Topic 6 · Immunity, Infection and Forensics — Edexcel A Level Biology A (SNAB) 9BI0 Revision Notes & Topical Questions

Section 1 of 27Time of death: factors

Forensics

• Forensics use of science in investigating crime, collecting and analysing evidence from a scene.

Time of death (TOD)

• Time of death (TOD) important for the circumstances of a death.

Factors to establish

• Extent of decomposition.
• Stage of succession.
• Forensic entomology.
• Body temperature.
• Degree of muscle contraction.

Caution

• TOD is an estimate; many factors affect a body, so it cannot be pinpointed exactly.

Section 2 of 27Decomposition

1Decomposers

• Decomposers bacteria and fungi secrete enzymes that break down biological molecules;
  • decomposition begins soon after death.

2Stages

• First few days
  • cells and tissues break down;
    • skin often greenish.
• Days to weeks
  • tissues and organs broken down, gases such as methane cause bloating;
    • skin blisters and falls off.
• A few weekssoft tissue liquefies and leaves the body.
• Months to yearsonly a skeleton remains.
• Decades to centuriesskeleton disintegrates.

3Rate

• Rate slower in anaerobic conditions and at lower temperatures; faster at high temperatures.

Section 3 of 27Succession on a body and forensic entomology

1Succession

• Succession change in the organisms present;
  • unlike ecology, early arrivals do not disappear as the body decomposes.

2Sequence above ground

• Bacteria immediately after death.
• Flies lay eggs and larvae hatch as tissue decomposes.
• Beetles establish as soft tissue is consumed.
• Flies leave as tissue dries (prefer moisture); beetles remain and eat dry tissue.
• When all tissue is gone most organisms leave.

3Location

• Location buried in soil, in a coffin, or under water changes insect access and oxygen, altering the stages.

4Forensic entomology

• Forensic entomology study of insect colonies on a body.
• Flies arrive within hours; beetles later.
• Life-cycle stage gives clues: blowfly eggs hatch in about 24 hours,
  • so larvae mean more than 24 hours since death; only blowfly larvae suggests about 24 hours.

5Factors affecting insect life cycles

• Factors affecting insect life cycles drugs in the body, humidity, oxygen availability, temperature.

Section 4 of 27Body temperature and rigor mortis

Algor mortis

• Algor mortis cooling after death: metabolism stops producing heat,
  • so the body falls from about 37 °C to the surroundings at about 1.5–2.0 °C per hour.
• Rate affected by air temperature, SA:V ratio, clothing.

Rigor mortis

• Rigor mortis stiffening beginning about 4–6 hours after death.
• No oxygen, so muscle cells respire anaerobically and make lactic acid.
• Lactic acid lowers pH, denaturing enzymes that make ATP.
• Without ATP myosin heads cannot detach from actin, locking the muscle contracted.

Timing

• Timing starts in small muscles of the head and ends in the large muscles of the lower body;
  • complete in every muscle by 12–18 hours;
  • wears off after about 24–36 hours;
  • faster at higher temperatures;
  • affected by muscle development.

Section 5 of 27Decomposition and nutrient recycling

Decomposers

• Decomposers bacteria and fungi secrete enzymes that break large organic molecules into smaller ones such as glucose, which are respired.

Releases

• CO₂ and methane to the atmosphere.

Recycling

• Recycling green plants absorb CO₂ and fix the carbon into carbohydrates in photosynthesis.

Section 6 of 27DNA profiling overview

Uniqueness

• Uniqueness every person except identical twins has a unique DNA sequence.

Uses

• Uses identifying individuals in forensics;
  • paternity and maternity tests;
  • ancestry kits;
  • evolutionary relationships.

1

Isolate DNA from saliva, skin, hair or blood.

2

Amplify with PCR.

3

Carry out gel electrophoresis.

4

Analyse the pattern of fragments.

Section 7 of 27Polymerase chain reaction

1

PCR

• PCR in vitro DNA amplification used in DNA profiling and genetic engineering; billions of copies in hours from a single molecule; each cycle doubles the DNA.
  • (20 cycles gives a million molecules)

2

Thermal cycler

• Thermal cycler controls temperatures and times automatically.

