DNA profilling, PCR & Gel Electrophoresis Flashcards
What can DNA profiling also be known as?
Genetic fingerprinting and DNA fingerprinting
What does DNA profiling rely on?
Everyone’s DNA being unique
Describe the features of STR sequences
. Can contain 2–50 base pairs
. Short DNA sequences can be repeated many times.
Describe what is special about the location of STR sequences
- 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
Why are STR sequences analysed for a DNA profile?
- 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
Making the DNA profile:
State some possible sources of DNA for forensic purposes
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
Outline how DNA is extracted from cells
- 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
How are restriction enzymes used in the original DNA profiling procedure?
To cut out the STR sequences from the DNA sample
Describe how they identify where to cut DNA
They cut at a specific recognition site, at a specific basic sequence 4 to 6 base pairs long
Why were restriction enzymes so useful in producing a DNA profile?
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
State the technique used to copy STR sequences in modern DNA profiling
Polymerase Chain reaction
State the technique used to separate the STR sequence fragments
Gel electrophoresis
What are the two techniques used to visualise fragments?
Southern blotting and fluorescent primers
Describe the technique of southern plotting
- Transfer onto Nilan membrane
- Membrane and incubated with DNA probe which complimentary base pass to the STR fragments (hybridisation)
Describe the process of fluorescent primers for visualisation of fragments
Fluorescent primers in PCR remain attached to DNA after last cycle and passed down gel to be detected and analysed by a computer
State possible uses of DNA profiling
- 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
How was a conclusion reached from the profile?
- 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
Describe why DNA profiling is not infalliable ( not never wrong )
- 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
What is PCI used for?
Amplification of DNA/making more copies of DNA
What is gel electrophoresis used for?
Separation of different sized fragments of DNA
Describe how and why DNA moves through the gel and gel electeropheresis
- 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
