LS15 - Week 5 notes
LS15 - Week 5 notes Life Science 15
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This 3 page Class Notes was uploaded by AK315 on Thursday March 10, 2016. The Class Notes belongs to Life Science 15 at University of California - Los Angeles taught by Professor Phelan in Winter 2016. Since its upload, it has received 38 views. For similar materials see Life: Concepts and Issues in Biology at University of California - Los Angeles.
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Date Created: 03/10/16
Week 5 DNA ﬁngerprinting Why are we interested in DNA ﬁngerprinting? ◦ Real ﬁngerprints not left behind in many crimes ◦ They are essentially unique What are the uses? ◦ Solve unsolvable crimes ◦ Resolve paternity issues ◦ Can prove innocence of people unjustly convicted ◦ Immigration issues About DNA • DNA is a long sequence of ‘letters’ that carry the info of how to build molecules for the body • It’s essentially 2 strands (the backbones) and the rungs. • Comparing the DNA ﬁngerprints means seeing how much ‘spelling’ diﬀerence there is between individuals. ◦ Almost no diﬀerence - 99.9% identical ◦ 0.1% of base pairs is diﬀerent (0.1% of 3 billion base pairs = 3 million base pairs and this is enough for identifying individuals) • On average, individuals diﬀer in their DNA sequence at 0.1% of their bases; 3 million diﬀerences out of 3 billion base pairs. • We have DNA from several samples. How can we tell if the DNA came from the same person. Short Tandem Repeats (STR’s) • What are STR’s (Short tandem repeats) which is repeating units 4-5 nucleotides long (so it’s a small sequence within the DNA somewhere) • Everyone has STR 1 (for example ATGCC) but it repeats a diﬀerent time for each person (5/11 repeats OR 3/6 repeats OR 13/7 repeats etc.) STR 2 (for example may be AGAT). • For an STR locus - many (2 or 3 dozen or even more!) alleles exist within the population, but each individual can only have two of them. • Simply comparing 1 STR for an allele won’t yield any results when matched up because odds are low. ◦ But if choose 6 features for which individuals vary a lot. ◦ Compare DNA sample for those 6 features. If they are the same at all of them, then the samples came from the same individuals. • Creating a DNA ﬁngerprint ◦ The DNA fragment containing each STR region is ampliﬁed using PCR. This results in huge number of those fragments. ◦ The fragments are separated by size using electrophoresis (using gel - the big STR’s move slow through the gel as a result of electric current pulling it through the gel to the positive charge and the small STR’s move fast, essentially this forms a pattern). ◦ PCR > You take a strand > Heat it up > Stands separate and you add more nucleotides (throw in alphabets - PRIMERS that attach to the strand) > cool it down > you now have doubled the DNA > do it multiple times • Human paternity testing ◦ Analyzing one’s locus ◦ What do the bands represent? ▪ A band tells you what allele it represents and it tells you how many time an STR repeats itself based on the distance it moves down the gelatin. ▪ The width of the band is the same, however. Important issues • Diﬃculties and controversies with DNA ﬁngerprinting ◦ Overlap of STR’s (it takes judgment to determine if STR’s are same or slightly diﬀerent since they are overlapping) ◦ How many STR’s should we compare? ◦ How likely is a match? p(one band)*p(another) …. can juries understand this? ◦ Are there sub-population diﬀerences (in allele frequencies)? ▪ P = (0.1)*(0.1)*(0.1)*(0.1)=1/10,000 2 loci, 10 alleles each P = (0.1)*(0.1)*(1/1)*(1/1) =1/100 ▪ Need to take into account geographical factor that many people of higher genetic relatedness will be living in same area ◦ Do humans make mistakes? ▪ Cops planting evidence ◦ How sure do we want to be? ▪ What is a ‘reasonable’ level of certainty to warrant imprisoning someone? ◦ Should we keep DNA databases? Who should have access to them? ▪ Probable cause (attack on privacy) vs larger database ▪ Insurance companies can exploit the data to identify known disease causing sequences and then deny insurance to those individuals (because they’d have to pay them if they get the disease)
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