Topic 6 Revision Notes
Transcript of Topic 6 Revision Notes
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Topic 6
INFECTION, IMMUNITY AND INFECTION
Fingerprint methods are used to identify a dead body with no identification papers
on them. Fingerprints are small ridges caused by folds in the epidermis of the skin.
Sweat and oil leave impressions on surfaces we touch. Oils are secreted from
sebaceous glands (non on palms/fingers) are and transferred to fingers when
touching other parts of our body. There are 4 types of finger prints: Arch, Tented
arch, whorl, loop. This is known as the Henry Classification. Fingerprints are made
visible by using fine aluminium, iron or carbon powders; using superglue; using
ninhydrin; magnets and iron flakes are sometimes used. The police have a national
database of biomedical information called IDENT1. Everyone who is arrested has
their prints taken. Dental records are used if someone has no fingerprints on file or
their body has been burnt. This is because teeth and fillings are more resistant to
burning and decay slowly.
DNA profiling relies on the fact that everyones DNA is unique (apart from identical
twins). The non-coding blocks on DNA are called introns and the coding blocks are
called extrons. In introns there are sequences of repeated bases known as short
tandem repeats (STRs) or satellites. The same STRs occur at the same place (locus)
on both chromosomes of a homologous pair. The number of repeats on eachhomologous pair can be different which causes variation in individuals. E.G.
A DNA profile is made by
cutting up DNA, separating
the fragments and then
comparing it to some
reference (e.g. a suspect, a
relative of the corpse).
To obtain a DNA sample
take any tissue sample
(blood, cheek cells, semen,
bone). Physically break it
down in a buffer solution that includes salt and a detergent to disrupt the cell
membranes. The DNA is separated from the rest of the cell debris by filtering or
centrifuging. Protease enzymes remove proteins, and then cold ethanol is added to
precipitate out the DNA. Washing with buffer solution then follows.
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Restriction enzymes (restriction endonucleases) will only cut DNA at specific base
sequences. The short tandem repeats remain intact but it will be cut away from the
rest of the genome. They are found in bacteria; they protect themselves by
changing the bases in their own sequences that are targeted by their own restriction
enzymes.
Polymerase Chain Reaction allows scientists to use tiny amounts of DNA, which is
copied numerous times. It uses DNA Primers which are short DNA sequences
complementary to the DNA adjacent to the STR. The DNA primers are marked with a
fluorescent tag. The forensic sample is placed in a reaction tube with DNA
polymerase, DNA primers and nucleotides .Once in the PCR machine, the tube
undergoes a cycle of temperature changes. The first separates the double stranded
DNA. The second temperature optimises prime binding to the target DNA sequence
in the sample. The polymerase attaches and replication occurs. The final
temperature is the optimum temperature for the heat stable DNA polymerase.
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DNA fragments can be separated by gel electrophoresis according to their size. The
gel (agarose or polyacrylamide) is submerged in a buffer solution and connected to
electrodes. The negatively charged DNA fragments move across the solution with
the small fragments moving further and faster (closer to the positive electrode).
Southern blotting is used to transfer the fragments to a more resilient nylon or
nitrocellulose. The membrane is placed on top with a wad of dry absorbent paper
on top, which draws up the buffer solution carrying the DNA fragments onto the
membrane. The membrane is placed in a bag with DNA probe. Single-stranded DNA
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probe binds to fragments with a complementary sequence. If the DNA probe is
radioactive, X-ray film is used to detect the fragments. If the DNA probe is
fluorescent it is viewed using UV light.
The STR is inherited like genes so are used for identification purposes.
Determining Time of Death
The temperature of the body, the degree of rigor mortis and the state of
decomposition can be used to estimate time of death. In addition,
entomological (insect) evidence can provide further clues about when the
person died.
