene, could change the protein that the gene codes for. Explain the connection between the sequence of bases in a gene and the shape and function of the protein it produces.

   [3 marks]

   (b)

   The scientist explains that although a liver cell and a muscle cell in the same person contain the same genes, they produce very different sets of proteins. Explain why cells containing the same genes can produce different proteins and therefore have different specialised functions.

   [4 marks]

   Total for question 3: 7 marks

4. 4
   A researcher discovers that a particular inherited disorder is caused by a single-base mutation in a gene coding for an enzyme, resulting in a non-functional enzyme, and later investigates a different mutation in a gene coding for a membrane carrier protein instead of an enzyme.

   (a)

   The enzyme is involved in an important metabolic reaction, and patients with the mutated gene produce a version of the enzyme that cannot function properly. Explain, tracing the process from the gene through to the effect on the cell, how a single-base mutation could lead to a non-functional enzyme and disrupt the metabolic reaction it normally catalyses.

   [6 marks]

   (b)

   The researcher also investigates a different mutation, this time in a gene that codes for a membrane carrier protein rather than an enzyme. Patients with this mutation are found to be unable to take up a particular ion by active transport in their kidney cells. Explain, using your understanding of how DNA controls the production of proteins, why a mutation in this different gene could specifically affect the function of a membrane carrier protein rather than an enzyme, and why this could prevent active transport of the ion from occurring.

   [6 marks]

   Total for question 4: 12 marks

End of questions