6A/B - Forensics + Microorganisms and Immunity

How to use the extent of decomposition in order to determine a time of death.

• Autolysis - tissues break down due to the actions of enzymes

• Putrefaction begins - skin turns green around abdomen reddish-green → purplish black

• Bacteria causes gas + liquid bubbles → bloating

• bloating passes - liquified tissues seep out of body + body dries out

How to use stage of succession for TOD

• immediately after conditions are optimum for bacteria

• bacteria decompose tissue → favourable for flies and larvae

• fly larvae feed on body → favourable for beetles

• body dries → less fav for flies, beetles stay to decompose dry tissue

• no tissue=not favourable for most

How can forensic entomology be used to determine the time of death.

• TOD cane be estimated by the life cycle of the insect

• blowfly larvae hatch from eggs about 24 hours after being laid

• If only eggs are found on a body you could estimate a TOD no more than 24 hours.

• Different conditions will affect an insects life cycle.

How is body temperature used to determine a TOD.

• From TOD metabolic reactions slow down and eventually stop

• Temperature falls + equals surroundings=Algor mortis

• Human bodies cool at around 1.5-2.0°C /hr

• Conditions such as: air temperature, clothing and body weight affect temperature.

How can degree of muscle contraction be used to determine TOD.

• about 4-6 hours after death the body stiffens=rigor mortis

• respiration still takes place in the cell buts its anaerobic → lactic acid buildup

• pH decreases → inhibits enzymes that produce ATP

• No ATP means bonds in muscle cells fix and body stiffens.

• smaller muscles first + is faster at higher temp

What’s the role of micro-organisms in the decomposition of organic matter and the recycling of carbon.

• Important part of the carbon cycle

• microorganisms on+in the body secrete enzymes that decompose dead organic matter int o small molecules they can respire

• Methane and Co2 are released→ recycles carbon back to atmosphere.

How is DNA profiling used for identification and determining genetic relationships between organisms

• A DNA profile is a fingerprint of someone’s DNA

• Everyone’s DNA is different therefore everyone’s DNA profile is unique

How can DNA be amplified using PCR

• The DNA mixture is heated to 95°C → break hydrogen bonds between the strands

• Mixture cooled 65°C so primers can anneal to the strands (primers start signal where to start synthesising the DNA

• heated to 72°C so polymerase can work (lines up+ complementary base pairing)

• Two new copies are formed+1 cycle complete

• Cycle restarts and strands double each time.

Where are virus’s genetic material stored

They contain a core of nucleic acid which is either DNA or RNA

What are attachment proteins?

They stick out from the virus’s capsid and allow it to cling on to a host cell

What is an envelope?

An extra outer layer stolen from the previous host cell.

What is the capsid

The protein coat surrounding the core

Compare the structures of bacteria and viruses

• Bacteria are single celled whilst viruses are non cellular particles

• both have no nucleus

• both can cause disease

• bacteria reproduce by binary fission, viruses invade host cells

Explain the role of DNA primers in the production of the amplified 345bp sequence (2)

• Primers have a specific base sequence

• Bind to complementary bases

• Therefore providing a site for DNA polymerase

Explain why the amplified DNA fragments for the G20210A allele and the wild type allele are different (3)

• The restriction enzyme cuts at a specific site

• that is only present in the G20210A allele

• therefore a shorter fragment is produced for G20210A

Devise an investigation to determine the optimum number of cycles for the polymerase chain reaction used to amplify the DNA for this test.

• Make the mixture, DNA polymerase added in excess

• Control the temperature

• change the number of cycles

• use gel electrophoresis to determine amount of DNA produced

• choose the smallest number of cycles that produce a band

Describe a study for the changes in insect species on a body after death with pigs (3)

• Control variable: same mass/size/age

• Temperature monitored to make sure its the same

• record the presence of different species every 24 hours

Explain how the results of a pig study could be used to help establish the TOD of a human (insects) (3)

• Record which insects are present on the human and compare with the results of the pig

• Take into account which stages of the life cycle are present

• When using stage of succession to determine TOD environmental variables need to be taken into account

Explain the effect of ambient temperature on the rate of decomposition (3)

• The higher the ambient temperature the higher the rate of decomposition

• Enzymes in bacteria will have more kinetic energy and decompose faster

• Increases growth rate of bacteria/fungi

How does (HIV) infect human cells, causing a sequence of symptoms that may result in death.

• HIV invades T-helper cells within the immune system

• GP120 bind to the CD4 receptors on the surface of the t-helper cel

• The viral enveleope fuses with teh T-heleper cell membrane enabling viral RNA to enter

• Reverse transcriptase turns RNA into DNA → integrates into the hosts DNA

• This allows it to be transcribed to produce new virus proteins

• Those new viruses leave cell killing it and taking some cell membrane

Describe the progression of HIV into AIDS. (Acute→Latent→disease)

• HIV antibodies appear in blood after 3-12 weeks

• rapid replication of virus plus loss of T-helper cells

• T-helper cells are recognised by killer cells which start to destroy them and reduces rate of replication

• dormant diseases like TB can reactivate

• immune system vulnerable (below 200 WBC per mm² blood)

How do pathogens enter the body

• Cuts

• Digestive system (food)

• Respiratory system

• Mucosal surfaces

What barriers are in the body to protect from infection

• Stomach acid → kills pathogens

• Skin → physical barrier

• Gut+skin flora → Compete with pathogens for nutrients

• Lysozyme→ mucosal surfaces

Describe the non-specific immune response.

• Site becomes inflammed (immune system recognise foreign material

• vasodilation → increasing blood flow to site also increasing permeability