Forensic Analysis of DNA Chapter 9

DNA  Genetic Material  Double stranded; two strands of nucleotides   Nucleotides have phosphate group, deoxyribose sugar, and nitrogenous base (adenine, thymine, guanine, and cytosine)  DNA is copied into new DNA through DNA replication in nucleus   DNA is transcribed into mRNA in nucleus; mRNA is converted to proteins through translation at the ribosome

DNA Replication

Transcription—DNA to mRNA

Restriction Enzymes  Cut DNA has specific recognition sequence  Extracted from bacteria  Gives fragments of different sizes  gyfQ gyfQ

Restriction Enzymes

Polymerase Chain Reaction  Makes many copies of small collected DNA samples  Done prior to other testing  Uses DNA polymerase, 2 kinds of primers, free nucleotides, and thermal cycler  Uses heating to denature and cooling for annealing  Can get 2 n DNA double helices where n is the number of heating and cooling cycles  Y Y

PCR

Repetitive DNA  When during DNA typing, want to use non- coding repetitive DNA; not coding DNA  Much more variation in non-coding DNA  Tandem repeats; non-coding repetitive DNA  VNTR—variable number tandem repeats; often used for DNA typing  STR—short tandem repeats; used most often for DNA typing  RFLP—restriction fragment length polymorphism; cut with restriction enzymes to make many fragments

VNTR

STR

RFLP

Electrophoresis  Load negative DNA at black side of chamber into gel (agarose or polyacrylamide)  DNA moves to positive electrode  Small fragments move farther through the gel  Buffer is used as electrolyte to help send current through gel  8e0&feature=related 8e0&feature=related

Electrophoresis

DNA Typing--Identification tHK4&feature=fvw

Crime Scene DNA Analysis

DNA Typing—Paternity Testing

Paternity Testing

Hybridization  DNA from electrophoretic gel is transferred to nylon membrane or paper filter  DNA can be heated to be denatured  Radioactive or fluorescent probe are used to find specific DNA section

Hybridization

DNA Typing  Amplify with PCR  Use restriction enzymes to make RFLPs of STR or VNTR sequence of genome  Separate fragments with electrophoresis  Match bands with known samples to determine identity  Can be used for crime scene matching, paternity testing, and identifying corpses or body parts

CODIS  STR data for first 13 STRs are put into CODIS  If use at least 6 STRs, matching process is very precise; 1 in 2,000,000 probability; Using all 13 makes the probability of an incorrect match being 1 in trillion

CODIS STRs

Capillary Electrophoresis  Use glass capillary tube with gel wrapping and buffer reservoir  STRs move through column and as pass through column peak appears on attached computer instrument  Peak diagram is called an electropherogram

STR used for gender identification  Uses amelogenin gene whose length is different in X and Y chromosome  STR electrophoresis shows two separate bands for presence of X and Y chromosome in males  Shows one band for 2 X chromosomes in females

Gender ID with STR

mtDNA  Found outside nucleus is mitochondria  Easier to get and can be taken from any relative of the same maternal lineage  Same in all relatives from same maternal line; so not as specific of a match  Can be gotten when burning, age, or environmental degradation has damaged genomic DNA

Mitochondrial DNA

DNA sources for collection  Any cells from skin, in blood, cheek cells from saliva, epithelial cells in hair follicles  Sweat, semen, ear war, mucus can also be used to extract cells for DNA typing

Collection of Biological Evidence  Photograph, sketch, describe, and collect  Collect from body fluids, tissues, trash, laundry, placed often touched (handles, light switches; places licked (envelopes, lipstick, cigarettes, partially eaten food for drink); places where body fluids might be (clothing, tissues, sheets, pillows, condoms)

DNA Evidence  Package each DNA stained item separately in paper bag or well-ventilated container; closed containers can lead to moisture and growth of DNA digesting bacteria  Obtain reference DNA—buccal cells or blood  Avoid contamination—wear gloves; never cross-contaminate DNA from one piece of evidence with another; use different instruments for every piece of evidence