3.8.4.3 - GENETIC FINGERPRINTING

Question 1

short tandem repeats (STRs) are short sequences of DNA, usually 2 to 7 base pairs. STR are repeated a number of times, one after another. for example, the STR D5S818 is made up of AGAT repeated 7 to 16 times.

the repeated sequences in STRs are common to all humans. due to variation in the number of repeats, STRs can be used in genetic fingerprinting. during genetic fingerprinting, the different STRs are separated by gel electrophoresis.

give two features of STRs which enable them to be separated by gel electrophoresis (2)

Answer 1

1. number of nucleotides/bases OR length
2. negative charge

Question 2

explain how genetic fingerprinting may be carried out on a sample and how it can be used to identify a person (6)

Answer 2

1. DNA is cut using restriction endonucleases
2. polymerase chain reaction used to amplify the fragments
3. use electrophoresis
4. separates according to length/mass/size
5. DNA made single-stranded
6. transfer to nylon membrane
7. apply probe
8. radioactive/fluorescent marker
9. detected by autoradiography/UV light
10. reference to VNTRs
11. pattern unique to every individual

Question 3

suggest two advantages of using faeces, rather than blood samples, to obtain DNA from pandas (2)

Answer 3

1. taking samples from animals may cause stress/injury to animal
2. difficult to find animals
3. pandas are dangerous to humans

Question 4

explain how DNA could be present on a toothbrush (2)

Answer 4

1. cells in toothbrush
2. DNA present in cell

Question 5

explain why the PCR was used on the sample of DNA from the toothbrush (2)

Answer 5

1. toothbrush gives a small sample of DNA (need more)
2. for analysis
3. PCR creates many copies

Question 6

explain one way in which the PCR differs from DNA replication in a cell (2)

Answer 6

ANY PAIR:

1. uses heat
2. to separate strands

OR

1. PCR replicates pieces of DNA
2. because DNA has been cut

OR

1. primer added in PCR
2. to initiate replication

Question 7

tests for use in criminal cases often take much longer because samples are very small or contaminated.

explain why it takes longer to obtain a genetic fingerprint if the sample is:

1. very small (1)
2. contaminated (1)

Answer 7

1. PCR needed to amplify the sample
2. other DNA will be present, so need to identify the ‘required’ DNA from the rest

Question 8

the length of the replicated DNA fragment is different for each species.

explain why this is important in identifying the shark species involved (3)

Answer 8

1. electrophoresis separates DNA
2. so they can be identified by their position on the gel
3. smallest/shorted fragments travel furthest/quicker