Body Temperature: Core temperature in a human is usually 36.2 to 37.6o
c,but when a person dies the body cools down due to no heat producing
reactions taking place. Temperature is measured through the rectum or an
abdominal stab wound, with a long thermometer (normal ones are too short
and have a lower temperature range). However, environmental conditions
must be noted as they can change the rate at which the body cools. The
cooling of a body follows a sigmoid curve which shows that normal body
temperature lasts for 30-60 mins after death. But if the person had a feveror had hypothermia then the body temperature would not be normal.
Many factors will effect post-mortem cooling including; body size, body
position, clothing, air movement, humidity, surrounding temperature. For
example if a body is in water it will cool much quicker than on land as water
is a better conductor of heat than air.
Rigor Mortis: After death muscles totally relax and then stiffen again, this is
rigor mortis (stiffness of death). After a period of time the muscles become
relaxed again. The sequence in which this happens is
1) after death, muscle cells become starved of oxygen, and oxygen-
dependant reactions stop.
2) Respiration in the cells becomes anaerobic and produces lactic acid.
3) The pH of cells fall, inhibiting enzymes and thus inhibiting anaerobic
respiration.
4) The ATP needed for muscle contraction is no longer produced. AS a result,
bonds between the muscle proteins become fixed.
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5) The proteins can no longer move over one another to shorten the muscle,
fixing the muscles and joints.
Smaller joints stiffen before larger ones. Rigor mortis passes off as muscle
tissue starts to break down, in the same order in which it developed.Decomposition: After death, tissues start to break down due to the action of
enzymes. Autolysis occurs first, which is where the bodys own enzymes
from the digestive track, break down cells.
First sign is a greenish colour (discolouration) of the skin, in the lower
abdomen. Due to the formation of sulphaemoglobin in the blood.
Due to action of bacteria, gases including hydrogen sulphide, methane,
carbon dioxide, ammonia, hydrogen form in the intestines and tissues. Thiscauses the body to smell and become bloated, which deflates as further
tissue decomposes and gas is released.
Decomposition varies but in average conditions, discolouration occurs 36-72
hours after death. Gas formation about 1 week.
Low temperatures means low decomposition rate. Warm temperatures speed
up decomposition.
Injuries to the body allow the entry of bacteria that aid decomposition.Forensic Entomology
The presence of insects allows forensic entomologists to make an estimate
of how much time has elapsed since death.
The temperature of the air, ground, body and maggot mass are measured in
order that the rate of maggot development can be determined.
For the commonest bluebottle species found on bodies, Calliphora Vicina, a
graph is seen to determine age. (only used if temperature conditions havestayed constant).
Fly lifecycle... egg: 1 day. Lava: 9 days. Pupa: 6-12 days.
Other factors e.g. toxins in the body will affect the results cocaine would
accelerate development.
There is succession on a dead body. Normally eggs are laid in wounds or at
openings to the body (mouth or nose) .
The season, weather conditions, size and location of the body will allinfluence the type and number of species present.
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Cause of Death
A post-mortem may be done. This is an internal and external examination of
the body to help determine cause of death.
Viruses consist of a strand of nucleic acid (RNA or DNA) enclosed in a protein
coat. Some viruses can have an outer envelope taken from the host cells
surface membrane. They have glycoproteins from the virus itself. These are
antigens, molecules recognised by the hosts immune system as not beingits own self.
Viruses lack some internal structures required for growth and reproduction.
This means that they have to enter the hostscells and use the hosts
metabolic systems to make new viruses. After reproducing inside the hosts
cells, new virus particles may bud from the cell surface or burst out of the
cell splitting it open. This splitting kills the cell and is called lysis.
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Transmission of TB bacterium
It is carried in the droplets of mucus and saliva released into the air when an
infected person talks, coughs or sneezes. This is known as a dropletinfection. The droplets can stay in the air for several hours and as dust for
several weeks making bedclothes potentially dangerous. Close contact, poor
health, poor diet and overcrowding living conditions increase the risk of
developing the disease.
Transmission of HIV
It cant survive outside the body. It can be passes on by bodily fluids but not
saliva or urine. For infection to occur, the body fluids have to be transferreddirectly into the body of the nest host (sharing needles while taking drugs,
unprotected sex, blood to blood transfer through cuts and grazes, maternal
transfer from mother to child or in breast milk).
