Topic 6

Question 1

6.1 How to determine the time of death of a mammal?

Answer 1

-extent of decomposition
-stage of succession
-forensic entomology
-body temperature
-degree of muscle contraction

Question 2

Decomposition

Answer 2

• The breakdown of tissues after death due to microorganisms and enzyme activity
  -Signs include -Putrefaction (green colouration of the skin), Gas/liquid blisters appearing on the skin, unpleasant odour and body bloating.
  -The rate of decomposition can be affected by the availability of oxygen and the surrounding temperature.
  -Slower at anaerobic conditions and lower temperatures and faster at aerobic conditions and higher temperatures.

Question 3

Stage of Succession

Answer 3

-Refers to the change in types of organisms found in a habitat over time. (habitat = dead body)
-Bacteria will be found on the body directly after time of death. Flies will start laying eggs, larvae hatch and eat the soft tissue, Beatles start appearing.
-Flies and larvae prefer a wet/moisture-rich environment.
-No tissue remaining, conditions are no longer favourable for most organisms
-SOS will differ depending on where the body is located as the accessibility of insects and availability of oxygen will be affected eg…. Buried in soil, buried in a coffin, under water.

Question 4

Body temperature

Answer 4

Respiration and other metabolic processes produce heat in living organisms.
When a person dies…
- metabolic reactions will come to an end
- body temperature drops until it reaches the temp of the surrounding environment (algor mortis)
- decreases by 1.5-2.0 degrees Celsius per hour
- Air temperature, surface area:volume ratio and presence of clothing will affect the rate at which the body heat is lost.

Question 5

Degree of muscle contraction

Answer 5

Muscles start to contract about 4-6 hours after TOD leading to a general stiffening of the body (rigor mortis)
Happens between 12-18 hours - wears off between 24-36 hours after TOD
Rigor mortis
- Muscles receive no oxygen, respire anaerobically producing lactic acid.
- Lactic acid decreases the pH, denaturing the enzymes that produce ATP.
- Myosin heads cannot be released from the actin filaments locking the muscles in a contracted state leading to the body stiffening.
Rigor mortis begins in smaller muscles and ends in larger muscles.
Level of muscle development and temperature of the surroundings affect rigor mortis.

Question 6

6.2 Know the role of micro - organisms in the decomposition of organic matter and the recycling of carbon.

Answer 6

• Dead plants and animals are broken down by microorganisms e.g. bacteria and fungi.
• Microorganism secrete enzymes which break large organic molecules into smaller molecules.
• These small molecules such as glucose can be broken down further during respiration.
• Micro organisms involved in decomposition produce methane CH4 and CO2 carbon dioxide released back into the atmosphere.
• Carbon dioxide can then be absorbed by green plants which can fix the carbon back into carbohydrates during photosynthesis.

Question 7

6.3 Know how DNA profiling is used for identification and determining genetic relationships between organisms (plants and animals)

Answer 7

DNA PROFILE
- very useful in forensic science as it provides a way to identify individuals
- can be used to determine the genetic relationships between different organisms e.g.
—> paternity and maternity testing
—> ancestry kits
—> determining evolutionary relationships between different species.

Question 8

6.4 Know how DNA can be amplified using the polymerase chain reaction (PCR)

Answer 8

PCR is a common molecular biology technique used in most applications of gene technology e.g.
- DNA profiling
- Genetic engineering
Produces many copies of a piece of DNA ; this can be referred to as DNA amplification.
1. Sample is taken and DNA is extracted.
2. Primers, DNA nucleotides and DNA are added. DENATURATION
3. Sample is raised to 90c to separate the DNA strands. (breaks the hydrogen bonds that hold the two DNA strands together) ANNEALING
4. Sample is lowered to 55c to allow the primers to bind - (primers can anneal to the ends of the single strands of DNA)
5. Sample is raised to 70c to allow DNA polymerase to replicate the strands. ELONGATION/EXTENSION
6. Repeats many times making millions of copies of a small sample of DNA
Each PCR cycle doubles the amount of DNA

Question 9

The genetic relationship between these two species of grey tree frog has been studied using DNA profiling (DNA fingerprinting).
A small sample of DNA was taken from each species of grey tree frog. This DNA was amplified, fragmented and used to produce a DNA profile (DNA fingerprint) for each species.