3

Requirements

• DNA or RNA to amplify.
• Primers: short single-stranded DNA complementary to the 3' end, defining the region and where polymerase binds.
• DNA polymerase, usually Taq polymerase from Thermus aquaticus,
  • which does not denature at high temperature and whose high optimum prevents re-annealing of uncopied strands.
• Free nucleotides.
• Buffer for optimum pH.

Denaturation

• Denaturation heat to 95 °C to break hydrogen bonds between the strands.

Annealing

• Annealing cool to 50–60 °C so primers anneal.

Elongation

• 72 °C, the optimum for Taq polymerase to build complementary strands.

After PCR

• After PCR restriction enzymes cut the DNA into fragments of different lengths and a fluorescent tag lets fragments be seen under UV.

Note

• Note forward and reverse primers need not be known.

Section 8 of 27Practical: gel electrophoresis

Principle

• DNA is negatively charged and moves to the anode; small fragments move faster through the gel pores and so further.
  • (phosphate groups)

Apparatus

• Apparatus agarose gel, tank, electrolyte, micropipette, electrodes, DNA samples, DNA standard (control), probes or stain.

1

Make an agarose gel in a tank with wells at one end.

2

Submerge it in electrolyte (a salt solution that conducts electricity).

3

Pipette samples into wells, with the standard in the first well, for a set of known results.

4

Connect the negative electrode at the wells end and the anode at the far end.

5

Smaller fragments move faster and further.

6

Add probes then take an X-ray or use UV to show the band pattern and compare with the standard.

• Probes
  • single-stranded DNA complementary to regions of interest, with a radioactive label or fluorescent dye (coloured bands under UV).
    • (dark bands on X-ray film)

Section 9 of 27Analysing DNA profiles

Basis of bands

• Basis of bands restriction enzymes cut between variable number tandem repeats (VNTRs);
  • people have different numbers of repeats,
• so fragment lengths and banding patterns are unique; comparing crime scene DNA with suspects is reliable.

Paternity

• Paternity child's profile compared with candidate fathers; half the DNA comes from each parent,
• so the candidate sharing the most bands is the most likely father.
  • (example: child shares bands 1, 3, 5 with the mother and 2, 4, 6 with candidate B)

Breeding programmes

• Breeding programmes compare profiles to pair the most genetically different individuals and avoid inbreeding.

Inbreeding problems

• Accumulation of harmful recessive alleles otherwise masked by dominant ones.
• Smaller gene pool and reduced ability to adapt to change.

Detail

• Detail you need to explain how to run electrophoresis and how profiles are analysed, not the restriction enzyme detail.

Section 10 of 27Structure of bacteria and viruses

Structure of bacteria and viruses

Bacteria

• Bacteria small single-celled prokaryotes: no membrane-bound organelles, 70S ribosomes, no nucleus, murein (peptidoglycan) cell wall, membrane folds (mesosomes).
  • (one circular chromosome free in cytoplasm)

Extra structures

• Plasmids (DNA loops).
• Capsule (slime capsule)protects from drying and immune attack.
• Flagellarotate to move.
• Pili
  • attach to cells or surfaces;
    • involved in gene transfer.

Viruses

• Viruses non-cellular infectious particles, much smaller than prokaryotes.
• Nucleic acid core (DNA or RNA, single or double-stranded) inside a capsid.
• No plasma membrane, cytoplasm or ribosomes.
• Some have an envelope made of phospholipids from the host cell.
• May contain enzymes, for example HIV reverse transcriptase.
• Attachment proteins let the virus bind host cells.
• Reproduce only inside living cells using their machinery.

Section 11 of 27Infectious disease and tuberculosis

Disease

• Disease illness with signs and symptoms; infectious diseases are caused by pathogens and are transmissible.
  • (certain bacteria, some fungi, all viruses; not all viruses are human pathogens)

TB

• TB caused by Mycobacterium tuberculosis.
• Active TB patients cough or sneeze droplets; inhaled by others; spreads faster in overcrowded conditions.
• Bacteria engulfed by phagocytes may survive and reproduce inside.
• A healthy immune system walls infected phagocytes in tubercles, with bacteria dormant.
• Reactivation when immunity is weakened (for example HIV) gives the active phase.
• Time to disease from weeks to years.

