Forensic Science Exam Flashcards

What’s the importance of the identification of semen?

Evidence in sexual assault cases

Can help prove crime committed

Can identify perpetrator

What is semen a mixture of?

Cells, enyzmes, other organic/inorganic material

What cell is only specific to semen?

Sperm cells

What’s the structure of sperm cells?

Sperm cell (spermatozoan)

Tail (flagella) for movement

Head which contains DNA

Acrosome: Structure at front of head

What color do semen stains appear?

Yellowish-white

Why is just glowing under U.V not an indicator of semen by itself?

Even though it glows under U.V so do other materials

Acid Phosphatase

Area class of enzymes found in various living tissues

Places where acid phosphatase found

Urine, vaginal secretions, perspiration, cauliflower stem juice

AP Spot Test

Reacts with semen diluted 500 times due to semen containing high levels of seminal acid phosphatase

Levels high in males until 40, then decrease

AP Spot Test Method (Swab)

Swab moistened with water

Rubbed over semen stain

Method for swab

Filter paper moistened with water. F.P used to blot area

Reagent with substrate and chromogen dropped on filter paper

Positive reaction is purple

False Positive for AP test

Other materials with low AP levels (urine, vag secretions, perspiration) may produce weak reaction

What type of reactions will fresh semen stains produce?

Immediate strong color change reaction

Christmas Tree Stain

Microscopic Exam for Sperm Cells

Red dye stains sperm heads

Green dye stains sperm tails

Staining helps distinguish sperm cells from epithelial cells (blue)

Method for Christmas Tree Stain

Collect portion of stain on moist swab or take small cutting of stain

Tease apart fibers in drop of water on microscopic slide and dry.

Stain with red dye and rinse. Stain with green dye and rinse

Prostate Specific Antigen (PSA)

Protein produced by the prostate gland

Can confirm semen in samples that don’t contain sperm cells

PSA relies on an antigen-antibody reaction

Method for PSA

Swab or cutting agitated in water

Drop of solution added to test well

Liquid moves through result well

Antibody already bound to membrane

Visible test line (T) means PSA present

What is saliva

Water, mucus, enzymes

What does saliva aid in?

Digestion by lubrication food and begins breakdown of starches

How to detect saliva?

Look for presence of amylase

Found in many body fluids

Amylase

Enzyme Breaks down starch

What are the 2 presumptive tests for saliva

Starch-iodine and phadebas reagent

Phadebas Reagent Reaction

Sold as blue pellet

Blue dye linked to tiny pieces of starch which are pressed together to form pellet

When exposed to saliva, presence of amylase breaks down starch and releases dye

Phadebas Reagent Method (Tube)

Small piece of stain cut and placed in tube

Water and crushed phadebas tablet added and tube heated

Sample centrifuged

Searching with phadebas paper

Phadebas paper contains Phadebas reagent on one side of paper

With blue side down placed on area to be searched and sprayed with water and pressed against area

Amylase in saliva breaks down starch releasing blue dye

What are the limitations of phadebas reagent?

Since amylase present in other fluid, it may cause a false positive detection

Test not very sensitive

What is the phrasing for comparing unknown and known samples?

Compare leftover (unknown) blood from crime scene to blood “from the suspect”

What does DNA stand for?

Deoxyribose Nucleic Acid or Deoxyribonucleic Acid

What does DNA do?

Contains genetic info

Codes for proteins our bodies make that are necessary for survival

Code for making protein Order of amino acids determine what type off protein made

Hemoglobin

Carries oxygen from lungs to cells

Insulin

Regulates metabolism

What does DNA determine in regards to proteins?

How much proteins each cell makes

What is the structure of DNA?

Double helix, like twisted ladder

Paired nitrogenous bases

Sugar phosphate backbone

What is a nucleotide?

Building blocks that DNA ladder is made up of

What are the four bases of DNA

Adenine pairs with thymine

Cytosine, which pairs with Guanine

They form rungs of DNA ladder

DNA Sequence

DNA written as sequence of bases

Only one side of ladder written

How many base pairs are there in one copy of DNA for humans?

3 billion

What percentage of the three billion bases in DNA sequence are same in humans.

99.8%

Where is DNA found?

In cells, mitochondria, and nucleus (brain of cell)

What cells are important to forensic scientists?

White blood cells, sperm cells, and Cheek Cells

How many chromosomes and pairs are there in each cell?

46 chromosomes, 23 pairs

What are sources of DNA at crime scene?

Blood, semen, saliva, tissue, bone, teeth, etc.

DNA Sequence

AGCT

Sequence of four letters

1. One base pair can be different

2. Base pairs can be added/removed

3. Repeated DNA

Can DNA change over time?