Describe how a DNA profile was produced from this small sample of DNA? (6 marks)

Answer 9

• Multiple copies of DNA made
• Using PCR/polymerase chain reaction
• Use of primers/DNA polymerase/nucleotides, many repetitions
• restriction enzymes produce DNA fragments
• gel electrolysis
• load the DNA onto the gel (agar gel)
• electric current is applied
• use of fluorescent tag/UV light

Question 10

How to compare DNA profiles? (3 marks)

Answer 10

• compare total numbers of bands
• compare position of bands
• comparing size/width of bands

Question 11

Gel electrophoresis CORMS

Answer 11

• Multiply DNA using PCR
• Make fragments using restriction enzymes to cut the DNA at specific
sequences
• Pour buffer solution over the gel
• Add the DNA fragments and fluorescent markers to the sample wells using a pipette loading
• Apply a current across the gel
• DNA is negatively charged due to the phosphate in its backbone
• So moves towards the positive electrode
• Compare samples, more similarities indicate more closely related

Question 12

EXAMPLE - A deer was found dead on National Trust land. Some people thought that the wounds that led to the deer’s death could have been caused by a big cat such as a black panther.
The DNA produced by PCR was analysed to find out if a black panther was involved.

Explain how gel electrophoresis could be used to find out if this DNA came from a black panther. (5 marks)

Answer 12

• detail of loading of electrophoresis tank e.g. use of agarose gel, use of a buffer, sample placed in wells.
• current/potential difference applied across the gel
• use of gene probe/DNA stain e.g. methylene blue.
• use of STRs/DNA of black panther
• compare bands/DNA profiles
• a match would indicate that DNA from a black panther was present.

Question 13

How to create a DNA profile?

Answer 13

• isolating a sample of DNA e.g from saliva, skin, hair or blood.
• producing more copies of the DNA fragments in the sample using the polymerase chain reaction (PCR).
• The fragments are separated and visualised using gel electrophoresis.

Question 14

What are DNA profiles and how can they be useful?

Answer 14

• Can be used to determine the genetic relationships between people e.g. in paternity tests
• Can be useful in selective or captive breeding programmes of animals or cultivation of plants.
• DNA profiles can be compared to determine relationships in paternity testing.

Question 15

Devise an investigation to determine the optimum number of cycles for the polymerase chain reaction used to amplify the DNA for this test.

Answer 15

• DNA, polymerase, primers (appropriate reagent to be provided)
• Temperatures used are 90, 55 and 70.
• Change the number of cycles.
• Use gel electrophoresis to determine the quantity of DNA produced.
• Choose the smallest number of cycles that produces an observable band.

Question 16

6.5 Be able to compare the structure of bacteria and viruses.

Answer 16

• Bacteria have DNA, Viruses have DNA and RNA.
• Bacteria have circular genetic material, Viruses have linear/straight.
• Bacterial DNA is double stranded, Viral DNA/RNA is single or double stranded
• Bacteria may have plasmids, Viruses do not.

Question 17

6.6 Understand how Mycobacterium tuberculosis (TB) infect human cells, causing a sequence of symptoms that may result in death.

Answer 17

• Inhale Mycobacterium tuberculosis.
• Macrophages ingest the bacterium but cannot digest it due to the thick waxy layer.
• Tubercles form in response to the infection – immune system cells surrounded by a fibrous membrane.
• Some Mycobacterium tuberculosis may survive at the center of the tubercles & lie dormant for years.
• When the immune system is compromised the bacteria multiply rapidly and destroy the lung tissue.
• In too high numbers for the immune system to be able to cope.
• They may also travel in the blood to other organs and destroy those too.

The active TB bacteria can inhibit T-cells
T-helper aren’t activated to produce cytokines
So B-cells aren’t activated to produce antibodies
T-killer cells aren’t activated to kill infected cells
Weakened immune system leads to opportunistic infections.

