Using gene sequencing and PCREdexcel A-Level Biology B: Subtopic test

1. 1
   A forensic laboratory receives a swab from a crime scene that contains only a few cells' worth of DNA. Technicians mix the DNA with primers, free nucleotides, a heat-stable DNA polymerase and buffer, and place the tubes in a thermal cycler that repeatedly heats and cools the mixture.

   (a)

   Which enzyme is most suitable for the polymerase chain reaction in the thermal cycler?

   [1 mark]

   • ADNA ligase
   • BTaq DNA polymerase
   • CA restriction endonuclease
   • DRNA polymerase

   (b)

   The mixture is first heated to about 95 °C. What is the purpose of this step?

   [1 mark]

   • ATo join the primers to the DNA
   • BTo add nucleotides to the new strands
   • CTo break the hydrogen bonds between the two DNA strands
   • DTo break the phosphodiester bonds in the sugar-phosphate backbone

   (c)

   Explain why primers are needed in the reaction mixture.

   [2 marks]

   Total for question 1: 4 marks

2. 2
   A baby has symptoms that suggest sickle cell disease. Doctors amplify and sequence part of the baby's gene for the beta chain of haemoglobin. In the healthy allele the template-strand triplet is CTC, which codes for glutamic acid. In the baby's allele the same triplet reads CAC.

   (a)

   Which statement best describes the genome of a person?

   [1 mark]

   • AThe complete set of DNA in the cells of that person
   • BAll the proteins that the person can make
   • CAll the alleles of one gene in a population
   • DThe genes that are expressed in one tissue

   (b)

   What is the mRNA codon transcribed from the template-strand triplet CAC?

   [1 mark]

   • ACAC
   • BGTG
   • CCUC
   • DGUG

   (c)

   Explain how the sequence of the baby's gene allows doctors to link the baby's symptoms to a genetically determined condition.

   [2 marks]

   Total for question 2: 4 marks

3. 3
   Police recover a bloodstain at a burglary that contains a very small amount of DNA. A forensic scientist amplifies several short tandem repeat (STR) regions of this DNA and produces a DNA profile from the sample. She compares it with DNA profiles from three suspects.

   (a)

   Explain why the DNA is amplified by PCR before the profile is produced.

   [3 marks]

   (b)

   Explain how the profile from the bloodstain can be used to identify a suspect.

   [4 marks]

   Total for question 3: 7 marks

4. 4
   A clinic is asked to test paternity of a child. DNA profiles are produced for the mother, the child and two men, X and Y. Each profile is a pattern of bands numbered 1 to 7 from the largest fragment to the smallest. Mother: bands 1, 2 and 5. Child: bands 1, 2, 3 and 4. Man X: bands 3, 4 and 6. Man Y: bands 2, 5 and 7. The clinic also amplifies and sequences the child's CFTR gene to test for the allele that causes cystic fibrosis.

   (a)

   Describe how DNA profiles are produced and use the information to deduce which man is the father of the child.

   [6 marks]

   (b)

   Explain how the child's CFTR gene can be amplified and sequenced, and how the result can be used to link the child to cystic fibrosis.

   [6 marks]

   Total for question 4: 12 marks

End of questions