No

What are types of unknown samples?

Blood stain, Semen stains, Saliva residue

When can contamination occur when collecting DNA samples?

Occurs when DNA from another source gets mixed with collected sample

What type of bags does DNA go in

Paper bags

What does moisture do to DNA?

Degrades DNA so air dry

In what cases is mitochondrial DNA used?

Cases of severely degraded DNA

Short Tandem Repeats

Most used forensic markers

Located in nuclear DNA (chromosomes)

Individual identification possible

Type used in FBI CODIS database

People differ in length at loci

Locus

Location of STR (DNA) on chromosomes

Allele

Type of DNA. For STRs it’s the number of repeats

How many alleles are there at each locus?

Two

What are the three steps in DNA Analysis

1. Extraction 2. Amplification (PCR) 3. Separation

Extraction

Separates DNA from sample

Amplification/PCR

Amplifies small portions of DNA (STR regions)

Separation

Separates amplified fragments according to size

When reporting final results of DNA findings what is it presented as?

A statistic (ie. Frequency of DNA profile in a given population is 1 in 300 trillion)

Mitochondria

Powerhouse of cell

Have own DNA

Double Helix

No chromosome

Has ring of DNA

How many letters long is mitochondrial DNA

16,569

How many D loop bases does it have?

900 with 1.7% difference

What are the three steps for mitochondrial DNA analysis?

1. Extraction

2. Amplification

3. Sequencing

Firearms Summary

Guns (manufactured/type), ammunition, firing gun, collecting evidence, analyzing evidence

Smooth barrel

Hollow tube

Rifled Barrel

Barrel of gun can be rifled to put spin on bullet; increases accuracy

What does the inside of a rifled barrel have?

Lands (hills) and Valleys (grooves) which twist down length of gun

What do minor differences in manufacturing lead to?

Differences in scratches (striations) on fired bullets

What are the two different handguns?

Revolver and semiautomatic pistol which can be fired with one hand

How does a revolver work?

Hammer pulled back manually or when trigger pulled

Hammer hits back of bullet casing which causes explosion

Barrel rifled

How does a semiautomatic pistol work?

Hammer strikes bullet case when trigger pulled

Casing automatically ejected

Barrel rifled

What are the two types of long guns?

Rifle and shotgun

How does a rifle work?

Hammer strikes bullet case when trigger pulled

Bullet casing manually/automatically ejected

Barrel rifled; fires bullet

How does a shotgun work?

Hammer strikes shot shell when trigger pulled

Bullet casing manually/automatically ejected

Barrel smooth; fires shot pellet or slug

What are the three types of bullets?

Lead/Lead Alloy, Semijacketed, and Full metal jacket

Lead/Lead Alloy

Soft bullet; scraped up by lands/grooves of barrel

Semijacketed

Brass jacket covers sides of a lead bullet; except for tip

Full metal jacket

Bullet completely covered by metal

What does primer do?

When hit with primer it creates small explosion which then lights propellant creating larger explosion (propels bullet)

Propellants

Modern ones smokeless powders composed of nitrocellulose and some contain both nitrocellulose and nitroglycerin

How do you collect firearms evidence?

Either collect item containing embedded bullet or use rubber coated forceps to remove bullet

What containers should small firearms evidence be collected in?

Sealed marked containers

Hard containers to prevent scratching (pillbox, glass vials, etc.)

What are the class characteristics of bullets?

Caliber (size), # of lands/grooves, Width of lands/grooves, Direction/ degree of rifling twist

What are the class characteristics of bullet casings?

Caliber (size), firing pin location, size, shape, extractor/ejector sizes, and relative location of ejector/extractor

Comparison microscope

Used to compare striations on bullets

Striations

Tiny scratches on bullet that can be specific to a single gun

Gun Shot Residue

Mixture of materials that originate from firing gun which falls on hands of person firing gun

What are the two elements in Gun Shot Residue

Antimony and Barium

How is GSR collected/analyzed?

Elements barium/antimony found in high levels

Hand, glove, paw swabbed to collect GSR

Use of powerful microscope

What can range of fire do/ tell you?

Distance between gun and victim can be estimated by looking at GSR patterns

Long distance

No Gun Shot Residue on victim

Close-Range

GSR on victim spread out

Near Contact

GSR on victim concentrated circle

Contact Shot

“Blown out”

Conclusion of Firearm Analysis

If class characteristic and striations match, can say bullet fired by suspect

If don’t match suspect excluded

If class characteristics match but striations don’t, results may be inconclusive (Up to firearm analyst to determine)