Forensics DNA Chapter

What year was DNA discovered?

1868

Did scientists understand DNA’s role by 1868?

no

Who were the two researchers of DNA in the 1950’s that discovered its structure?

James Watson and Francis Crick

What is DNA?

It’s a polymer

What is a polymer?

a large molecule made by linking a series of repeating units; called monomers

What is the purpose of DNA?

It gives the cell “instructions” to make proteins.

What are chromosomes?

Threadlike structures in the cell’s nucleus (the center) which are composed of DNA.

It is where genes are located.

What are genes?

They are the FUNDAMENTAL unit of HEREDITY.

• made up of DNA

  • instruct the body’s cells to make proteins

• each chromosome contains many genes

What is a nucleotide?

Nucleotides are monomers that join together to form the structural units of RNA and DNA.

• composed of:

  • sugar molecule (either ribose in RNA or deoxyribose in DNA)

  • phosphate group

  • nitrogen-containing base

What are DNA’s bases? How do they pair?

The bases are: thymine & adenine; guanine & cytosine

• A & T

• C & G

What determines the inheritable traits controlled by DNA?

Its availability to produce proteins.

How are proteins produced?

connecting groups of amino acids

How does the production of proteins occur?

the combination of 20 known amino acids

What determines the shape and function of the protein?

the sequence of amino acids

Where is DNA found?

• white blood cells

• semen

• urine

• hair roots

• teeth

• bone tissue

• buccal cells

Where is DNA most abundant?

buccal cells

What is DNA or RNA replication?

the synthesis of a new DNA or RNA from an existing one

How does DNA or RNA replication work

• enzymes and proteins unwind the DNA double helix

• the two individual strands are then exposed to free nucleotides (monomers)

  • the pair up with new bases

    • new double helixes are created

After the double helix is assembled by connecting base pairs… are the copies identical or similar of the original strand?

they are identical

Why do forensic scientists use DNA?

1. Identify potential suspects

2. Exonerate individuals

Other reseasons:

• establish paternity

• match organ donors

What is DNA Typing or Fingerprinting?

a general term for a wide range of methods for studying genetic variations

What are the two methods studied for DNA typing?

1. Restriction Fragment Length Polymorphism (RFLP)

2. Short Tandem Repeat (STR)

What is PCR?

polymerase chain reaction

Explain Polymerase Chain Reaction

It is a technique used by scientists to replicate or copy a portion of a DNA strand outside a living cell

Why would forensic scientists use PCR?

They can copy a small quantity of DNA or broken DNA strands found at the crime scene in order to test through DNA typing.

How is PCR done. Explain in steps

1. DNA is heated to 94 degrees celsius

   • (the strand separates)

2. DNA temp is lowered to 60 degrees celsius

   • (short sequence of base pairs will link with separated strand)

3. DNA is reheated to 72 degrees celsius

   • (DNA polymerase will attach to the primer and free nucleotides)

Outcome: New DNA strands

How much times is the PCR process repeated.

28-32 times

How long does a single PCR cycle last?

2 minutes

What are the advantages of PCR?

With small amounts of DNA, scientists can gather a large sum of DNA samples to test

What happens after PCR?

The new strands are moved to testing area.

How much percent of our DNA doesn’t really code anything?

>30%

What makes up this thirty percent of our DNA whose purpose is unknown?

Repeated segments

What is another name for the repeated segments in our DNA?

tandem repeats

What is the purpose of the tandem repeats?

Scientists still don’t know.

Usually… how long are tandem repeats

15-35 bases long

Where do people get their repeating sequence?

they inherit them from each parent

What changes from person to person in terms of TRs?

• the combination of base pairs

• the amount of times the bases are repeated

How do forensic scientists use tandem repeats?

• They compare the DNA in chromosomes found in the crime scene to the one being tested.

• When both share two or more chromosomes with the same repeating sequence… they are most likely a match.

What is RFLP?

Restriction Fraction Length Polymorphism

What is the purpose of RLFP?

distinguish one person from another

What key characteristic of RFLP is used to complete its purpose (distinguish one person to another)

the length differences

How can the process of Restriction Fragment Length Polymorphism be studied and observed?

Through the use of:

• restriction enzymes

• electrophoresis

• hybridization

What does the restriction enzyme do?

Its splices DNA into the fragments for them to be placed individually in the electrophoresis gel.

Explain the process of electrophoresis.

After the DNA is sliced into fragments by the restriction enzyme:

1. Voltage is applied to the gel

   • this separates fragments

2. smaller fragments will move faster and end up at the end

3. larger fragments will move slower and end up at the start

Explain the process of hybridization.

Once electrophoresis is completed:

1. fragments of DNA are chemically treated to separate them

2. fragments are transferred to a nylon membrane

3. fragments are treated with radioactively labeled probes

   • they contain complimentary base sequence that will attach with the fragment

4. They are place in an X-Ray film to be developed

What is another name for the transference of DNA to a nylon membrane?

Southern Blotting

How is DNA Typing done through the observation of the nylon sheets?

• Bonds appear in the nylon sheet where the radioactive probes stuck to the fragments

LENGTH is measured by where they are located. This is the individual characteristic.

What is STR?

Short Tandem Repeats

What is the purpose of the Short Tandem Repeats

To profile DNA

What is the most successful form of profiling DNA?

Short Tandem Repeats

Where are STRs located?

They are located in the chromosomes that have short segments of 3-7 repeating base segments.

Why are STRs more reliable?

They are found in great abundance in strands that are shorter.

• although PCR is still done STR can be studied from DNA that is almost decomposed

How many types of STRs are found?

hundreds

How does multiplexing help scientists?

allows them to simultaneously detect more than one STR

What is mitochondrial DNA?

• DNA found outside the nucleus and produced from mitochondria (the powerhouse of the cell)

• It’s used when DNA is not available

• If the suspect does not comply on giving the mitochondrial DNA a sample can be taken from a relative

What is the advantage of MDNA?

• can be used when DNA is not found or is usefull

What are the disadvantages of MDNA?

• time consuming

• costly

• more rigorous

What is CODIS?

Combined DNA Index System

Who is CODIS from?

It is the United States’ national DNA database

What is CODIS purpose?

It is to maintain DNA profiles of:

• criminal offenders

• unsolved crime-scene evidence

• arrestees

• profiles of missing people

Why is CODIS usefull?

• allows comparison to match suspects to crime scene DNA