Biology topic 6

Forensics

The application of scientific knowledge to legal matters, as in the investigation of crime

Fingerprints

Impressions left by the friction ridges of a human finger

Development of fingerprints

• mostly formed between weeks 6-10 of foetal life

• Raised portions of the epidermis

• Movement of baby in the womb, speed of growth affect the fingerprint pattern

Detecting fingerprints

• carbon,aluminium or magnetic iron powder (sticks to grease)

• Ninhydrin (becomes purple with amino acids in sweat)

• Superglue vapour

Dental records

Teach survive longer than other parts of the body, unique

Methods used to identify a body

• ID papers

• Fingerprints

• Skeletal records

• Dental records

• DNA profiling

Non-coding DNA

• Doesn’t code for proteins

• About 99% of the genome

Short tandem repeats

• non-coding DNA

• Usually 3-7 bases long and repeated

• Make up 3% of the human genome

• Likely involved in chromatin folding and transcription regulation

STR locations

• same locus on both homologous chromosomes

• Number of STRs at a locus can vary

• Inherited like genes

DNA profiling

• also used in parentage testing, genealogical and medical research

• Developed by Alec Jeffreys in 1984

• uses STRs

DNA profiling steps

• obtain DNA sample

• Create fragments

• Separate fragments

• Visualise fragments

DNA samples

• cheek cells from swabbing

• White blood cells from blood sample

Restriction enzymes

= enzymes that cut DNA at specific recognition sites

• discovered by Werner Arber in 1970

• Found in bacteria

• More than 600 available commercially

PCR

Polymerase chain reaction

Amplifies DNA

DNA primer

Short DNA sequences complementary to the DNA either side of the STR

Step 1 of PCR : denaturation

DNA strands separated (denatured) (hydrogen bonds are broken)

95°C for 1min

Step 2 of PCR: annealing

Small primers anneal at the start and end of STR sequence via complementary base pairing

55°C for 2 mins

Step 3 of PCR: extension

• Taq DNA polymerase synthesises complementary DNA strands using free nucleotides(dNTPs)

  70°C for 2 mins

Why is Taq polymerase used in PCR?

Thermostable so can withstand temperature changes

dNTPs

Deoxynucleotide triphosphates

Used in PCR

Amplifaication in PCR

• steps 1-3 are repeated for 25-30 cycles

• In every cycle more DNA is present to act as a template - increases exponentially

• Performed by PCR machine/thermal cycler

Gel electrophoresis

a method of separating DNA fragments according to size in an agarose gel by applying an electric field

Step 1 of gel electrophoresis: preparing the gel

• mix agarose and buffer

• Microwave to melt agarose

• Cool and pour into mole

• Remove comb when gel has set

Step 2 of gel electrophoresis: loading the gel

• the gel is put into a tank with buffer

• DNA samples are loaded into the wells with a pipette

Step 3 of gel electrophoresis: running the gel

Electrodes attached - cathode near wells, anode on opposite side because DNA is negatively charged

Staying the DNA directly in gel electrophoresis

• can be stained using ethidium bromide and visualised under a UV lamp

• Most commonly used dye

• Inserts itself between the base pairs in the double helix

• Glows in UV light

Evidence for time of death

• body temperature

• Rigor mortis

• Decomposition

• Entomology and succession

how to measure core body temperature?

long thermoprobe via rectum or abdominal stab

point A on curve

• sigmoid curve

• plateau, 30-60 mins

• metabolic reactions not fully stopped yet

point B on curve

linear decline of temperature can be used to estimate time of death (approx -1.5C/h)

point C on curve

body temperature reaches ambient (environmental) temperature

factors that affect initial body temperature

• fever

• hypothermia

factors that affect post-mortem cooling

• environmental temperature

• air movement (wind)