Question 18

6.6 Understand how Human Immunodeficiency virus (HIV) infects human cells, causing a sequence of symptoms that may result in death.

Answer 18

• Glycoproteins on the surface of the virus
• bind to the CD4 receptors on the surface of the T helper cells
• Viral envelope fuses with cell membrane of T helper cell
• Viral RNA enters the cell

Question 19

HIV symptoms

Answer 19

• First symptoms - flu like including fevers, tiredness and headaches
• After several weeks HIV antibiotics appear in blood, thus making a person HIV positive
• After this, the symptoms disappear until the immune system becomes weakened again, thus leading to AIDS.
• Weight loss, diarrhoea, dementia, cancers and opportunistic infections such as TB. These opportunistic infections can lead to death.

Question 20

6.7 Understand the non-specific responses of the body to infection including Inflammation, lysozyme action, interferon and phagocytosis. (Non - specific response)

Inflammation

Answer 20

Inflammation -
- Histamine is released
- causes arterioles to dilate/vasodilation
- which increases the blood flow (to the site causing redness)
- histamine also causes the permeability of capillaries to increase
- allowing blood plasma, white blood cells and antibodies to leave the capillary (enter the tissues) causing Edema/swelling and pain.

Question 21

Lysozome action (Non - specific response)

Answer 21

Lysozyme is an enzyme found in secretions such as tears and mucus which kills bacterial cells by damaging their cell wall (causing lysis)

Question 22

Interferon (Non specific response)

Answer 22

• When cells are invaded by viruses they produce a group of chemicals called interferon’s.
• An interferon diffuses from the cell where it is made into the surrounding cells.
• It then binds to receptors in the surface membrane of uninfected cells.
• This stimulates a pathway which makes the cells resistant to infection by viruses by stopping them reproducing.
• This prevents the infection of more cells when the virus breaks out of the first cell.

Question 23

Phagocytosis (Non-Specific response)

Answer 23

• Often seen in association with inflammation.
• It involves 2 groups of white blood cells, the neutrophils and macrophages. These are both known as phagocytes as they ingest pathogens.
• The phagocytes can sometimes be seen as pus where they accumulate.
• Macrophages ingest and partially digest the pathogen, then display the antigens.
• Neutrophils ingest and fully digest the pathogen.

Question 24

Fever (Non-specific response)

Answer 24

• When a pathogen invades, the hypothalamus sets a higher running body temperature.
• This reduces the pathogens ability to reproduce quickly.
• The specific response system works better at slightly higher temperatures so will be more successful at fighting the infection.
• In viral infection the temperature often “spikes” very high when viruses burst out of the cells and then drops again.
• A fever can be damaging and even fatal!

Question 25

Difference between production of interferons and lysozymes

Answer 25

• interferons are involved in viral infections, lysozymes affects bacteria.
• interferons are produced by infected cells, lysozymes are present in phagocytes.
• interferons inhibit replication, lysozymes destroy/kill bacteria by affecting the cell wall.

Question 26

X

Answer 26

X

Question 27

6.8 Understand the roles of antigens and antibodies in the body’s immune response including the involvement of plasma cells, macrophages and antigen-presenting cells.

Answer 27

Antigens
- Antigens are molecules (usually proteins or polysaccharides) found on the surface of cells.
- When a pathogen invades the body, the antigens on its cell surface are recognised as foreign, which activates cells in the immune system.
The body has two types of immune response — specific and non-specific.

Question 28

MHC - Major histocompatibility complex

Answer 28

• are proteins which bind to fragments of antigens
• when a pathogen is ingested by a macrophage
• the antigens are presented on the surface by the MHC
• the cell is now on APC - antigen presenting cell

Question 29

Structure of Antibodies

Answer 29

(Look at photo on notion)
-Glycoprotein
- Y shape with disulphide bridges between peptides
- made of 4 polypeptide chains - two heavy chains and two light chains.
- Each chain has a variable region and a constant region.
- Variable region form the antigen binding sites.
- Shape is complimentary to a specific antigen.
- The hinge region allows flexibility when the antibody binds to the antigen.
- Constant regions allow binding to receptors on immune system cells e.g. phagocytes.
- Constant region is the same in all antibodies.
- Produced by plasma cells
- Present on B cells
- Opsonisation, immobilisation, agglutination, lysis

Question 30

How do antibodies work?

