Topic 6: Immunity, Infection and Forensics Flashcards
What is Polymerase Chain Reaction (PCR)?
PCR is controlled DNA replication PCR specifically targets and amplifies a single sequence from w in a complex mixture of DNA
What is Taq polymerase?
Thermus aquaticus DNA polymerase, aka Taq polymerase is NOT permanently destroyed at 94 degrees C. The optimal temp is 72 degrees. This is used in PCR
Describe the process of PCR
Mix the double stranded DNA sample, primers, free nucleotides and DNA polymerase. Heat the mixture to 95 degrees to denature the DNA, separating the 2 strands.
Cool to 55 degrees to allow primers to anneal (hybridise/bind) to the strands. Increase to about 70 degrees (optimum for Taq polymerase).
The Taq polymerase forms comp base pairs w each DNA sample strand using the free nucleotides. This produces TWO double stranded DNA samples. This cycle is repeated around 30 times and gives rise to an amount of DNA sufficient to create a DNA profile.
What happens to cells after death?
After death autolysis first occurs, where lysosomes breakdown cells. Enzymes break down tissues.
Anaerobic bacteria part of the body’s normal flora thrive in the lactic acid rich environments of the muscles. As enzymes break down cells bacteria spread.
Give the signs of decomposition
Putrefaction is a greenish discolouration of the skin on the abdomen caused by formation of sulfhaemoglobin. Discolouration spreads, then darkens to red to black.
Bacteria produce gases resulting in blisters and bloating. In a temperate climate discolouration of the abdominal wall occurs 36-72 hrs after death.
What factors affect decomposition rate?
Low temps slow decomposition rates, warm temps increase decomp rates. Decomposition rate is highest between 21-38°C.
Intense heat denatures enzymes involved in autolysis, delaying the start of decay.
Injuries to the body allow entry of bacteria that aid decomposition.
Give and explain features some bacteria have
Flagella enabling the cell to swim
Slime capsule= slimy surface layer to protect and prevent dehydration, allows bacteria to form colonies.
Pili= thin protein tubes allow bacteria to adhere to surfaces. Involved in conjugation
What are viruses and why are they classified as living?
Viruses are the smallest of microorganisms, 0.02-0.3 micro meters across.
Viruses are arrangements of genetic material w a protein capsid. Viruses are classed as obligate intracellular parasites.
Viruses invade other living cells and hijack their biochemistry to make more viruses. For this reason they are classified as living.
Describe the structure of viruses
Viruses are usually geometric shapes w variation. The protein capsid is made of repeating units called capsomeres.
Capsomeres minimise the amount of genetic material needed to code for coat production and ensures simplicity when assembling the protein coat in the host cell.
Some viruses are also covered by a lipid envelope produced from the host cell. This makes it easier for the viruses to pass from cell to cell but makes them vulnerable to substances which will dissolve the lipid membrane.
What are the 2 types of viral genetic material?
Viral genetic material can be double stranded DNA or single stranded RNA. Viral DNA like in smallpox or adenovirus directly synthesises viral proteins.
Viral RNA synthesises reverse transcriptase which makes proviral DNA. This can be inserted into the host genome using integrase. This DNA is transcribed and translated to produce viral protiens.
Eg= tobacco mosaic virus and HIV.
How do viruses attach to their host cells?
Viruses attach to their hosts cells by antigens called virus attachment particles (VAPs) which target particular proteins in the host cell membrane.
Therefore viruses are often specific in the tissue they attack.
Describe the lysogenic pathway
Viral DNA is inserted into the host DNA to form a provirus.
Viral DNA recombines w host cell chromosome.
The cell carrying the viral genome (aka prophage) divides normally. The virus is dormant and non virulent.
It can switch to the lytic pathway
Describe the lytic pathway.
Viral genetic material is replicated independently of the host DNA. The virus is virulent.
Virus attaches to host cell and inserts its nucleic acid. Viral nucleic acids replicate.
DNA is packaged to make new viral particles. Viral protein coats are synthesised.
New viruses are made and the host cell bursts in lysis, releasing many viruses to infect more cells.
Describe TB
Caused by mycobacterium tuberculosis. It is a robust bacterium. Droplet infection from mucus and saliva.
