DNA profiling Flashcards
Define a polymorphic site.
A point in the genome that s highly variable between individuals.
mtDNA is highly variable and good for DNA profiling. Give one other advantage of using it.
It is available in extremely high copy number per cell so even small specimens from a crime scene can be useful.
Why are Y-chromosomes helpful in sexual assault cases?
Only males carry the Y so if the victim is female her DNA will not contaminate the evidence.
Even though mtDNA and Y-chromosomes are useful in DNA profiling, using autosomes is preferred. Give 4 reasons why.
- Most of our genome is autosomal, thus there is more variation
- Autosomes are highly variable at some loci
- Each locus has 2 copies
- Most loci are un or weakly linked, meaning they assort independently. This creates further variation.
Is there recombination in mtDNA?
No.
Is there recombination of the Y-chromosome?
Very little.
What does VNTR stand for?
Variable number tandem repeats.
Define a VNTR.
A point in the genome where a short nucleotide sequence or ‘motif’ is repeated. The number of repeats vary between individuals.
A.R. Wyman and R. White discovered VNTRs in 1980. When were they first used in forensics?
1984, applied by Alec Jeffreys.
VNTRs come in 2 forms. What are they?
- Minisatellites
2. Microsatellites
Define a minisatellite.
The repeat motif is between 10-50 bp in length.
Define a microsatellite.
The repeat motif is between 2-5bp in length.
Mini and microsatellites are generated in the same way. True or false?
False.
What are satellite bands?
When DNA is centrifuged at high speed it separates according to density. Highly repetitive DNA forms satellite bands.
What solution is the DNA centrifuged in?
CsCl (caesium chloride)
Approx. how many minisatellites are there in the human genome?
1000.
Minisatellites are always different. True or false?
False - they often contain a core sequence that varies in one base position.
The number of VNTRs varies between parents and offspring. This high mutation rate causes variation in the form of what?
Alleles.
Where are most VNTRs found?
In introns (non-coding ‘junk’ DNA).
What can happen if a microsatellite is found in the coding region of a functional gene?
It can create genetic disease.
Give 2 examples of a genetic disease caused by a microsatellite mutation.
- Huntingdon’s disease
2. Fragile X syndrome
What are trinucleotide expansion mutations?
VNTRs containing 3 nucleotides responsible for numerous diseases.
What is characteristic about diseases caused by trinucleotide expansion mutations?
They appear with early onset and increase in severity down the generations.
Give 2 mechanisms by which VNTR numbers can be changed.
- Unequal crossing over
2. Slipped-strand mispairing
Explain unequal crossing over.
Because there are so many repeats of the same sequence the chromosomes line up in the wrong place and exchange unequal sections of chromosome in crossing over. Then each chromatid will end up with a different number of VNTRs.
Explain slipped-strand mispairing.
Because there are so many repeats the strands do not line up in the correct place. Then at DNA replication the enzymes may remove or add extra repeats.
What is unusual about slipped-strand mispairing?
Whole repeats, not nucleotides are removed/added. That is why only areas of the genome that can tolerate high levels of variability have VNTRs.
Why are minisatellites hard to genotype? Give 3 reasons.
- The repeat unit is longer
- There are more repeats than in microsatellites
- They extend over greater distances (up to a few kbp), meaning often they cannot be reliably PCRed.
How are VNTRs detected?
- Genomic DNA is cut with REs
- The fragments will differ in length as they have different VNTRs
- Southern blotting is done, using a VNTR probe for visualisation
What is a multilocus probe?
Many minisatellites have similar core sequences. They are located in different points around the genome but the same probe can be used to identify them.
What are single locus probes?
Those that are specific to a VNTR. The probe either contains sequence flanking the core region of the VNTR or a non-core repeat sequence.
You inherit your VNTRs from one parent. True or false?
False: you inherit some from each parent.
Microsatellites can be assayed with PCR to find out their length. How?
- Primers designed to anneal to sequences flanking the microsatellite region
- Thus PCR product length will be the length of the VNTR repeat number
- The PCR products can be visualised with dyes
- PCR product length can be easily measured with GeneScan (automated sequencers)
What is n(n+1)/2?
If there are ‘n’ possible numbers of VNTR, that is the number of possible genotypes.
What is (n(n+1)/2)x?
If there are ‘n’ possible numbers of VNTR, and ‘x’ is the number of different VNTRs, then that is the probability of 2 unrelated individuals having the same genotype for all VNTRs.
How are vagina and sperm cells separated in forensic anaylsis rape cases?
Sperm can withstand lysis in solution that causes vagina cells to burst.
What is the NDNAD?
The national DNA database. It contains ~7million profiles taken from individuals and crime scenes over England and Wales. Has produced a match in ~450,000 cases.
Define the Protection of Freedoms Act in 2012.
States rules on the retention and use of DNA to ensure human rights.
N.B. there are also bare laws bout using DNA in court
Give 4 other applications of DNA profiling.
- Paternity testing
- Ecological studies
- Population studies
- Linkage studies for disease/medicine