Answer 30

Agglutination :
Antibodies can cause microbes to stick together
This makes it easier for phagocytes to engulf them

Neutralisation :
Some pathogens make us ill by producing toxins
Some antibodies work by neutralising these toxins

Opsonisation or the complement cascade :
The binding of an antibody to the surface of a pathogen can set off a chain reaction with blood proteins, which causes the pathogen to swell up and burst.

Question 31

X

Question 32

X

Question 33

6.9 Understand the differences between the roles of B cells (B memory and B effector cells) in the body’s immune response.

Answer 33

B Lymphocytes are made & mature in the Bone marrow.

B Lymphocytes can clone into 2 types of cells:

B EFFECTOR/PLASMA CELLS – which flow around the body, making and releasing antibodies
B MEMORY CELLS – that remains in the body for a number of years and act as immunological memory. Produce antibodies faster & in greater number/concentration.

Question 34

6.9 Understand the differences between the roles of T cells (T helper, T killer and T memory cells) in the body’s immune response.

Answer 34

T cells mature in the Thymus

T lymphocytes develop and differentiate into 3 different cells:

T HELPER CELLS – release cytokines (chemical messengers) that stimulate the B- cells to develop and stimulates phagocytosis
T KILLER CELLS – attack and kill infected body cells
T MEMORY CELLS – kept as a memory for use if same pathogen infects the body again.

Question 35

Humoral Response

Answer 35

• A phagocyte engulfs and destroys a bacteria. This is phagocytosis.
• The antigen displayed by antigen-presenting cells goes to the surface of the phagocyte on the major histocompatibility complex.
• The phagocyte/macrophage presents the specific antigen to a T helper cell where it binds with CD4 receptors.
• The T helper cell is activated and divides to produce clones of itself (clonal expansion).
• T memory cells are produced.
• B cells are activated by a T helper cell which binds to it and produces cytokines.
• B cells have specific antibodies and binds to a specific antigen to form an antigen-antibody complex.
• B cells divide by mitosis to form B memory and B plasma cells by clonal expansion.
• Plasma cells produce antibodies that attach to the antigen of the current type of pathogen.
• B and T memory cells remain in the system & produce antibodies faster and in greater concentration on reinfection with the same pathogen.
• Macrophages identify the pathogens marked with antibodies and engulf/destroy them. Antibodies also clump the bacteria together which helps to stop them spreading and can neutralise toxins.

Question 36

Cell Mediated

Answer 36

• Body cells act as antigen-presenting cells when infected.
• T helper cells have CD4 receptors which bind to the APCs (antigen presenting cell) and are activated.
• Activated T helper cells produce cytokines to active T killer cells.
• Activated T killer cells become cytotoxic, killing cells.
• T memory cells remain ready to respond to further infection.

Question 37

6.10 Understand how one gene can give rise to more than one protein through post - transcriptional changes to messenger RNA (mRNA) (CHECK)

Answer 37

• Exons contain the protein code and introns are non - coding.
• The primary mRNA transcript is where exons and introns have been transcribed
• Splicosomes move along the primary transcript, cutting out the introns and splicing the exon sequences together to form mRNA.
• The exons can be spliced in different combinations to give different mRNAs and are translated to produce different sequences of amino acids which result in different bonds stabilising different proteins tertiary structure.
• 1 gene can code for more than 1 protein.

Question 38

6.11 Know the major routes pathogens may take when entering the body.

Answer 38

• Cuts in the skin
• Digestive system via contaminated food or drink
• Respiratory system by being inhaled
• Mucosal surfaces e.g. nose, mouth, genitals
• Ears, eyes, nose, mouth, trachea, bronchi, stomach, skin, urethra

Question 39

6.11 Understand the role of barriers in protecting the body from infection, including skin, stomach acid, and gut and skin flora.

