Using gene sequencing and PCREdexcel A-Level Biology B: Mind map

Exam questions on Using gene sequencing and PCR

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.
   Explain why primers are needed in the reaction mixture.2 marks
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.
   Explain how the sequence of the baby's gene allows doctors to link the baby's symptoms to a genetically determined condition.2 marks
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.
   Explain why the DNA is amplified by PCR before the profile is produced.3 marks