The Bodys Response to Infection
Non-specific responses help to destroy any invading pathogens, whereas
specific immunity is always directed at a specific pathogen.
If dust lands in your eye, tears appear. These contain lysosome that killsbacteria by breaking down their cell walls. The same enzyme is found in
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saliva and nasal secretions.
An injury enables microbes to enter the body. The inflammatory response
destroys these foreign organisms. Damaged white blood cells and mast cells
releases special chemicals such as histamines, causing the arterioles todilate, increasing blood flow to the area. Plasma fluid, white blood cells and
antibodies leak from the blood causing oedema (swelling) the microbes can
now be attacked by intact white blood cells.
Phagocytosis
Phagocytes are white blood cells that engulf bacteria and other foreign
pathogens. They include both neutrophils and macrophages.
Neutrophils = 70% of WBC leave blood capillaries by squeezing betweenthe cells of capillary walls.
Monocytes = Circulate in the blood for a day or two before they move into
the tissue by squeezing between the cells of the capillary walls. Here they
become macrophages and engulf bacteria, foreign matter and cell debris.
Neutrophils are first to arrive (5-20 bacteria) followed by macrophages (up
to 100). The ingested material is enclosed within a vacuole. Lysosomes
containing digestive enzymes fuse with the vacuole the enzymes arereleased and destroyed.
Sometimes bacteria get carried away. The lymphatic system tries to stop this.
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Interferon provides non-specific defence against viruses.
Lymphocytes are white blood cells that help to defend the body against
specific diseases. They circulate in the blood and lymph and gather at the
site of infection.
B and T cells (Lymphocytes)
This is the specific immune response.
B cells = secrete antibodies to antigens. Antibodies are known as
immunoglobulins (acts as labels). B cells are specific. Produced in bone
marrow.
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T cells + produced in bone marrow but
mature in the thymus gland. They are
specific.
There are 2 types of T cell. T helper cells(stimulate B cells to divide, also enhance
phagocyte activity). T killer cells = destroy
anything with an antigen on its surface
membrane (could include transplanted
tissue).
When foreign material is engulfed by a
macrophage, an antigen is added to the cellsurface membrane. Macrophages displaying
non-self peptides are antigen-presenting
cells (APCs).
A T helper cell with a complementary shape
(called a CD4 receptor) binds to the antigen.
This binding activates T helper cells to divide
and produce moreT helper cells and a
clone of T memory
cells, which stays in
the body for years
in case the
pathogen enters
the body again.
Cloning of B cells
Antigen-presenting
B cells bind to a T
helper cell which
release a chemical
called cytokines.
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These stimulate division and differentiation of B cells into 2 types.
B effector cells differentiate into plasma cells (release antibodies into the
blood and lymph).
B memory cells long lives, enable body to respond more quickly to thesame antigen in the future.
The process of B cell division is called Clonal Selection. It takes about 10-17
days to produce sufficient antibodies called the primary immune response.
T killer cells bind to the anti. It divides to form a clone (stimulated by
cytokines). T killer cells release enzymes that create pores in the membrane
of the infected cell which allows water and ions into it. It swells and burst
and the pathogens in the cell are released so they can be labelled byantibodies for destruction by macrophages.
The secondary immune response
Involves memory cells. B memory cells produce
plasma cells immediately and release antibodies.
There is a greater production of antibodies and
it lasts longer.
Membrane proteins on the surface of our cells
act as bar codes and mark the cell as self.
Any lymphocytes with self membrane proteins
are destroyed by apoptosis (programmed cell
death) only lymphocytes with receptors for
foreign antigens remain.
Tuberculosis
TB is caused by the bacterium Mycobacterium
tuberculosis. Infection may occur when the
bacteria are inhaled and lodge in the lungs.
There are 2 phases of the disease
Primary Infection: The immune system responds
by an inflammatory response. In a healthy person, macrophages engulf thebacteria. A mass of tissue known as a granuloma forms. These are anaerobic
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and have dead bacteria and macrophages in the middle. They are called
tubercules. After 3-8 weeks it is controlled and the infected area heels over.
