Topic 6 Flashcards
What is base A?
Adenine
What is base T?
Thymine
What is base C?
Cytosine
What is base G?
Guanine
What is a DNA nucleotide made up of?
Deoxyribose sugar, a phosphate and a base
What kind of bonding joins DNA bases?
Hydrogen
What does non-overlapping mean?
Each triplet is read in sequence, separate from those before or after, they don’t share bases
What does degenerate mean?
More possible combinations of triplets than amino acids, so some amino acids are coded for by multiple triplets
How many amino acids are there?
20
Which base is replaced in RNA and what by?
T is replaced by U, Uracil
Differences between DNA and RNA?
- Uracil replaces Thymine
- Ribose not Deoxyribose
- Single stranded not double stranded
Name the sections of DNA that don’t code for amino acids
Introns
Name the sections of DNA that do code for amino acids
Exons
What is the process of removing introns called?
Splicing
How is RNA modified before translation?
Splicing
Where does RNA modification take place?
Inside nucleus
How can more than one protein be formed from a gene?
After splicing, mRNA sections have to rejoin. This can happen in different orders, meaning different amino acid sequences and therefore different proteins
What is a DNA profile?
A genetic fingerprint
How is a DNA profile made?
1) A DNA sample is obtained
2) PCR is used to amplify the DNA
3) A fluorescent tag is added
4) Gel electrophoresis is used to separate DNA
5) Gel viewed under UV light
What is PCR?
Polymerase chain reaction
What happens in PCR?
1) Reaction mixture set up with DNA sample, free nucleotides, primers and DNA polymerase
2) Mixture heated to 95C
3) Mixture cooled to between 50 and 65C
4) Mixture heated to 72*C
5) DNA Polymerase lines up free nucleotides alongside each template strand, forming new complementary strands
6) Two new copies of fragment of DNA formed
7) Cycle starts again
How much does the amount of DNA increase by for each cycle of PCR?
Doubles
What are primers?
Short pieces of DNA that are complementary to the bases at the start of the fragment you want
What is DNA polymerase?
Enzyme that creates new DNA strands
Why is PCR mixture heated to 95*C?
To break the hydrogen bonds between the two strands of DNA
Why is the PCR mixture cooled to 50-65*C?
So that primers can bind (anneal) to the strands
Why is PCR mixture heated to 72*C?
So DNA polymerase can work
How does gel electrophoresis work?
- DNA inserted into well in slab of gel and covered in conductive buffer solution
- Electrical current passed through gel, negatively charged DNA fragments move towards positive electrode
- Shorter fragments move faster and travel further through the gel in a set time, thereby separating fragments according to length
How can time of death be determined?
Body temperature Degree of muscle contraction Forensic entomology Extent of decomposition Stage of succession
How is TOD determined by body temperature?
After death, body temperature falls from 37*C to the temperature of the surroundings
What rate do bodies cool at?
1.5 to 2*C per hour
What affects cooling rate of a body?
Air temperature, clothing, body weight
How is TOD determined by muscle contraction?
Riger mortis starts 4-6 hours after death and it takes different length of time for different muscles to contract
What happens in riger mortis?
- Begins when muscle cells deprived of oxygen
- Anaerobic respiration still takes place, lactic acid builds up
- pH decreases, inhibiting enzymes that produce ATP
- No ATP means bonds between myosin and actin become fixed and the body stiffens
How is TOD determined by forensic entomology?
Different insects at different times, different stages of lifecycle
What is forensic entomology?
Study of the insects that colonise a dead body
What conditions will affect the lifecycle of an insect?
Drugs, humidity, oxygen, temperature
How is TOD determined by extent of decomposition?
- Immediately after death bacteria and enzymes begin to decompose body
- Hours to few days - cells and tissues being broken down, skin turns greenish
- Few days to few weeks - decomposition produces gases which bloat body, skin begins to blister and fall off
- Few weeks - tissues begin to liquify and seep out
- Few months to a few years - only skeleton remains
- Decades to centuries - Skeleton begins to disintegrate
What conditions affect rate of decomposition?
Temperature and oxygen availability
How is TOD determined by stage of succession?
