DNA profilling, PCR & Gel Electrophoresis

Question 1

What can DNA profiling also be known as?

Answer 1

Genetic fingerprinting and DNA fingerprinting

Question 2

What does DNA profiling rely on?

Answer 2

Everyone’s DNA being unique

Question 3

Describe the features of STR sequences

Answer 3

. Can contain 2–50 base pairs
. Short DNA sequences can be repeated many times.

Question 4

Describe what is special about the location of STR sequences

Answer 4

• The same STR occur at the same local on both chromosomes of a homologous pair
• However the number of times the sequence is repeated on each of the homologous chromosome differs

Question 5

Why are STR sequences analysed for a DNA profile?

Answer 5

• Each person has a large number of Intron with lots of STR Loki
• as there is large amounts of variation in the number of repeats for each STR sequence, it is unlikely that two individuals will have the same number of repeats at every STR locus in their DNA
• The profile will be unique for each individual

Question 6

Making the DNA profile:
State some possible sources of DNA for forensic purposes

Answer 6

Any biological tissue from plants or animals can be used
- cells from cheek
-White blood cells from blood smear
- Bone marrow from skeleton
- Sperm from sexual assault

Question 7

Outline how DNA is extracted from cells

Answer 7

• cells are treated with detergent and salts
• Disrupting cell membranes and releasing DNA
• dna separated from rest of cellular components by filtering or centrifugation
• Incubated with proteins to remove proteins from DNA
• DNA is precipitated out and washed by using ice cold ethanol

Question 8

How are restriction enzymes used in the original DNA profiling procedure?

Answer 8

To cut out the STR sequences from the DNA sample

Question 9

Describe how they identify where to cut DNA

Answer 9

They cut at a specific recognition site, at a specific basic sequence 4 to 6 base pairs long

Question 10

Why were restriction enzymes so useful in producing a DNA profile?

Answer 10

If same restriction enzyme is used to cut the same DNA sample or two DNA samples from the same individual, the fragments will be identical in every case

Question 11

State the technique used to copy STR sequences in modern DNA profiling

Answer 11

Polymerase Chain reaction

Question 12

State the technique used to separate the STR sequence fragments

Answer 12

Gel electrophoresis

Question 13

What are the two techniques used to visualise fragments?

Answer 13

Southern blotting and fluorescent primers

Question 14

Describe the technique of southern plotting

Answer 14

• Transfer onto Nilan membrane
• Membrane and incubated with DNA probe which complimentary base pass to the STR fragments (hybridisation)

Question 15

Describe the process of fluorescent primers for visualisation of fragments

Answer 15

Fluorescent primers in PCR remain attached to DNA after last cycle and passed down gel to be detected and analysed by a computer

Question 16

State possible uses of DNA profiling

Answer 16

• prosecution of a suspect in a murder case
• Checking variation within a species
• Identification of a body
• Paternity testing
• Genetic screening for diseases
• Identifying evolutionary relationships between species

Question 17

How was a conclusion reached from the profile?

Answer 17

• each STR fragment shows up as a band on the gel
• Compare the bands in the test sample and reference sample
• a match is confirmed if the same number of bands are seen in exactly the same positions

Question 18

Describe why DNA profiling is not infalliable ( not never wrong )

Answer 18

• The profiling procedure has many steps with cross contamination could occur
• Only some STR sequences are analysed and not entire DNA sequence of an individual
• there is a chance that another individual has an identical profile
  Closely closely related individuals could have related profiles

Question 19

What is PCI used for?

Answer 19

Amplification of DNA/making more copies of DNA

Question 20

What is gel electrophoresis used for?

Answer 20

Separation of different sized fragments of DNA

Question 21

Describe how and why DNA moves through the gel and gel electeropheresis

Answer 21

• A potential difference is generated across gel between two electrodes
• The cathode(negative) is where Wells are, the anode (positive) is the other end
• dna fragments move through gel based on their charge and separated based on their size (small fragments can move more easily through the gel and move faster and further through the gel)
• DNA is negatively charged to move towards the positive anode
• A reference sample with fragments of known lengths may be added to the gel to allow for comparison