Invading the immune system
M.tuberculosis can survive inside macrophages, resisting killing mechanismsby having a thick waxy cell wall which makes them difficult to break down
and also hides their antigens from other macrophages. They also reproduce
inside the macrophage.
They can be dormant for years and also target immune system cells. TB
bacterium can suppress T cells, reducing antibody production and attach
killer cells.
Active Tuberculosis (2nd phase)If patients immune systemscant contain the diseases when it first arrives
or an old infection may break out if the immune system isnt working
properly.
Activity of immune system is reduced in old and very young ages, also by
malnutrition and poor living conditions.
Most significant factor is AIDS. HIV the virus that causes AIDS directly targets
white blood cells and reduces patients ability to fight infection.Bacteria in lungs destroy tissues creating holes and cavities. The lung
damage will eventually kill the sufferer.
Symptoms of active TB
Coughing (blood sometimes)
Shortness of breath
Loss of appetite and weight
Fever and extreme fatigueRole of Fever
Fever occurs because of inflammatory response causes fever-causing
chemicals to be released from neutrophils and macrophages. The chemicals
affect the hypothalamus and alter core body temperature by the use of
effectors.
Raised temperature is thought to increase immune function and
phagocytosis while also making it harder for bacteria to reproduce.Glandular TB
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TB can move to other parts of the body.
Main sites are the bones, lymph glands and central nervous system.
enlarged lymph glands are signs of glandular TB (neck or armpits)
DiagnosisA history is taken from the patient of their symptoms.
Skin and blood tests: Tuberculin (extracts from several species of
Mycobacteria) is injected under the skin. A positive result shows inflamed
skin showing antibodies are already present.
This can be wrong so a blood test is done to find specific T cells.
Identification: a sample of sputum is coughed up and cultured. Using
staining techniques different bacteria can be identified.X rays can also be used on the lungs to discovered extent of the disease.
Treatment is usually antibiotics and a better lifestyle.
AIDS (acquired immune deficiency syndrome is caused by infection with the
human immunodeficiency virus, HIV. A syndrome is a collection of symptoms
related to the same cause. HIV is an example of an enveloped virus, with the
envelope coming from the hosts cell membrane.
Particular glycoprotein molecules called gp120 (located on the virus surface)
bind to the CD4 receptors on the T helper cells. They fuse with another
gp120 receptor which enables the envelope surrounding the virus to fuse
with the T helper cell membrane, allowing the viral RNA to enter the cell.
(can also happen with macrophages).
Once inside the T helper cell the virus needs to make viral components. This
means making DNA from the virus RNA. They do this by using an enzyme
called reverse transcriptase. Viruses that contain RNA and use reverse
transcriptase in this way are known as retroviruses. Once the HIV DNA
strand is produced, it is integrated into the hosts DNA by another enzyme,
integrase. Once the HIV genome is integrated into the hosts cell genome it
can be transcribed and translated to produce new viral proteins.
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DNA in the nucleus unwinds its double helix to form a sense strand and an
antisense strand. Free nucleotides in the nucleus line up on the antisense
strand, due to complementary base pairing, making a mRNA strand. It then
travels out of a nuclear pore to the ribosomes. This is transcription.Once on a ribosome, the mRNA is read in codons (3 base pairs make a
codon). For each codon there is a complementary anticodon on a tRNA
molecule. Each anticodon codes for a specific amino acid which joins
together in the ribosome by a peptide bond.
Between transcription and translation mRNA s often edited with the introns
(non-coding sections of DNA) being removed. This means that severalproteins can formed from one length of mRNA if it is spliced in different
ways.
The new virus is assembled and bud out of the T cell taking some of the cell
surface membrane with it and killing the cell as it leaves.
Infected T helper cells will be destroyed by T killer cells. As the number of T
helper cells decrease, macrophages, B cells and T killer cells are notsuccessfully activated an the immune system therefore doesnt work
properly.