*Types of organism on body change over time
*Bacteria -> Flies and larvae -> Beetles -> Nothing
Affected by ability of insects to get to body
Structure of bacteria
- Few micrometers long
- Plasma membrane
- Cytoplasm
- Ribosomes
- Flagellum sometimes
- Cell wall
- Sometimes mesosomes (folds in membrane)
- Sometimes capsule
- Plasmids sometimes
- No nucleus - one long coiled up chromosome
Structure of viruses
- Smaller than bacteria
- No plasma membrane
- No cytoplasm
- No ribosomes
- Do have nucleic acids - either RNA or DNA
- Protein coat called capsid
- Some have envelope
- Some have proteins in capsid
How can pathogens enter the body?
- Cuts in the skin
- Digestive system
- Respiratory system
- Other mucosal surfaces e.g inside nose, mouth, genitals
What physical barriers prevent infection?
- Stomach acid
- Skin
- Gut and skin flora - compete for space with pathogens
- Lysozyme - produced by mucosal surfaces, kills bacteria by damaging cell walls
What triggers an immune response?
Antigens on the surface of an invading cell are recognised as foreign
What three mechanisms are part of the non-specific immune response?
- Inflammation at the site of infection
- Interferons
- Phagocytosis
What does inflammation at the site of infection do?
- Immune system cells recognise foreign antigens and release molecules that trigger inflammation
- Molecules cause vasodilation, increases blood flow to area
- Also increase permeability of blood vessels
- Increased blood flow brings immune system cells to area, increased permeability allows them into infected tissue
- Immune cells can start to destroy pathogen
Signs of inflammation?
Red, warm, swollen, painful
What does interferons do?
Cells infected by viruses produce proteins called interferons.
Prevent viruses spreading to uninfected cells by:
*Prevent replication by inhibiting production of viral proteins
*Activate cells involved with specific immune response
*Activate other mechanisms of non-specific immune response e.g. promote inflammation
What is a phagocyte?
White blood cell that carries out phagocytosis
How do phagocytes work?
- Recognises antigens on a pathogen
- Phagocyte engulfs pathogen
- Pathogen contained in phagocytic vacuole in cytoplasm
- Lysosome fuses with vacuole, enzymes break down pathogen
- Phagocyte presents pathogen’s antigens to activate other immune cells- called antigen presenting cell
What is a T cell?
A type of white blood cell which surface is covered in receptors
What do the receptors on T cells do?
Bind to antigens presented by the phagocytes
What do receptors/antigens have to be to bind?
Complimentary
What happens to the T cell when an antigen binds?
It is activated
What happens when a T cell is activated?
It divides and differentiates into different types of T cells that have different functions
What different types of T cell are produced when a T cell is activated?
- T helper cells
- T killer cells
- T memory cells
What is the function of T helper cells?
Release substances to activated B cells, T killer cells and macrophages
What is the function of T killer cells?
Attach to antigens on a pathogen-infected cell and kill the cell
What is the function of T memory cells?
Gives immunity
How to T helper cells activate B cells?
Antibodies bind to antigens when complimentary
T helper cell releases substances that combined with the binding activate the B cell
What are B cells?
A type of white blood cell covered with antibodies
What are antibodies?
Proteins that bind to antigens to for antigen-antibody complexes
What happens to an activated B cell?
Divides by mitosis into plasma cells and B memory cells
What is the other name for plasma cells?
B effector cells
What is the other name for B effector cells?
Plasma cells
What are plasma cells?
Clones of B cells (identical) that secret large quantities of antibody into the blood
What is the structure of an antibody?
- Variable region - this is the antigen binding site - shape is complimentary to a specific antigen
- Hinge region allows flexibility when the antibody binds to the antigen
- Constant regions allow binding to receptors on immune system cells - same in all antibodies
- Disulfide bridges hold the polypeptide chains together
How do antibodies help to clear an infection?
1) Agglutinating pathogens
2) Neutralising toxins
3) Preventing the pathogen binding to human cells
How does agglutinating pathogens help clear an infection?
Each antibody has two binding sites, so it can bind to two pathogens at once. Pathogens can become clumped together. Phagocytes bind to antibodues and phagocytose many pathogens at once
How does neutralising toxins help to clear an infection?
Antibodies bind to toxins produced by pathogens, preventing them affecting human cells. There are therefore neutralised. Toxin-antibody complexes are phagocytosed.