Symptoms

• Symptoms fever, fatigue, coughing, lung inflammation; untreated damages the lungs and can spread to other organs.
  • (respiratory failure)

Section 12 of 27HIV and AIDS

HIV and AIDS

HIV

• HIV human immunodeficiency virus; an RNA retrovirus.
  • (not "HIV virus")

Transmission by body fluids

• Sexual intercourse.
• Blood donation.
• Shared needles.
• Across the placenta.
• Mixing of blood during birth.
• Breast milk.

Replication

1

Viral glycoproteins bind CD4 receptors on helper T cells.

2

The viral envelope fuses with the cell membrane and viral RNA enters.

3

Reverse transcriptase makes a complementary DNA strand, which becomes double-stranded.

4

The DNA inserts into the host DNA.

5

The host cell's enzymes make viral components, which assemble into new viruses.

6

New viruses bud from the cell into the blood and infect more helper T cells.

Course

• Initial infection with rapid replication and flu-like symptoms.
• Latency period (often years) without symptoms while helper T cells gradually fall.
• No B cell activation, no antibodies, immunocompromised.
• AIDS when opportunistic infections occur constantly and helper T cell count falls below a critical level.

AIDS illness

• AIDS illness begins with mild mucous membrane infections, then more severe (diarrhoea, TB), finally serious opportunistic infections that cause death.

Progression depends on

• Progression depends on existing infections, HIV strain, age, access to healthcare.

Terms

• HIV is the virus; AIDS is the disease it causes.

Section 13 of 27Entry routes and barriers to infection

Entry routes

• Entry routes broken skin, digestive system, respiratory system, mucosal surfaces (nose, mouth, genitals).
  • (contaminated food and drink)

Barriers

• Skin
  • physical barrier;
    • clotting seals damage but takes time.
• Gut and skin floracompete with pathogens for resources.
• Stomach acidhydrochloric acid in gastric juice kills many pathogens, though a few reach the intestines.
• Lysozyme in tears, saliva and mucus damages bacterial cell walls, making bacteria lyse.

Section 14 of 27Non-specific responses

Non-specific

• Non-specific same for any pathogen; specific — directed at a particular pathogen.

Antigens

• Antigens surface proteins or glycoproteins acting as ID tags; pathogens have non-self antigens.
  • (self or non-self)

Inflammation

• Inflammation swollen, warm, painful area.
• Mast cells secrete histamine (a cell signalling molecule).
• Vasodilation increases blood flow.
• Capillary walls become leaky; fluid enters tissues giving swelling; some plasma proteins leave.
• Phagocytes enter the tissue.
• Cytokines are released to trigger an immune response.

Interferons

• Interferons made by virus-infected cells.
• Prevent viruses spreading to uninfected cells.
• Inhibit viral protein production.
• Activate white blood cells of the specific response to destroy infected cells.
• Increase the non-specific response, for example inflammation.

Section 15 of 27Phagocytosis

1

Chemicals from pathogens and from damaged cells (histamine) attract phagocytes.

2

The phagocyte recognises non-self antigens.

3

Its membrane extends round the pathogen (endocytosis) forming a phagocytic vacuole.

4

Lysosomes fuse with the vacuole, releasing digestive enzymes including lysozyme.

5

The pathogen is digested.

6

Pathogen antigens are presented on the phagocyte surface, making it an antigen-presenting cell.

7

This starts the specific immune response.

• Phagocytes
  • leave blood through capillary walls and are released in large numbers during infection.

Section 16 of 27Antigens and antibodies

Antigens and antibodies

Antigens

• Antigens markers on cell membranes, bacterial walls or viruses;
  • glycolipids and glycoproteins act as antigens.
• Self antigens do not stimulate an immune response.
• Non-self antigens do (pathogens, a transplanted organ).

Antigen-presenting cells

• Antigen-presenting cells macrophages (phagocytic) display digested antigens;
  • lymphocytes bind them with specific receptors.