Answer 39

Skin : contains keratin, which is a fibrous protein, it is tough/strong which provides and impermeable/impenetrable barrier.

Skin oils : contain anti microbial chemicals that kill microorganisms.

Skin and gut flora : non pathogenic microorganisms that grow on our skin and in our gut, prevent growth of other microorganisms, due to competition for nutrients, space and release of antimicrobial chemicals.

Stomach acid : low pH from hydrochloric acid, denatures enzymes in microbes, damages cell surface membrane/protein capsid. Kills microorganisms.

Question 40

Understand how individuals may develop immunity (natural, artificial, active, passive).

Answer 40

Natural active immunity - arises from being exposed to an antigen/getting the disease.
Natural passive immunity - antibodies are provided via the placenta or breast milk from mother to foetus.
Articulate active immunity - acquired through vaccinations which stimulate the immune system and lead to production of antibodies. Produces t helper cells which activates B cells to produced antibodies and T killer cells to destroy pathogens. T and B memory cells are produced so if reinfected, antibodies are produced faster and in higher concentration.
Artificial passive immunity - antibodies are injected into the body.

Question 41

Passive Immunity

Answer 41

• Antibody mediated
• No immune response, antibodies not made, come from other source
• Immediate protection
• Short - term protection
• No memory cells produced

Question 42

Active immunity

Answer 42

• Exposure to antigen
• Lag phase before protection develops
• Long - term protection
• T and B memory cells produced

Question 43

Why are antibody levels higher when getting vaccinated again?

Answer 43

• Antibodies are present from the previous vaccination.
• Secondary immune response.
• T and B memory cells are present due to the first vaccination.
• Memory cells produce antibodies faster and in higher concentration when exposed to the same antigen.
• So virus/pathogen is destroyed quicker.

Question 44

Herd immunity definition

Answer 44

Occurs when a large portion of a community (the herd) becomes immune to a disease.

Question 45

6.13 Understand how the theory of an ‘evolutionary race’ between pathogens and their hosts is supported by the evasion mechanisms shown by pathogens. (LOOK AT)

Answer 45

• An evolutionary race is an evolutionary struggle between competing sets of co-evolving genes, traits or species.
• Pathogens are constantly evolving mechanisms to evade the hosts immune systems, whilst host species are constantly evolving more complex methods of defending against, and defeating, invading organisms.
• Ultimately, in this type of evolutionary race pathogens have the advantage - their higher reproductive rate means they evolve quicker, and are more prone to mutations.

Question 46

How does the possession of the evasion mechanisms of TB support the concept of an “evolutionary race” between the pathogens and their hosts? (LOOK AT)

Answer 46

• The pathogen has to survive inside its host and must stop the host’s immune system from attacking it.
• Its thick waxy covering survives enzyme attack by macrophages.
• TB can survive inside macrophages and therefore is protected as it is inside a cell that the body regards as “self”.
• Natural selection means that only the most successful bacteria survive and their alleles get passed on.
• Lay dormant in tubercles until immune system is compromised
• Further infections can occur even years later
• Can then disrupt production of cytokines and prevent controlled cell death (apoptosis) of the macrophage
• Host needs more potent enzymes to digest the waxy coat or mechanisms that prevent the TB from living inside the macrophages to win the race.

Question 47

How do the mechanisms that allow HIV to evade the human immune system support the idea of an evolutionary race between pathogens and their hosts? (LOOK AT)

Answer 47

• HIV virus integrates its genes into the DNA of T-helper cells
• Uses host cell’s manufacturing systems to make viral protein & RNA
• T-helper cells are destroyed and more virus is produced.
• Destruction of T-helper cells compromises specific immune system
• HIV targets cells with high mutation rates, leading to errors in the copying of the genes
• Leads to viral mutations happening quickly, so virus can quickly develop different antigens and become resistant to drugs/vaccinations/antibodies
• HIV viruses “bud” out of the host cell and become wrapped in the lipids of the host cell
• Therefore macrophages and T-killer cells will recognise them as “self” and so will not attack.
• Only successful pathogens that can evade the host immune response will survive and pass on their alleles to future generations

Question 48

Explain why the evolution of the virus might reduce the effectiveness of any vaccine being developed.

