Topic 6- Notes Flashcards
What are fingerprints?
Fingerprints are small ridges caused by fold in the epidermis. Sweat and oil secretions- from rest of body, that we touch with our hands- leave imprints on surfaces. Everyone’s is unique, even identical twins.
How are fingerprints made visible/ matched?
Fine line aluminium, iron or carbon powders- stick to print.
Superglue- reacts w/ water & other substances left by the prints.
Ninhydrin- reacts w/ amino acids in sweat producing purple impression on absorbent surfaces.
Vacuum metal deposition to cover print w/ thin layer of non-reactive metal.
Magnets and iron flakes.
Min. 16 points must be identical for prints to match.
What is IDENT1?
National computerised database of biometric information to match fingerprints.
How can dental records be used in identifying people?
Teeth & fillings decay v. slowly.
Forensic dentist makes accurate chart of teeth, incl. fillings, dental work & missing teeth. Compared w/ records of missing people. Per records and X-rays used.
Development of teeth & roots used to determine age.
1) What are intragenic regions?
2) What are expressed regions?
3) What are short tandem repeats?
1) The non-coding (doesn’t code for proteins) blocks, introns. Within introns, short DNA sequences are repeated many times. They are inherited in the same way as any…
2) …genes within the coding regions, exons.
3) The sequence of repeated bases (STRs or satellites). Can contain from 2-50 base pairs & can be repeated from 5-100’s of times.
What is DNA profiling?
Also known as genetic fingerprinting and DNA fingerprinting- relies on fact that everyone, except identical twins, have unique DNA.
How does STR variation help w/ DNA profiling?
Same STRs occur at same place on both chromosomes of a homologous pair. No. times they’re repeated varies.
Each person has a large no. of introns w/ lots of STR loci + variation in no. repeats= 2 individuals v. unlikely to have same combination of STRs- enables scientists to create a virtually unique DNA profile.
How is DNA profiling made?
Tissue sample must be obtained & DNA extracted. Fragments of different lengths created by cutting DNA. Fragments separated & visualised, compared w/another.
How is the DNA obtained?
Can be obtained from almost all biological tissue. Tissue sample is broken down in buffer solution that includes salt and a detergent to disrupt the cell membranes.
The small suspended particles, including DNA, are separated from rest of cell debris by filtering or centrifuging.
Protease enzymes are incubated with the suspension to remove proteins & then cold ethanol is added to precipitate out the DNA.
Several stages of washing DNA in a buffer solution then follow.
How are the DNA fragments created?
Alec Jeffreys technique involves treating DNA sample w/ restriction enzymes/ endonucleases. These are found naturally in bacteria, function- cut up invading viral DNA. Enzymes will only cut DNA at specific base sequences (4~6 base pairs long). If restriction sites are either side of a short tandem repeat sequence that fragment of DNA will remain intact, but it’ll be cut away from rest of the genome.
Are bacteria’s own DNA affected by their own restriction enzymes?
No- their DNA doesn’t contain sequences that’re targeted by their restriction enzymes. In lab, their used as target ‘scissors’. Cut DNA sample into fragments only where their specific restriction sequence occurs. If same restriction enzyme’s used to cut two identical DNA samples, identical STR fragments are produced.
What is the polymerase chain reaction (PCR)?
Tiny deposits of hair, skin, body fluid are used for identification purposes DNA is copied numerous times using the PCR and DNA primers.
DNA primers are short DNA sequences complementary to DNA next to STR. DNA primers are marked w/ fluorescent tags.
Forensic sample is placed in reaction tube w/ DNA polymerase, primers & nucleotides. When in PCR machine, tube undergoes cycle of temp changes.
First separates double stranded DNA. Second optimises binding of primer to target DNA sequence in sample. Then polymerase attaches and replication occurs. Final temp change, opt temp for heat stable DNA polymerase.
Cycle continues- many targeted DNA fragment produced.
What are micro-satellites?
In forensic cases, DNA samples are generally analysed for presence of 10 short tandem repeat sequences-micro satellites.
Each target STR is 4 bases in length, an additional primer is used to determine gender; it targets a sequence on the sex chromosome.
How are the fragments separated?
Gel electrophoresis.
DNA placed on gel of agarose or polyacrylamide, both provide a stable medium through which the fragments can move. Gel is submerged in a buffer solution, & connected to electrodes that produce a potential difference across the gel.