Antibody structure

• Y-shaped immunoglobulin with four polypeptide chains: two heavy and two light, linked by disulfide bonds.
• Constant region does not vary within a class (five classes with different roles).
• Variable region has the antigen binding site, with a different amino acid sequence in each antibody,
  • giving specificity.
    • (complementary to the antigen epitope)
• Hinge region gives flexibility (not in all classes).

Membrane-bound or secreted

• Membrane-bound or secreted membrane-bound antibodies have an extra section of heavy chain;
  • alternative splicing of the heavy-chain gene removes the exons coding for it in secreted antibodies.

Functions

• Block pathogen receptors so pathogens cannot infect host cells.
• Act as antitoxins, neutralising toxins (diphtheria, tetanus).
• Cause agglutination, limiting spread and letting phagocytes engulf many pathogens at once.

Section 17 of 27T cell response

T cells (T lymphocytes)

• T cells (T lymphocytes) made in bone marrow, mature in the thymus;
  • each has T cell receptors (antibody-like) specific to one antigen.

Activation

• Activation binding the antigen on an antigen-presenting cell.
  • (macrophage, infected body cell or pathogen)

Division

• Division mitosis gives clones with the same receptor, which differentiate into three types.

Type	Role
T helper cells	Release signalling molecules that help activate B cells
T killer cells	Bind to and destroy infected cells displaying the specific antigen
T memory cells	Remain in the blood for a faster response next time

Section 18 of 27B cell response

1

B cells mature in bone marrow and have antibody receptors on their surface, each B cell for a different antigen.

2

The B cell binds its antigen, forming an antigen–antibody complex.

3

Binding plus signalling molecules from T helper cells activates the B cell.

4

It divides repeatedly by mitosis, producing clones.

5

Clones differentiate into effector cells that become plasma cells (making specific antibodies) and memory cells (staying in the blood).

Section 19 of 27Post-transcriptional modification

Antibody heavy chain

• Antibody heavy chain one gene;
  • post-transcriptional modification decides whether the extra section is present (membrane-bound) or absent (secreted).

Splicing

• Splicing pre-mRNA contains introns (non-coding) and exons (coding);
  • introns are removed and exons joined before the mRNA leaves the nucleus.

Alternative splicing

• Alternative splicing exons can be joined in different ways,
  • so one gene can code for more than one polypeptide, for example membrane-bound or secreted antibody.

Section 20 of 27Primary and secondary responses, active and passive immunity

Primary response

• Primary response first encounter with an antigen: non-specific then specific.
• Few T and B cells with the right receptors, so time is needed to activate, divide and differentiate.
• Several days before plasma cells make antibodies, so symptoms appear.
• Memory cells remain, giving immunity.

Secondary response

• Secondary response much faster and stronger;
  • more memory cells than the original lymphocytes;
  • antibodies quicker and in larger quantity;
  • often removes the pathogen before symptoms.

Type of immunity	Features
Active, natural	Exposure to the pathogen; memory cells; long-term
Active, artificial	Vaccination; memory cells; long-term
Passive, natural	Antibodies across the placenta or in breast milk; no immune response and no memory cells
Passive, artificial	Injection or transfusion of antibodies (tetanus antitoxin) from vaccinated people or animals; no memory cells, so another infusion is needed if reinfected

• Vaccines
  • contain antigens put in the body to induce artificial active immunity; may be dead or weakened pathogens, less harmful strains, antigens alone,
    • or genetic material coding for antigens; given by injection or mouth.
  • Create memory cells, so a faster and stronger secondary response and usually no symptoms.
  • Antigenic variation (from mutation) means vaccines may need updating.

Section 21 of 27Evolutionary race between pathogens and host

Evolutionary arms race

• Evolutionary arms race vertebrate immune systems evolved to deal with many pathogens;
  • pathogens evolve ways to evade them;
  • each gains new advantages.

HIV evasion

• Kills helper T cells, reducing detection and antibody production.
• High mutation rate in antigen genes gives new strains needing new primary responses;
  • memory cells for one strain do not recognise another.
• Prevents infected cells presenting antigens, so they are hard to recognise and destroy.

TB evasion

• After engulfment, bacteria stop lysosomes fusing with the phagocytic vacuole, so they are not digested and multiply inside the phagocyte.
• They disrupt antigen presentation in infected phagocytes.

