Unit 4: DNA Evidencektennant.weebly.com/uploads/3/8/2/4/38248527/intro_to_dna.pdfWorked on...
Transcript of Unit 4: DNA Evidencektennant.weebly.com/uploads/3/8/2/4/38248527/intro_to_dna.pdfWorked on...
Reminders
Paper outlines due next Wednesday (April 1st)
Can be handwritten or typed
Rough drafts due April 15th (Wed. after Spring
Break) – must be typed
Next Tuesday is last day of 3rd quarter
All late assignments from Units 1-3 must be
turned in by this Friday!
Unit 4: DNA Evidence will count towards Q4
What is DNA?
Deoxyribose Nucleic Acid (DNA)
The genetic material!
Discovered in the late 19th century
Established as genetic material in mid
20th century
How did we determine that DNA
is the genetic material?
Two experiments:
1. Avery, MacLeod, and McCarty
2. Hershey and Chase
Avery, MacLeod, and McCarty
Worked on transformation using two strains of
bacteria
S strain – pathogenic, mice died when injected
R strain – non-pathogenic, mice did not die
Question: was the transforming factor a
protein or nucleic acid
Avery, MacLeod, and McCarty
They attempted to transform the R strain
into the S strain
Incubated live R strain with heat-killed S
strain
Pre-treated the heat-killed S strain with a
protease (enzyme that degrades proteins) or
with DNAase (enzyme that degrades DNA)
Avery, MacLeod, and McCarty
If the transforming factor was a protein then
treatment of the heat-killed S strain with a
protease would destroy the protein and inhibit
transformation
Similarly, if the transforming factor was a
nucleic acid then treatment of the heat-killed
S strain with a DNAase would destroy the
DNA and inhibit transformation
Avery, MacLeod, and McCarty
Results:
Protease did NOT affect transformation
DNAase did affect transformation
i.e. DNA (a nucleic acid) is the transforming
factor
Hershey and Chase Experiment
1952 – DNA IS THE GENETIC MATERIAL!!!
Sources of DNA Evidence
Biological fluids (blood, semen, saliva, urine)
Hair (roots)
Teeth
Bone
Other tissue
Key features of DNA for identification
Our DNA does NOT change during our lifetime
DNA is the same in every cell
DNA is relatively stable
DNA (nuclear) is the only forensic evidence
statistically able to show ‘uniqueness’
DNA for Identification
We are all 99.9% the same genetically
To be forensically useful – need variability
Variability must have high frequencies
i.e. it is not super rare
Every cell has ~6 billion base pairs (AGCT)
Where is DNA found?
Nucleus
Mitochondria
What is the difference? Nuclear DNA
Double helix
2 copies/cell
Inherited from both
parents
Unique to individual
99.75% of total DNA per
cell
Low mutation rate
Genome size: 3.2 billion
base pairs
Mitochondrial DNA
Circular
>1000 copies/cell
Maternally inherited
Not unique to
individual
0.25% of total DNA
per cell
Higher mutation rate
Genome size: 16,569
base pairs
Forensic Advantage
Since nuclear DNA is unique to each
individual it has a greater forensic
advantage that mtDNA
However, there are some circumstances
where it is not possible to get nuclear
DNA
Mitochondrial DNA advantage
Highly degraded samples (old)
Higher copy number per cell – more likely to
survive
Mitochondrial DNA
Maternally inherited At conception only the sperm’s nucleus
enters the egg and joins with the egg’s
nucleus
So, when the zygote cell divides it only
contains the mother’s mitochondrial DNA
Heteroplasmy
Presence of more than one mtDNA type in an
individual
1. May have more than one type in a single tissue
2. May have one type in a certain tissue and
another type in a different tissue
Mitochondrial DNA (mtDNA)
Control region (D-Loop)
Contains 2
hypervariable regions
HV1
HV2
These are the regions
that mutate frequently
i.e. the ones we look at in
forensics
How do we identify mutations to mtDNA?
Human mtDNA was first sequenced in 1981
by Fred Sanger in England
(Sanger sequencing)
The 1st persons mtDNA that was sequenced
was Anderson
This sequence has been used as the
reference sequence since then
i.e. what we compare all other mtDNA sequences
to
How do we identify mutations to mtDNA?
Polymorphisms – single base mutations
i.e. genetic variant
Questions to answer from 2 articles
1. Why do you think mitochondrial DNA testing was used for the unknown soldier versus nuclear DNA testing?
2. Why didn’t they use the mtDNA from Prince Phillip to try and identify Tsar Nicholas II?
3. What was the significance of heteroplasmy in this case?
4. They tested bone from the Tsar and blood from the relative, do you think if they were able to test blood from the Tsar they would still see heteroplasmy at that location?