dna analysis - STR typing

STR

short tandem repeat

str sequence

very short. 3-5 nucleotides as the core repeat, entire str sequence 450 bases max, vary in number of repeats among the population, due its small size pcr can be used to amplify this region, only target 25 for profiling, if used for forensic typing tetra or penta repeats

advantages of STR

1, STR profiling by pcr is faster than vntr analysis 2, is applicable to much smaller quantities of dna ( 100 times less required) 3, allows use of dna that has been degraded significantly because even small pieces of dna may contain intact str sites

str profiling method - overview

genomic dna us analysed at mutiple loci, analysed simultaneously using pcr (multiplexing), the more strs included in the profile the more discriminating the test, fragments of dna containing strs are amplified and labelled with fluorescent syes, then analysed in capillary gel electrophoresis system to measure their length and determine which alleles are present

interpreting electropherograms

str region or locus appears as one or two peaks, x-axis amount of time taken for the amplified dna fragments to migrate through the capillary and is proportional to allele size, left smaller strs, right larger strs, y-axis amount of fluorescence detected and is proportional to the amount of dna present so the peak heught reflects the amount of fluoresence detected and will vary somewhat for each locus as some amplification reactions will be more efficient than others

amelogeinin gene

the sex of the dna contributor can be determined by analysing the amelogenin gene, present on both X and Y chromosomes, X 6 bases shorter then Y, therefore males have two different sized fragments

Y strs

under 20 strs identified on the Y chromosome, useful when sample is a mix originated from less then one male, only one allele per str, females do not have these strs so useful analysis in sexual assult cases where dna profile of victim may complicate results

issues with str analysis

factors can introduce ambiguity into str evidence leaving the results open to interpretation, mixed dna samples - very difficult to interpret, degradation - samples can be difficult to type, allelic drop out - only one allele detected, occurs occasionally if sample us limited or degraded, technical artifacts - due to imperfections in analysis process - presence of peaks that shouldnt be detected