The -ve DNA fragments migrate through gel according to charge and size. Smaller ones w/ smaller no. repeats move faster. They end up close to +ve electrode.
Reference sample w/ fragments of known length may be added to gel- DNA ladder or marker- fragment lengths are measured in base pairs.
How are the fragments then removed from the gel?
Gel is fragile & DNA fragments are double-stranded after electrophoresis if restriction enzymes have been used to produce the fragments. Southern blotting used to transfer them to more resilient nylon or nitrocellulose membrane.
Membrane placed directly onto gel & wad of dry absorbent paper placed on top- acting as a wick- drawing buffer solution up through the gel carrying DNA onto membrane. Fragments maintain their positions relative to each other and are denatured into single strands, exposing base sequences.
How are the DNA fragment (/probes) labelled?
Membrane incubated with excess labelled DNA probe- short section of DNA with base sequence complementary to target DNA sequence that needs to be located. Probe’s given time to bind to complementary sequence (hybridised)- any unbound is washed away. Probes can be labelled w/ radioactive phosphorus (32P), or w/ fluorescent marker.
Radioactive probe- membrane dried & placed next to X-ray film- film blackens where probe has bound.
Fluorescent probe-position on membrane can be visualised under UV.
Explain the bands after the fragments have been visualised:
Single band occurs on the profile where a person’s maternal and paternal chromosomes have the same no. repeats at particular locus. 2 bands occur on profile if 2 chromosomes have different no. repeats at a locus.
Unique banding for each individual if probes for many different repeated sequences are used. Pattern produced is the familiar banded DNA profile.
How can fragments of DNA produced by PCR be analysed?
Gel electrophoresis.
DNA primers have fluorescent tags- allows system to be automated. As DNA fragments with their tags move through gel they pass a laser, the dye in tag fluoresces & coloured light is detected- gives time taken for fragments to pass through gel- calibrated w/ size using fragments of known length. size= no. base pair it has. Several can be analysed at once using different wave lengths.
What can be used to help determine the time of death?
Temperature of the body, the degree of rigor mortis, and the state of decomposition.
Also any entomological (insect) evidence can help.
How can body temperature be used to determine time of death?
Body starts to cool down after death- useful method for up to 24 hours after death. Measured through the rectum or through an abdominal stab w/ long thermometer/ electronic temp probe. Cooling of body allows for a sigmoid curve. Initial temp plateau ~lasts 30-60 mins. Graph assumes temp at death was 37degree C.
As near linear part of cooling curve temp decline per hour can be used. Factors that effect cooling: size, position, clothing, air movement, humidity and temp of surroundings.
How can Rigor Mortis be used to determine time of death?
After death muscles relax then stiffen then relax- stiffening known as Rigor Mortis.
1) muscle cells starved of O2
2) respiration in calls becomes anaerobic-lactic acid.
3) pH of cells falls, inhibiting enzymes and anaerobic respiration.
4) the ATP needed for muscle contraction no longer produced. Bonds between the muscle proteins become fixed.
5) proteins can’t move over one another to shorten muscle, stiff.
Rigor Mortis will be complete 6-9 hrs after death, shorter if temp is high or person has been physically active before death.
How can decomposition be used to determine the time of death?
Autolysis occurs first- body’s own enzymes, from digestive tract and from lysosomes, break down cells.
Bacteria from gut and gaseous exchange system rapidly invade the tissues after death, releasing enzymes that result in decomposition. Loss of O2 in tissues favours the growth of anaerobic bacteria. Low temp-slow rate. Heat denatures enzymes- delays decomposition.
What are the signs of decomposition?
First signs (putrefaction) in humans, greenish discolouration of skin of lower abdomen- 36-72 hrs after death- due to sulfhaemoglobin in blood - spreads and turns purple-black. Gas/ liquid blisters on skin- after a week. Smell, bloating- bacteria- H2, CO2, methane.
How can Forensic entomology be used to determine time of death?
Forensic entomologist- record info on location and condition of body- sample insects found- temps recorded- measure ‘maggot mass’.
Some maggots killed to determine age, some kept alive to take to lab. Live maggots feed on meat to complete development- establishes species and when they pupate. Bluebottle is most common species found on bodies.