• humidity/ body found in water

• body size, SA/vol ratio

• fat composition

• body location

• clothing

how factors affect post mortem cooling

• greater temperature gradient and wind, quicker cooling

• higher humidity, slower cooling

• a body in water will cool quicker due to the large temperature gradient

• small body cools quicker due to high SA/vol ratio

• high fat composition and clothing will insulate a body

rigor mortis

after death, muscles first relax, then stiffen and then relax again

rigor mortis steps

1) death: muscles become starved of oxygen —> aerobic respiration stops

2) respiration becomes anaerobic —> produces lactic acid

3) pH falls, inhibiting enzymes —> anaerobic respiration stops

4) ATP is no longer produced —>myosin and actin permanently fixed in contracted state

why does rigor mortis stop after about 36 hours?

decomposition - lysosomes break down and release enzymes that break down cells

factors that affect the onset of rigor mortis

• temperature

• O2 availability

• manner of death

decomposition

digestion of cells resulting in the breakdown of tissues and release of carbon and minerals (e.g. nitrate and phosphate)

five stages of decomposition

• fresh, initial decay

• bloating, putrefaction

• active decay

• advanced decay

• dry remains

fresh, initial decay

• 0 to 3 days after death

• autolysis

• anaerobic bacteria in gut start to digest tissues and release gasses (start of putrefaction)

bloating/putrefaction

• 3 to 10 days after death

• increased gas production by bacterial activity causes swelling of tissues and putrid odour

• breakdown of haemoglobin leads to venous marbling and green discolouration of abdomen

active decay / black putrefaction

• 10 to 20 days after death

• discolouration of skin - turns purple then black

• tissues start to soften and liquefy

• flesh looks creamy

• loss of fluid and deflation of body

advanced decay

• 20 to 50 days after death

• majority of internal tissue lost

• body starts to dry out

dry remains

• 50 - 365 days after death

• soft tissues lost leaving skin, bone and cartilage

autolysis

• self-digestion of cells

• starts 4 mins after death

• when respiration stops, lysosomes release digestive enzymes which digest cell components

• digestive enzymes secreted into gut

putrefaction

digestion of proteins and tissues by anaerobic bacteria

putrefaction in detail

• as proteins are broken down, gases are produced and secreted by anaerobic bacteria ( which cause putrid odour)

• gases diffuse to other parts of the body, leading to bloating of torso and then limbs

• increased pressure weakens and separates tissues

factors that affect decomposition rate

• weather

• exposure

• temperature

succession

insects arrive on a corpse in a predictable sequence based on the stage of decomposition

forensic entomology

study of insects and other small invertebrates in criminal investigations

fresh/initial decay entomology

• 0 to 3 days after death

• anaerobic bacteria

• blow flies arrive within minutes of death and lay eggs around wounds and body openings

• 24h later, eggs hatch and larvae (maggots) feed on body tissue

blating/putrefaction entomology

• 3 to 10 days after death

• young blow fly larvae feed on body

• other fly species (e.g. flesh flies) arrive

• beetles (e.g. rove and carrion beetles) arrive and feed on fly eggs and larvae

active decay entomology

• 10 to 20 days after death

• blow fly larvae are the dominant larvae

• parasitoid wasps and scavenger flies arrive

• beetles are the dominant adult insects present

advanced decay entomology

• 20 to 50 days after death

• as body dries out, blow fly larvae are no longer able to eat tough tissue so migrate away

• beetles remain - can chew through skin and ligaments

dry remains entomology

• 50 - 365 days after death

• mites and moth larvae feed on hair until only bones left

how does a forensic entomologist determine time of death?

• take samples of larvae

• take temperature of environment

• keep maggots to determine species

• kill maggots to determine age

factors that affect succession of insects

• location

• temperature of surroundings

• presence of drugs

circular DNA of bacteria

genetic code (not associated with proteins)

plasmid

small loop of DNA

food granules

glycogen granules, lipid droplets

mesosome

infolding of cell membrane, possible site of respiratopn

cell wall of bacteria

made of peptidoglycan (polypeptide + polysaccharide)

