Flashcards in DNA Profiling Deck (25)
Who first discovered DNA profiling?
What was the first use of DNA profiling?
Conviction of Colin Pitchfork, first mass DNA screening
What is DNA?
Unique to each person (except identical twins)
Nucleotides- base, sugar, phosphate- C, A, G, T
Genetic code- provides information, which produces proteins
How is DNA organised?
DNA forms a gene, intervening sequences
Genes determine proteins, incorporated in chromosomes (stored in nucleus)
What are possible sources of DNA?
Biological sources- saliva, hair root, cells, blood, semen, urine, tissues (i.e. skin, liver), bones, teeth
Trail of DNA- i.e. fingerprints, weapons, bitemarks
How much of a blood stain is required to obtain a DNA profile?
a very small amount
What is the method of RFLP to obtain a DNA profile?
Restriction Fragment length polymorphism
Variable number tandem repeats
Large amounts of DNA required
Requires under graded material
(Warm moist conditions increase degradation)
What is the method of PCR to obtain a DNA profile?
Polymerase Chain Reaction
Short tandem repeats (1-5 bases, repeated 17 times)
Less DNA required
Can be partially degraded
Limitations- contamination from other sources, more sensitive
How does RFLP work?
Variable number tandem repeats
Restriction enzymes- cut at points of DNA
Separated through gel electrophoresis
How does PCR work?
DNA goes through a number of cycles, each cycle repeats the last one
Heated to high temperature- 95 degrees, causing DNA to separate
Lower temperate- introduce primers (primmer annealing/join) bind to specific parts of DNA
Increase temperature to 72 degrees- polymerase called Taq polymerase synthesises new DNA strand
Lots of copies of DNA- useful in degraded samples, can increase number of DNA
What is PCR used for?
To make lots of copies of specific sections of the DNA taken from biological samples
What are these specific sections known as?
STRs (Short Tandem Repeats)
What does one cycle of PCR go through?
Through a series of temperature changes
This cycle is repeated several times (usually 28) to produce more copies of the DNA
What is done once lots of copies of DNA are made?
Analyse it using gel electrophoresis
Primers labelled with fluroescent labels
Products exposed to laser beams
General facts about results from gel electrophoresis?
Less repeats of locus- smaller it’s going to be, faster travels through cell
Need to look at more than one locus
Thicker bands- same number repeats for each allele, allele from mother and father the same
What are STRs?
DNA has variety of STRs
People vary in number of repeats at STR loci
Use PCR-recognise constant conserved sequences
Mutiplex STR- looking at multiple sites, more simple and direct, looking at multiple
What is the normal appearance of peaks from a DNA profile?
Peaks are coloured as they have fluorescent molecules attached during PCR
What are diagrammatic examples of DNA profiles?
Profiles from suspect and evidence samples
What are stutter bands?
DNA repeats slipping out of register during PCR
Usually 1 repeat length smaller than main band, i.e. mixtures of DNA (male and female)
Helped by DNA boost
Insertion caused by backward slippage
Deletion caused by forward slippage
What is LCN?
Low copy number
Same discriminatory power as routine DNA profiling
Copy 10 informative sites from smaller amounts of starting material (insuffienct amount)
Takes longer to process (have to increase the number of cycles, 34 cycles)
What is the use of partial profiles?
Used for familial searching (searching a DNA database, full profile looking for family member)
Prioritise using specific parameters (location and police intelligence)
Can use low copy number profiling
What is SNP?
Single Nucleotide Polymorphism
Difference in single base unit
Commonest form of genetic variation (90% of variations in genetics)
Happens during meiosis
SNP every 300 base pairs
Help with phenotyping, population specific
What is the use of the Y chromosome in STR testing?
Amelogenin locus- gene that is involved in mineralisation of enamel of developing teeth (X chromosome 6 bases shorter (one peak) than Y chromosome (two peaks)
STRs on Y chromosome
What is mitochondrial DNA?
1 mitochondrion has 5-10 identical circular molecules of DNA
Each 16569 base pairs- 37 genes
Useful in mass disasters to link family members- mitochondrial DNA only inherited from mother