Look at graph of development stages- compare to maggots found, see how old they are=min time of death.
Toxins and temp of body effect this.
How is succession present on corpses?
One group of organisms feed on body- conditions change- other species attracted-and more and more-finally left w/skeleton. Forensic entomologists use predictable sequences of organisms that feed on corpses to determine time of death.
Season, weather, size and location affect this.
Most early (pioneer) species remain throughout.
Some organisms only occur in one location- helps determine if body has been moved.
Which decomposers, other than insects, are involved in the decomposition of the body?
Bacteria from gut. Fungi and other bacteria.
Corpse= energy source. Proteins, fats, nucleic acid, organic carbohydrates= food source- bacteria and fungi multiply- CO2 released- sustains carbon cycle- used by plants in photosynthesis.
What can an internal examination show us about the cause of death?
Incision made down front of body-organs taken out for examination.
Heart and Arteries- atherosclerosis/ heart attack?
Cirrhosis of liver- death of liver cells & formation of fibrous tissue- suggests inadequate diet/ excessive alcohol consumption/ infection.
Blood & tissue- tested for toxins/ infections/ tumours.
Stomach analysed to see what was last eaten.
What does HIV stand for?
Human Immunodeficiency Virus (HIV).
What does AIDS stand for?
Acquired Immunodeficiency Syndrome (AIDS).
Describe some features of Bacteria:
- Prokaryotic
- No nucleus/membrane bound organelles.
- Diameter 0.5~5um
- Reproduce asexually- binary fission- DNA divides into two identical cells.
- Produce no spindle during cell division.
What are the pili (pilus)?
Protein tubes that allow bacteria to attach to surfaces and are involved in cell-to-cell attachment.
What is the capsule, on a prokaryotic cell?
A protective mucus layer which prevents cell dehydration; it also allows bacteria to from colonies.
What is the mesosome, in a prokaryotic cell?
An infolding of the cell surface membrane.
Describe the cell wall around a prokaryotic cell?
No cellulose- made of peptidoglycan- a polysaccharide cross-linked by peptide chains.
Describe the structure of viruses:
Small organic particles- consists of a strand of nucleic acid (RNA/DNA) enclosed within protein coat. Viral DNA can be single or double stranded
How do viruses reproduce?
Host cells- use their metabolic systems- hijacking it’s biochemistry. After reproduction- new virus particles may bud from cell surface/ burst out of cell, splitting it open. Splitting kills cell and is called LYSIS. Results in cell contents being released into surrounding tissues- many chemicals and enzymes released can damage neighbouring cells. These processes cause the disease symptoms produced by virus infections.
What are viral envelopes?
An outer envelope from the hosts cells surface membrane- envelope contains lipids and proteins.
Viral envelopes also have glycoproteins from virus itself- these are antigens- molecules recognised by host’s immune system as foreign. Envelope helps virus attach to cell and penetrate cell membrane. HIV is an example of an enveloped virus.
Describe the process of the virus entering, and exiting a host cell:
Virus attaches to host cell. Virus inserts nucleic acid. Viral nucleic acids replicate. Viral protein coats synthesised. New virus particles formed. Virus particles released due to cell lysis.
How is tuberculosis spread?
Droplet infection: talking, coughing, sneezing- droplets stay in air for a while in unventilated areas.
Can survive as dust droplets for several weeks as well- very contagious.
How can HIV be transferred?
Can be passed on through body fluids, vaginal secretions, semen or blood. Sharing needles, sex, maternal transmission- placenta, blood-to-blood contact.
What is an antigen?
A cell surface molecule (e.g. a glycoprotein) on a ‘foreign’ body which provokes an immune response (e.g. making antibodies).
What is an antibody?
Specific molecules made by lymphocytes in response to antigens.
Why are Viruses hard to get rid of?
Viruses have a high mutation rate, so their antigens change often. (by the time the body has made a specific antibody, the viruses antigen will have changed.)
What is a syndrome?
A collection of conditions and infections.
What is Tuberculosis?
A contagious disease caused by the mycobacterium tuberculosis bacteria.
What are the phases of TB?
Primary infection
And Active tuberculosis
How is tuberculosis transmitted?
Carried in droplets- sneezing/ coughing
What are TB’s favoured host cells?
Lungs and macrophages
