Topic 6 - Infection, immunity and forensics - Biology A2 Unit 4

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FORENSICS
Investigating time of death:
As soon as a person dies, chemical changes occur in order, so can be used to
estimate the time of death.
Rigor mortis/degree of muscle contraction:
Begins when muscle cells become deprived of oxygen
Respiration is anaerobic, causing the build up of lactic acid in muscles
The pH decreases due to the lactic acid, inhibiting enzymes to produce ATP
ATP is required to keep muscles relaxed, so no ATP means the bonds become
fixed, the muscles contract so the body stiffens
Smaller muscles contract first
Begins about 2 ­ 4 hours after death, full effect is about 6 ­ 8 hours. It passes
at about 36 ­ 48 hours after death
The bodies stages in rigor mortis can give a rough outline of time of death
Body temperature:
Metabolic reactions slow down so the core body temperature cools slowly
from around 37oC to room temperature over time.
If the temperature of the room is known, it is possible to create a cooling
curve so discover the time of death
The body reaches room temperature after about 18 hours
Body temperature can be influenced by ­ body size, body position, clothing,
air movement, humidity
Extent of decomposition
After death, tissues break down due to the action of enzymes.
Autolysis occurs ­ the loss of oxygen in tissues favours the growth of anaerobic
bacteria.
Bodies usually follow a standard pattern of decay. Enzymes in the gut start to break
down the wall of the gut and the surrounding area. As cells die, they release enzymes
which help break down tissues.
The signs of decomposition such as decolouration of the skin (putrefication) and gas
formation, combined with environmental conditions allow time of death to be
estimated.
The warmer the environment, the faster the decay
Injuries also allow bacteria to enter, aiding decomposition
After a few hours ­ cells and tissues are broken down by enzymes and
bacteria present before death ­ turns skin green

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Few days-weeks ­ microorganisms decompose tissues and organs ­
produces gasses which cause bloating.…read more

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Forensic entomology:
2 main ways ­ succession and life cycles of insects
Succession:
As a body decays, the populations of insects on it change ­ there is a succession in
species. The community of species present when the body is found allows the stage
of succession to be determined and the time of death estimated.…read more

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Genetic identification
Polymerase Chain Reaction (PCR):
Allows small samples of DNA to be amplified for use during DNA profiling, meaning
that forensics only need small DNA samples such as a strand of hair.
A cycle of temperature changes results in huge numbers of DNA fragments being
produced.…read more

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DNA profiling/DNA fingerprinting:
To identify genetic information
Everyone's DNA is unique because of the variety of the DNA sections not
used to code for proteins (introns)
Look for short repeated sequences in introns (short tandem repeats)
Mini satellites contain 20 ­ 50 base pairs
Micro satellites contain 2 ­ 4 base pairs
Can be several hundred copies of STR at a single locul ­ people vary
The STR at many loci build up a unique pattern for that individual
Double stranded DNA and restriction enzymes.…read more

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Teeth and fillings decay slowly, they are resistant to burning. A forensic dentist makes
a chart of the teeth, including dental work, fillings and missing teeth. This is
compared to dental records. The forensic dentist may also look at the development
of the teeth to determine the age.
Summary of forensics
When: rigor mortis, decomposition, forensic entomology,
temperature
Who: personal ID, finger prints, dental records, DNA profiling
BACTERIA AND VIRUSES
Infectious diseases are mostly caused by bacteria and viruses.…read more

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Conjugation ­ some bacteria can transfer their chromosome to a recipient bacteria
Viruses:
Smallest living organism
No cell wall, cell surface membrane, organelles or cytoplasm
Obligate parasites ­ cannot reproduce without a host.…read more

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TB ­ lung tissue is slowly destroyed by the bacteria
causing symptoms of: breathing problems, coughing up blood in sputum,
suppression of immunity, weight loss, poor appetite, fever and sweats
the TB bacteria also targets cells of the immune system so the patient
cannot fight other infections
HIV causes AIDS which directly targets white blood cells, reducing a patient's
ability to fight infection
The initial symptoms include fever, general weakness and severe coughing,
caused by inflammation of the lungs.…read more

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But as the virus leaves the T cell, it destroys the cell membrane killing the
host cell - this reduces the number of T cells in the body reducing immunity
Difficult to create a vaccine for AIDS because:
virus mutates rapidly
therefore antigens on the viral surface continually change
working on animals to develop vaccines not possible because HIV only
infects humans
the virus hides itself for years inside macrophages, therefore most may
not work properly
Q.…read more

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­ has to produce a
primary response for each new strain
The carbohydrate present in storage granules is glycogen.
BARRIERS TO PATHOGEN ENTRY
Pathogens enter through areas not covered by skin ­ nose, mouth, eyes, gas
exchange surfaces, gastrointestinal tract and genital tract and through wounds.…read more

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