Lecture 12: DNA analysis Flashcards
DNA bases
- DNA in all species is made up of 4 base pairs (bp)
> A – Adenine (Purine)
> T – Thymine (Pyrimidine)
> C – Cytosine (Pyrimidine)
> G – Guanine (Purine)
DNA structure
> Forms a double helix
Strands duplicate in 5’ – 3’ direction
Opposite strands duplicate in different direction
Length of DNA
> There are approximately 3,095,693,981 base pairs in the human genome.
Approximately distance in miles between the Earth and Pluto
98.4 years to count (assuming 1 every second)
Just under half the worlds population
Coding Vs non-coding
> Less than 2% is coding (~40% linked to coding regions)
- Contains the genes
- Undergoes transcription
- Makes the proteins
- Typically conserved
- Under selection pressure
Remaining is non-coding
Where is DNA found in the cell?
> DNA is found in two areas of the cell
- Nucleus
- Mitochondria
Both are used in Forensic DNA Analysis
- Different applications
- Different techniques used
Nuclear DNA
> Nuclear DNA is located on the chromosome
Packed tightly by histones
Chromosomes vary in size and (bp) and content
Somatic cells
> Each somatic cell can contain:
-22 pairs of autosomal chromosomes
-2 sex chromosomes
The somatic number is 2n:
-44 autosomes
-2 sex chromosomes
Chromosomes
> Chromosomes are duplicated before mitosis
-Generates two sister chromatids (identical)
-Joined by centromere
-Producing ‘common’ morphology
Becomes ‘single’ again during mitosis
How genetic variance occurs
> The genetic make-up of every autosomal cell in an individual is identical
One reason why individuals show genetic variation is genetic recombination
-Occurs during meiosis
-Genetic material is exchanged between parental chromosomes
Another source is the random assortment of chromosomes that can occur during meiosis I
-Homologous chromosomes can separate differently
-Results in four unique daughter gametes
Another source is the random pairing of gametes
-Either through the physical act of fertilisation
-Act of mate choice (may be prone to selection pressure)
These mechanisms explain why any cell that is left at a crime scene can be traced back to the individual, and not (necessarily) be mistakenly attributed to anyone else.
Why genetic variance happens
> Reasons for genetic differentiation between populations include
-Barriers to gene flow
»_space;Migration
-Genetic drift
»_space;Population size
-Natural selection
»_space;Non random mating
Sources of mutation
> Novel sources of variation include mutation
-The insertion or deletion (indel) of a single base-pairs of DNA
-Can result in a frameshift of the triplet code
Other sources of variation include mutation
-Sometime the mutation is a transversion or a transition
-Called point mutations
-Sometimes the mutation involves the removal of a region of DNA
-Occur at a high rate than point mutation
Genetic variation
> Explains why individuals share certain genetic sequences
Explains why individuals show genetic uniqueness
Explains why populations share certain genetic similarities
Explains why populations also show unique features
Explains how novel sources variation arise
Forensic science definition
“Forensic Science is the application of science to the law through rational and balanced inferences resulting from test results and measurements to either support or refute the defence or prosecution hypotheses”
-Evett & Weir 2008
DNA analysis
> Often requires comparison between two samples
Also if shown to match to a sample on the NDNAD
How to prove the prosecution hypothesis
In order to prove the prosecution hypotheses………
“We believe the sample belongs to the defendant”…….
You need to argue that the DNA sample does not belong to anyone else
What is a DNA profile
Wikipedia states that…..
‘A DNA profile is a small set of DNA variations that is very likely to be different in all unrelated individuals’
Genetic variation
> The genetic variation used to create a DNA Profile are from a class of molecular marker called Short Tandem Repeats (STR)
- 2-6 bp in length
- Repetitive
Show Mendelian inheritance
Alleles and locus
> Each allele is located at a specific locus
Each loci is inherited according to Mendelain laws
A single individual will always contain two alleles per locus
-Homozygous
-Heterozygous
There will be multiple alleles at each locus
How to detect alleles
When analysed using capillary electrophoresis the length differences are observed
Genotype frequencies
> In order to prove that the DNA sample does not belong to anyone else you need to know something about the probability that it would occur elsewhere in the population
We understand this by looking at the frequency each allele exists at within and between populations
By measuring multiple genotypes within populations we can calculate the Genotype Frequency
Proportion of population that is
- A1A1 – p2
- A2A2 – q2
- A1A2 (pq)
- A2A1 (qp)
- Pq + qp= 2pq
Allele frequency
By measuring multiple genotypes within populations we can also calculate the Allele Frequency
Probability of observing a matching profile
> It’s easy to measure the genotype frequency at a single locus with two alleles but the probability of observing a matching profile is high
Therefore Forensic DNA profiling uses more loci with greater polymorphism
Problems with matching profiles
> You don’t know what the genotype is before you sample so you add more loci
Also becomes more complicated as you don’t necessarily know which population the sample donor originates from
The probability of observing matching profiles isn’t always the same in different populations
Allele frequencies can differ between populations
DNA extraction process
> DNA extraction performs different processes
1. Extraction- Removal of DNA from cellular casing
2. Purification- removal of inhibits
