Forensics: DNA and DNA fingerprinting test review

What are the uses of DNA in forensics?

-identify potential suspects/victims whose DNA may match evidence left at scene

-clear persons wrongly accused of crimes

-identify crime + catastrophe victims

-establish paternity and other family relationships

-trace unethical organ donations/organ trafficking

-link crimes to known/unknown individuals

-determine traits about an individual

What are chromosomes?

long-chain DNA molecules that are tightly bound in a specific structure by wrapping around histones (proteins)

What is the human genome structure?

-46 total (23 pairs) chromosomes of genetic information in every cell’s nucleus

-chromosome pairs 1-22 are autosomal chromosomes

-chromosome pair 23 are sex chromosome

What is a gene?

-segment of DNA that codes for the production of a specific protein

What are regions between genes called?

non-coding regions or junk

What is an allele?

-variations in the genetic code

What do polymorphisms result from?

mutations

How does inheritance of nuclear DNA work?

-Structure: nuclear DNA exists as chromosomes

-Inherited from: one copy of each chromosome from mom and one from dad

-Can establish: identity of an individual specifically

How does inheritance of mitochondrial DNA work?

-because mitochondria were their own living organism, they contain their own unique DNA that is different than the DNA you have in your nucleus

-Structure: ring of DNA (plasmid) found in mitochondria

-Inherited from: Mom only

-Can establish: maternal lineages/familial relationships

How much DNA do humans share in common?

99.9%

What are the steps required for DNA characterization?

-Extraction: of DNA to separate from cell

-Fragment: with a restriction enzyme to make shorter base strands or copy smaller fragments with PCR

-Separate: the segments by size using an electrophoresis procedure

-Analyze: the resulting print by identifying specific alleles

How can DNA be obtained?

-skin

-blood

-saliva

-urine

-hair

-bone

How is DNA extracted?

-break down cell membranes and nuclear membranes

-uncoil DNA from histone proteins

-rinse cellular debris so DNA is dissolved in solution

-add nonpolar elute to cause DNA to precipitate out of solution

What is the basis of restriction enzymes?

-fragments DNA using restriction enzymes from bacteria

-fragment number and size varies from person to person

-separate using gel electrophoresis

-requires a large amount of DNA and is time consuming

What is the basis of PCR?

-copies parts of DNA using primers

-fragment number and sizes varies from person to person

-separate using gel or capillary electrophoresis

-requires a small amount of DNA and can produce millions of copies in just a few hours

What is the other varying information about restriction enzymes?

-will recognize a specific sequence of 4-6 and cut the DNA molecule at a specific point (restriction site)

-The number and sizes of the DNA fragments that result depend on the number and location of restriction sites for that enzyme in the given DNA sample

What is the basis of step three: separating DNA by size, gel electrophoresis?

-Separates DNA by size, using voltage. Since DNA is negatively charged, it is attracted to the anode (+ terminal) of the voltage

source

-The agarose gel acts like a molecular strainer, with smaller DNA fragments traveling farther in the same amount of time compared to larger fragments

-Once separated, the fragments are then made visible as bands using probes (aka dye that binds to the negatively charged DNA particles). These probes can be colored or fluorescent/X-ray capable

-Fragments can be measured using marker DNA –that contains fragments of known size (measured in base pairs, bp)

What is the basis of step three: separating DNA by size, capillary electrophoresis?

-Separates DNA by size, using voltage. Since DNA is negatively charged, it is attracted to the anode (+ terminal) of the voltage source

-A detector measures the size of the particles as they pass through the capillary tube

-Separated chemical compounds appear as peaks with different migration times, creating an electropherogram

-This technique allows for the analysis of multiple samples simultaneously

What is the basis of step four, analyzing results?

-individual to unknown identification: all bands or peaks will match exactly

-familial relationships: if nDNa, each band or peak of child must match either mom or dad. Child is 50 % percent from mom, 50% from dad

-if mtDNA, child’s mtDNA will match mother’s mtDNA exactly

What is the basis of Restriction Fragment Length Polymorphism?

-Involves cutting DNA using restriction enzymes (isolated from bacteria)

-The restriction sites for non-coding regions vary from person to person, resulting in different sized and number of fragments

-These fragments can be then separated by size using gel electrophoresis, then transferred to a membrane to create a Southern blot

-Developed in 1984, widely used in forensics, medicine, and genetics

-CONS: Requires a large amount of DNA that must be isolated (which can be difficult) and requires up to weeks of time to process samples

What is the repeats of short tandem repeats?

-Short tandem repeats are regions of non-coding DNA that repeat the same 2-5 nucleotide sequences “head-to-tail” a variable number of times ( i.e. the 16 bp sequence of "GATAGATAGATAGATA " would represent 4 tandem repeats of the short tetramer GATA = [GATA]4)

-20 loci (updated from 13) have been identified on human chromosomes that contain STRs of variable lengths

-Use PCR to copy these STRs, separate using gel or capillary electrophoresis, which allows you analyze multiple samples/loci simultaneously

What is the basis of single nucleotide polymorphisms?

SNP stands for "single nucleotide polymorphism". This means that there is a variation in one single nucleotide at a specific location in the genome (either in a gene or in a non-coding region).

-For example, sickle-cell anemia is a disease present in people who have two copies of the mutated beta-globin gene with a single A to T mutation.

-The advantage of SNPs over STRs is that PCR fragments for SNPs are much shorter than for STR (less DNA needed) so that degraded DNA can also be used. SNPs are also more abundant in the genome than STR and have a lower mutation rate.

-Due to the single nucleotide variation, most SNPs have two different forms (alleles) in a population. This makes a SNP less discriminatory than an STR and multiple SNP's are needed to study genetic variation. Conversely, microarrays can be used to assay millions of SNPs in parallel, so SNP typing can be extremely economical.

What is the basis of single nucleotide polymorphisms?

-NOTE: There is NO GENETIC BASIS FOR “RACE”.

A test called DNAWitness was developed to learn more about the physical

traits and biological origin of a person based on their genetic code

“The test surveys 176 Ancestry Informative Markers (AIMs) to provide an

inference of genetic ancestry or heritage. The AIMs were carefully selected

from large-scale screens of the human genome; and are characterized by

sequences of DNA that are more prevalent in people from one continent

than another. Using complex statistical algorithms, the test can determine

with confidence to which of the major bio-geographical ancestry groups,

Sub-Saharan African, European, East Asian or Native American, a person

belongs, as well as the relative percentages in cases of admixed peoples.

It's a great tool for those individuals or groups interested in more deeply

understanding their ancestry and lineage, or for certain people (i.e. some

adoptees of mixed heritage) learning about their genetic ancestry”

What is the basis of Mitochondrial DNA?

A haplogroup is an

allele (variation)

You inherit your

mtDNA haplogroup

(allele) from your

mother only

Maternal lineages can

be traced through

matches in the mtDNA

haplogroup you have

What is the basis of forensic genetic genealogy?

Used when there is no direct DNA

comparison available, and typically only in

cold cases

Compare genetic profile to profiles in

databases available to police/criminalists

(Bode, GEDMatch, etc.)

Once similar matches return, use

genealogy to figure out familial

connections to matched samples

Compare known DNA to unknown

sample to confirm match