topic 6 revision notes

7/31/2019 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.

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