How does preventing the pathogen binding to human cells help to clear an infection?
Antibodies bind to the antigens on pathogens, whereby they may block cell surface receptors that pathogens need to bind to host cells. Therefore the pathogen cannot attach and infect host cells
What is primary response?
Activation of the immune system in response to an antigen present for the first time
What do memory cells do in general?
Remain in the body for a long time after infection
What do B memory cells do?
Record specific antibodies needed to bind to a specific antigen. Divide into plasma cells that produce the required antibody
What do T memory cells do?
Remember and recognise specific antigens. Divide into correct type of T cells to kill the cell carrying the antigen.
What is secondary response?
Response to an antigen that has been present before. Quicker and stronger than primary response.
How does specific immune response progress?
- Phagocytes activate T cells by binding antigens to receptors on the T cell surface
- T cells divide and differentiate
- T helper cells activate B cells
- T killer cells attach to antigens on pathogen-infected cells and kill them
- B cells produce plasma cells and B memory cells
- Plasma cells produce antibodies
- Memory cells remain for a long time
What is active immunity?
Immunity acquired when the body makes its own antibodies
What are the two sources of active immunity?
- Natural - by catching a disease
* Artificial - vaccination
What is passive immunity?
Immunity acquired by being given antibodies produced by another organism
What are the two sources of passive immunity?
- Natural - baby recieving antibodies from mother via placenta and breast milk
- Artificial - Being injected with antibodies e.g. tetanus shot
What is an evolutionary race?
Organisms evolving in order to outdo each other
What are the evasion mechanisms of the HIV virus?
- Kills immune system cells it infects, reducing no. of immune system cells
- High rate of mutation in antigen coding genes
- Memory cells don’t recognise new strains
- Disrupts antigen presentation in infected cells, preventing immune system recognising and killing infected cells
What is antigenic variation?
High rate of mutation in antigen coding genes means structure changes frequently, producing new strains
What causes TB?
Mycobacterium tuberculosis
What are the TB evasion mechanisms?
- TB bacteria infect lungs and are engulfed by phagocytes
- Produce substances that prevent lysosome fusing with phagocytic vacuole so bacteria aren’t broken down
- Multiply undetected inside phagocytes
- Disrupts anitgen presentation, prevents immune system recognising and killing infected phagocytes
What are the two types of antibiotic?
Bacteriocidal and bacteriostatic
What do bacteriocidal antibiotics do?
Kill bacteria
What do bacteriostatic antibiotics do?
Prevent bacteria growing
How do antibiotics work?
Inhibit bacterial metabolism
In what two ways can antibiotics inhibit bacterial metabolism?
1) Inhibit enzymes needed to make chemical bonds in bacterial cell walls, so can’t grow properly. Can lead to cell death as weakened walls can’t take pressure as water moves in by osmosis so bursts
2) Inhibit protein production by binding to ribsomes. Can’t make enzymes, so can’t carry out processes vital for growth and development
How do you investigate the effect of different antibiotics on bacteria?
1) Bacteria spread on agar plate
2) Paper disks soaked in antibiotic placed on plate at intervals, various concs used including control of distilled water
3) Use aseptic technique for these steps
4) Incubate at 25-30*C for 24-36 hours
5) Inhibition zone forms where bacteria can’t grow
6) Larger inhibition zone shows more effective antibiotic
What are HAIs?
Hospital Acquired Infections
How are HAIs transmitted?
Poor hygeine e.g. lack of hand washing, coughs not contained in tissue, equipment not properly dissinfected after use
What codes of practice have been introuced to control HAIs?
- Hand washing encouraged before and after patient contact for staff and visitors
- Equipment and surfaces dissinfected after use
- People with HAIs moved to isolation ward
What codes of practice have been produced relating to antibiotic use?
- Not prescribed for minor bacterial or viral infections
- Shouldn’t be prescribed to prevent infection
- Narrow-spectrum antibiotics used where possile
- Rotate use of different antibiotics
- Patients should take full course
What are narrow-specturm anitibiotics?
Only affect a specific bacterium
What is an antisense strand?
Template stand for DNA transcription
Role of codons on mRNA?
Complimentary to anticodons on tRNA
Role of anticodons on tRNA?
Complimentary to codons on mRNA
