Genetic fingerprinting

Question 1

Variable number tandem repeats (VNTRs)

Answer 1

Repeating sequences of nucleotides/bases (eg. GATA) found within non-coding sections of DNA at
many sites throughout the genome
● Allow production of a virtually unique genetic fingerprint:
○ Probability of two individuals having the same VNTRs is very low
○ Because an organism’s genome contains many VNTRs and lengths differ between individuals

Question 2

The technique of genetic fingerprinting in analysing DNA fragments (VNTRs)

Answer 2

1. Amplification
   ● Extract DNA from sample eg. blood / cheek cells
   ● Amplify by the polymerase chain reaction (PCR)
2. Digestion
   ● Use restriction endonucleases to cut DNA into smaller fragments
   ● Which cut at specific sites near VNTRs, leaving VNTRs intact
3. Separation
   ● Separate DNA/VNTRs by gel electrophoresis
   ● According to length → shorter VNTRs travel further
4. Hybridisation
   ● Transfer from gel to nylon membrane (Southern blotting)
   ● Treat DNA to separate strands / make single-stranded (eg. add alkaline)
   ● Apply DNA probes complementary to VNTRs with a radioactive / fluorescent label
5. Development
   ● Use autoradiography (place on X ray film) or use UV light to visualise banding patterns

Question 3

Principles of gel electrophoresis - separating nucleic acid (DNA/RNA) fragments (or proteins) by size

Answer 3

1. Load DNA samples into wells in porous gel and
   cover in buffer solution (conducts electricity)
2. Pass electrical current through → DNA is
   negatively charged so moves towards positive
   electrode
3. Shorter DNA fragments travel faster so travel
   further, ie. separated according to size

Note: Proteins can also be separated by electrophoresis according to mass / length of polypeptide chain, and charge as variable R groups differ

Question 4

Use of genetic fingerprinting in Determining genetic relationships

Answer 4

● More closely related → more similar VNTRs → more similarities in genetic fingerprints
● Paternity testing - father should share around 50% of VNTRs/bands and ones that don’t come from
mother must be from father

Question 5

Use of genetic fingerprinting in Determining genetic variability within a population

Answer 5

Differences in VNTRs arise from mutations, more differences show greater diversity within a population

Question 6

Use of genetic fingerprinting in Forensic science

Answer 6

● Compare genetic fingerprint of suspects to DNA at crime scene
● If many bands match, person is linked to a crime scene / likely to be present

Question 7

Interpreting data showing results of gel electrophoresis

Answer 7

● Run a DNA standard under same conditions - DNA fragments of known lengths
● Compare to position of unknown fragments