Arises from overcrowding, poor health, poor diet and close contact w infected person. Pulmonary TB which is highly contagious and infects the lungs is most common.
Improved housing, hygiene and antibiotics saw decline in TB cases. 2 phases to TB: phase 1 is the primary infection and phase 2 is the secondary infection.
Describe HIV
HIV is less robust, it can only be passed on through bodily fluids (sharing needles/unprotected sex). The virus can enter the bloodstream of a partner through breaks in the skin or lesions caused by other infections- usually other STIs. Condoms prevent this.
Direct blood to blood transfer can occur through cuts and grazes. Maternal transmission can occur from mother to unborn child or through breast milk.
The risk occurs in the last few weeks of pregnancy mostly around the birth itself when mingling of infant and maternal blood is likely. Taking anti HIV drugs in the last 3 months of pregnancy and giving birth by C section reduces this risk.
Describe various non specific immune responses
Lysosome in eyes, saliva and nasal secretions breaks down bacterial cell walls. HCl in stomach and acidic vaginal secretions kill bacteria.
Instantly vomiting after consuming dodgy food rapidly expels pathogens from the body
Interferon proteins inhibit microbial ps
Blood clots rapidly seal cuts.
Describe and explain the types of white cells
Types of white blood cells: Neutrophils leave blood capillaries by squeezing between capillary cell walls. They ingest and destroy bacteria. They last only for a few days.
Lymphocytes: circulate in the blood and lymph and gather around infection sites. Involved in specific response. B and T cells identify and respond to foreign antigens.
Monocytes: Circulate in the blood for a day before moving into tissue by squeezing between capillary cell walls. Here they become macrophages. Found in the lungs, liver, kidney etc
Others: produce histamine involved in the inflammatory response
What happens at a site of infection?
Pathogens and cells damaged at the infection site release chemicals that attract phagocytes. Neutrophils arrive first, followed by longer lasting macrophages. They ingest debris from damaged cells and other foreign matter.
Ingested material is stored in a vesicle and fused w lysosomes that release lysozymes to destroy foreign material.
Tras a few days, the infection site is full of dead cells (mainly neutrophils) to form pus. This may break through the skin surface or get broken down and absorbed by surrounding tissue.
What is the lymphatic system and how does it work?
The lymphatic system: some bacteria that get carried away by the blood or lymph are stopped by macrophages in lymph nodes, spleen and liver.
Tissue fluid drains into lymphatic vessels. The fluid (lymph) flows along lymph vessels and nodes and then returns to the blood via lymphatic and thoracic ducts.
As lymph passes through lymph nodes any present pathogens activate lymphocytes and macrophages which destroy them. Failure of the lymphatic system leads to septic shock
What are interferons?
Interferons provide non-specific defence against viruses, some bacteria and protozoa.
Microbe infected cells produce the interferon protein; it diffuses to surrounding cells where it prevents microbes from multiplying as it inhibits microbial protein synthesis.
Interferons prevent viruses from attaching to host cells by binding to receptors. Therefore the virus cannot infect and replicate inside the host cell.
Describe B cells
B cells secrete antibodies (a class of immunoglobin protein molecules) which bind to foreign antigens. Like labels, they allow phagocytes to recognise and destroy the cell.
Each B-cell produces ONE type of antibody specific to ONE antigen. B cell receptors can also directly bind to a comp antigen. Microbes usually have many diff antigens on its surface, so each antigen type activates and binds different B cells.
B cells are capable of phagocytosis, antigen presentation, and load antigens on major histocompatibility complexes to display to T cells.
Describe antibodies, and draw an antibody structure
Antibodies are just the B cell receptor in a secreted form, so they can circulate freely in the blood - attaching to pathogens and tagging them for destruction, thus enhancing phagocytosis.
Describe the structure of the HIV virus
HIV is a retrovirus so its nucleic acid is RNA.
Its RNA is surrounded by a 20 sided (icosahedral) protein capsid enclosed in a layer of viral protein.
A lipid envelope is formed from the host cell membrane when new viral HIV particles emerge from the cytoplasm. Viral glycoproteins (Gp120) stick out of the envelope.