capsule

slime layer on surface for protection and to prevent dehydration

pili

thin protein tubes, allow bacteria to adhere to surfaces

flagellum

hollow cylindrical tail-like structure, rotates to move cell

bacterial cell wall structure

made of peptidoglycan - polysaccharides held together by oligopeptide crosslinks

Gram staining

crystal violet stain turns Gram-positive bacteria blue-violet

safarin turns Gram-negative bacteria pink

Gram-positive bacterial cell wall

• thick layer of peptidoglycan

• one cell membrane

Gram-negative bacterial cell wall

• thin layer of peptidoglycan

• cell membrane and another outer membrane

Gram positive bacteria

• blue violet stain due to crystal violet

• more susceptible to lysozyme and antibiotics

• tend to live on skin

Gram negative bacteria

• pink stain due to safranin

• tend to live in wet areas because susceptible to drying out

how do bacteria reproduce?

• asexual reproduction by binary fission

• vertical gene transfer

how do bacteria cause illness?

• endotoxins (in outer layer of Gram negative bacteria) cause vomiting, diarrhoea, fever

• exotoxins (soluble proteins released by metabolism) have a toxic effect on cells, inhibit neurotransmitters

viruses

submicroscopic infectious agents that replicate inside living cells

general structure of viruses

• 0.002-0.3 micrometres

• 50x smaller than average bacteria

• many morphologies

• protein coat (capsid) made of repeating protein units (capsomeres)

• genome (RNA or DNA, double or single stranded)

• genetic material linear or circular

helical structure of viruses

• e.g. tobacco mosaic

• RNA bound to protein helix by interactions between negative RNA and positive proteins

polyhedral virus structure

e.g. adenovirus

enveloped virus structure

• e.g. influenza and COVID-19

• surrounded by host cell membrane studded with viral proteins

complex virus structure

e.g. bacteriophages

how do viruses cause illness?

• destruction of host cell during lysis

• hijack host cell’s protein synthesis so slow down host cell’s metabolism

• produce toxins

methods of disease transmission

• skin to skin

• droplet infection

• blood to blood

• sexually transmitted

• stool to mouth

• maternal transmission

• food borne

• waterborne

• animal borne

immune system

a host defence system comprising many biological structures and processes that protect against disease

humoral

molecules that circulate in the blood

cell mediated

cell to cell contact

non-specific (innate) immune system

• non-antigen specific

• immediate maximum response

• no memory cells made

specific (adaptive) immune system

• antigen specific

• lag time between exposure and maximum response

• memory cells are made following exposure

anatomical barriers to infection

• mouth, nose and eyes - chemical lysozyme

• skin - physical - keratin, chemical - sebum, biological - microbiome

• ears chemical - earwax

• airways - ciliated cells

• stomach - stomach acid

• gut - microbiome

• vagina - acidic secretions

anatomical immunity of non-specific IS

• physical factors

• chemical factors

• biological factors

humoral components of non-specific IS

• complement

• cytokines

• coagulation

cell-mediated components of non-specific IS

• phagocytosis

• inflammation

how does lysozyme work?

hydrolyses bonds in peptidoglycan which causes lysing of bacterial cells due to osmotic shock

complement system

enhances the ability of antibodies and phagocytic calls to clear microbes and damaged cells from an organism - circulate in blood as precursors

examples of the complement system

• lysis - bind to and lyse target cells

• chemotaxis - attraction of phagocytes to sites of infection

• opsonisation - opsonise (mark) bacteria for digestion by phagocytes

• inflammation - stimulate mast cells to release histamine

cytokines

small proteins that initiate changes in gene expression

interferon

• cytokine

• produced by virus infected cells

• induces virus resistance in uninfected cells

mast cells and histamine in inflammation

• mast cells release histamine in response to a bacterial infection leading to inflammation

• vasodilation —> increased blood supply (redness and heat)

• increased vascular permeability (endothelial cells contract) —> swelling (oedema) and pain

• more white blood cells arrive to clear bacteria

phagocytosis

engulf and digest bacteria

phagocytic cells

• neutrophils

• monocytes and macrophages

neutrophils

• multilobed nucleus

• 70% of leucocytes

• produce free radicals to break down bacterial DNA and proteins

• activate specific IS