Answer 48

• vaccine stimulates immune response to make antibodies specific to viral proteins/antigens.
• mutations in the virus nucleic acid results in a change in the shape of the viral proteins.
• therefore antibodies can no longer bind to the virus.

Question 49

6.14 Understand the difference between bacteriostatic and bactericidal antibiotics

Answer 49

Bacteriostatic - prevent the multiplication of bacteria. The host’s own immune system can then destroy the pathogens.
Bactericidal - kill/destroy bacteria.

Question 50

Compare and contrast Bactericidal and Bacteriostatic antibiotics.

Answer 50

• Bactericidal antibiotics kill the bacteria and bacteriostatic prevent growth
• Reference to broad spectrum (bacteriostatic) or narrow spectrum (bactericidal) and range of bacteria affected
• Bacteriostatic antibiotics interfere with protein production/DNA
  replication/cellular metabolism
• Reference to these proteins found only in bacteria, not host cell
• Bactericidal antibiotics destroy the bacteria by inhibiting cell wall or cell membrane synthesis
• Antibiotics affect both bad and good bacteria
• Risk of antibiotic resistance developing

Question 51

Bacteria does not affect tubercules

Answer 51

• Bacteria needs to be accessible to antibiotic
• Bacteria is hidden in macrophage
• Bacteria have thick waxy layer

Question 52

CORE practical 15 : Investigate the effect of different antibiotics on bacteria

Answer 52

• Change: The type of antibiotic being used (antibiotic ring)
• Organism: E. Coli - same strain and colony is used to culture a bacterial lawn
• Repeat: Repeat 4 times for each antibiotic, calculate mean, discard anomalies, calculate standard deviation
• Measure: Area covered by the zone of inhibition. Using A=πr2
  
  • Stats test – T-test
• Same: Duration of exposure to antibiotic. Volume of antibiotic. Spread of bacteria. Spreader (to keep E.coli spread out consistently)/Inoculation loop. Sterilisation.

Question 53

Safety - aseptic techniques

Answer 53

• aseptic technique described e.g. flame necks of bottles
• do not seal plates when incubating them / tape lids down but allow air in
• use of disinfectants / ethanol to clean apparatus used
• safe disposal of culture / plates
• wash hands after handling equipment
• keep ethanol away from naked flames
• wear heat resistant gloves when handling molten agar
• incubator set at less than 30 degrees to discourage pathogenic bacteria from growing

Question 54

6.15 Know how an understanding of the contributory causes of hospital acquired infections have led to codes of practice regarding antibiotic prescription and hospital practice that relate to infection prevention and control.

Answer 54

Hospital protocols

• Codes of practice or conduct
• Appropriate antibiotics should be given to patients
• Educating patients about taking antibiotics/taking the full
  course of antibiotics
• Hand washing
• Increased washing of bed sheets
• Isolate patients

Question 55

Antibiotic resistance development

Answer 55

• There is a random mutation in the gene so there is genetic variation amongst bacteria
• Presence of antibiotics is the selection pressure
• Resistant bacteria are selected by Natural Selection as have the
  advantageous allele.
• Reduced “good” bacteria population reduces competition, allowing resistant bacteria to reproduce.
• As they multiply by binary fission, toxins are released by pathogens and the resistant allele is passed on.
• Resistant allele frequency increases so there is a large population of bacteria resistant to the antibiotic.
• The more resistant bacteria there are, the more likely new strains acquire the resistant gene by horizontal gene transfer.

Question 56

Why don’t antibiotics work against viruses?

Answer 56

• Antibiotics work by inhibiting the formation of the polymer peptidoglycan.
• As viruses do not have this polymer in their coating, antibiotics are ineffective.
• Antibiotics target organelles, structures or processes in bacteria.