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?