21.1 Summary Question & Exam Style Questions

Question 1

What is an intron? [3]

Answer 1

An intron is a large, non-coding (1); region of DNA (1); that is removed from messenger RNA before
it is translated into a polypeptide chain (1).

Question 2

State the purpose of the polymerase chain reaction and explain how it has advanced DNA profiling? [4]

Answer 2

Polymerase chain reaction (PCR) is a process by which a small piece of DNA is amplified/replicated
many times (1); need relatively large DNA sample for DNA profile/sequencing (1); in forensic
cases/criminal investigations samples available are often extremely small (1); PCR amplifies samples
so DNA profiling can be carried out (1).

Question 3

Discuss the benefits and limitations of DNA profiling. [6]

Answer 3

Benefits: can be used in criminal cases to prove guilt or innocence (1); tiny amounts of DNA can be
used (1); DNA lasts a long time so ‘cold’ cases can be revived by DNA evidence (1); can be used to
prove paternity / to prove or disprove familial relationships in immigration cases (1); can be used to
identify species / can be used to find evolutionary relationships (1). Limitations: can be too dependent
on it and ignore other evidence in criminal cases (1); DNA profiles can be done at different levels and
mistakes can be made (1); contamination of samples with DNA from other organisms (1). Any other
sensible points (6 max, up to 3 marks for benefits and up to 3 marks for limitations).

Question 4

Produce a flow diagram to show the main stages of the modern DNA profiling process. [6]

Answer 4

Variety of ways students might do this but must include: extract DNA from sample → PCR to amplify
DNA if necessary → use restriction endonucleases to cut DNA sample into fragments at recognition
sites → place fragments in wells of agarose gel electrophoresis plate with buffer, identifying fragments
→ pass electric current through plate so fragments move dependent on size and charge → transfer
gel plate to alkaline buffer to denature DNA fragments → carry out Southern blotting by placing nylon
filter of nitrocellulose paper over gel and using absorbent material to draw fluid through leaving DNA
fragments on filter → fix fragments using UV light → add excess of radioactive or fluorescent gene
probes to hybridise with fragments → wash off excess probes → use X-ray photography or UV light to
show up bands of DNA profile (max 6).