BIOL114 - Lecture 12 - Genes and Identify

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  • Created on: 17-03-16 17:57
List the key features that make DNA profiling a useful forensic tool
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Describe how to make a DNA fingerprint
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Differentiate between DNA fingerprinting and DNA profiling
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What applications come to mind when thinking of DNA testing?
Forensic analysis, paternity analysis, missing person/disaster victims, gnealogy and deep ancestory, population migration patterns.
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What are the goals of forensic science?
To unambiguously match biological specimens left at the scene of a crime to specific individuals.
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What type of specimens could be used?
Blood, semen or hair. It is important to match with 100% accuracy to confirm a guilty verdict. Can also eliminate innocent suspects.
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What does genetic identification require?
Variable DNA
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What are the types of variable DNA? These differ largely between individuals
Mnisatellites, microsatellites, single nucleotide polymorphisms
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What are minisatellites known as?
Variable number tandem repeats - VNTRs
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What are microsatellites known as?
Short tandem repeats - STRs
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Who discovered minisatellites?
Alec Jeffreys - at Leicester Uiversity in 1985
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What did the discovery of Minisatellites lead to?
The development of DNA fingerprinting
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Where is each minisatellite located?
At a locus on a particular chromosome.
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Are minisatellites inherited?
Yes - in Mendelian fashion - one from each parent
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Does the number of repeats at a locus stay the same between individuals?
No - the variants are known as alleles
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How can we use minisatellites in analysis?
The original way of anaylsisng these was to extract DNA from an individual or a scene of crime sample and digest this with a restriction enzyme on either side of the sequence. This releases the minisatellite from the genomic DNA.
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Different minisatellites share what?
A core sequence
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What can the core be used as?
As a probe to detect multiple VNTRs simultaneously (Multilocus probe)
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What is measured
The difference is in te repeat number at each locus - this results in a unique DNA fingerprint
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What is a DNA fingerprint
A complex pattern of bands
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How can DNA fingerprints be used in paternity analysis??
You compare the bands of both the parents to the child and look for an overlap
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What are the limitations of multi-locus fingerprinting method?
Bands produced cannot be assigned to any particular locus, DNA fingerprint not readily amenable to statistical analysis which hampered its use in court. You cannot work out the probability of a match happening by chance.
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In order to work out how likely it is due to chance, what do you need to know?
You need to know the probability of each pair of alleles occurring in a population.
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What are the disadvantages to southern blotting technique?
You need to have sufficient biological material (typically DNA content of 1000 cells) It is a multistep procedure - isolate DNA, digest DNA, run DNA on gel, soutern blot, probe membrane, identify band. Good quality DNA required.
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Why is PCR better in forensics?
You need little starting material, starting material doesn't need to be of high quality (works best with STRs), it is simple and quick, and individual loci are analysed - allele frequencyy and statistical data available.
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What are STRs?
Microsatellites, the repeat sequence is only a few nucleotides in length. These are scattered like minisatellites throughout the human genome in both coding and non coding regions at known loci. STRs typically not as hypervariable as VNTRs.
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In PCR amplification of VNTRs/STR why do you use a polyacrylamide gel, rather than an agarose gel?
Because the difference between the chromosomes could be as small as 2 nucleotides
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Typically in Britain, how many minisatellites are analysed simultaneously?
10, it is a larger number in other countries.
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What is multiplex PCR?
Where several STRs can be analysed simultaneously using fluorescent dye labelled primers
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Where does the STR analysis go?
It can easily be stored on a database. It is populated with DNA samples from crime scenes.
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What are the ethical issues associated with the DNA database?
Until recently, all DNA samples were taken during criminal inquiries, even those of innocent individuals. The 2012 freedom acts have outlawed retention of samples of these people now
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It is also possible to analyse Y chromosome, why would this be useful?
If there is a small amount of male DNA being mixed with a large amount of female DNA. E.g in semen from vasectomised male, touch DNA on skin or clothings, male skin cells beneath female nails. Y STRS inherited together.
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What is the difficulty associated with Y chromosomes
No recombination of Y chromosomes, so all are inherited as a block together, so you can't calcualte indiviudal and therefore you have to looks at a population of men and determine how often this would occur.
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The other type of DNA that can be used in forensic analysis is...
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Why is mitochondrial DNA good?
Because it is useful in old or degraded samples (resistant to degregation) -useful looking at disaster victims. This is because the nucleus is enclosed and small
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What are the issues with mtDNA analysis?
It is expensive and slow. It doesn't have STRs, but Hypervariable regions.
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The future of forensic analysis -
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If there is no suspect, what can you determine from DNA?
Ethnic origin (by SNP profile), physical characteristics (specific SNPs), Surname (if male - Y chromosome SNPs and STRs), and face prediction
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How is ethnic origin determined from DNA?
There are particular SNPs that associate with different origins.
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Is it possible to identify clusters of SNPs according to geographical location?
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How can surname be determined from DNA?
Men with the same (less common) surname tend to share Y chromosome markers. E.g most sykes men share a common ancestor and Y chromosome. Exceptions arise through non paternity.
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How can you predict face shape from SNPs in 12 genes?
There is a connection between face morphology and SNPs. A DNA smample could be taken from a blood sample and this information could be used to build up a picture of the individual. (This can't be done yet - but could help identify suspects.).
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Card 2


Describe how to make a DNA fingerprint



Card 3


Differentiate between DNA fingerprinting and DNA profiling


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Card 4


What applications come to mind when thinking of DNA testing?


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Card 5


What are the goals of forensic science?


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