Section 22 of 27Types of antibiotics

Types of antibiotics

Antibiotics

• Antibiotics chemicals that damage bacterial cells with little or no harm to human tissue;
  • penicillin discovered by Fleming in 1928.

Bactericidal

Bactericidal kills bacteria; bacteriostatic inhibits growth (at high enough doses can kill).

Modes of action

• Inhibit enzymes forming cell wall bondswalls weaken and burst by osmosis (penicillin causes lysis).
• Bind ribosomes and stop protein synthesis.
• Damage cell membranes, causing loss of metabolites or uncontrolled water entry.
• Stop bacterial DNA coiling into rings so it no longer fits in the cell.

Why not harmful to human cells

• Why not harmful to human cells eukaryotic cells lack cell walls and have different enzymes and ribosomes.

Viruses

• Viruses lack enzymes, ribosomes and cell walls, so antibiotics do not affect them.

Section 23 of 27Practical: comparing antibiotics

Aseptic technique

• Aseptic technique sterile equipment, disinfected benches, and a Bunsen burner for an updraught.
  • (washed hot or with antibacterial chemicals)

Apparatus

• Apparatus bacterial culture, disinfectant, Bunsen, agar plates, pipettes, spreader or inoculating loop, antibiotics and paper discs, distilled water, forceps.
  • (or pre-treated discs)

1

Disinfect surfaces and light the Bunsen burner.

2

Spread bacterial culture on an agar plate with a sterile pipette and spreader or loop (metal loops can be flamed).

3

Soak similar discs in different antibiotics (for example methicillin, tetracycline, streptomycin) or different concentrations.

4

Add a negative control disc soaked in distilled water.

5

Space discs apart with sterile forceps.

6

Lightly tape the lid, invert and incubate at 25 °C for 24–48 hours.

Why

• Why light taping lets oxygen in;
  • inverting stops condensation dripping on to the agar;
  • about room temperature avoids growing harmful pathogens.

Results

• Results bacterial lawn with clear zones;
  • larger zone means a more effective antibiotic;
  • no zone means resistance;
  • compare sizes between antibiotics and concentrations.

Section 24 of 27Hospital acquired infections and antibiotic resistance

1Hospital acquired infections (HAIs)

• Hospital acquired infections (HAIs) contracted in hospital; increased by poor hygiene; patients are already ill with weak immunity.
  • (not washing hands, uncontained coughing and sneezing, not disinfecting equipment)

2Controls

• Staff and visitors wash hands.
• Infected patients isolated.
• Surfaces and equipment disinfected after each use.

3MRSA

• MRSA methicillin-resistant Staphylococcus aureus;
  • resistant infections are hard to treat and can cause serious illness or death.

4Why hospitals risk resistance

• Why hospitals risk resistance widespread antibiotic use is a selection pressure.
  • (a factor driving natural selection)

5Practices to reduce resistance

• No antibiotics for minor infections or viral diseases.
• No preventative use.
• Prescribe narrow-spectrum antibiotics (active against a narrow range of bacteria),
  • so resistance genes passed to other bacteria by horizontal gene transfer cause fewer problems.
• Rotate different antibiotics.

6Origin

• Origin resistance arises by natural selection, more likely where antibiotics are used often, such as hospitals.

Section 25 of 27Worked examples

1. Describe how time of death can be estimated from a body found outdoors. [5]5 marks

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2. Explain why rigor mortis occurs. [4]4 marks

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3. Explain the role of the DNA fragments and electrophoresis in DNA profiling. [4]4 marks

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4. Describe how HIV leads to AIDS. [5]5 marks

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5. Explain why the secondary immune response is faster than the primary response. [3]3 marks

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Section 26 of 27Key terms

Term	Meaning
Algor mortis	Cooling of the body after death.
Rigor mortis	Stiffening of muscles after death because ATP is unavailable to detach myosin from actin.
VNTR	A variable number tandem repeat: a repeated DNA sequence differing in number between people.
Antigen	A surface molecule that identifies a cell as self or non-self.
Antigen-presenting cell	A cell displaying antigens of a digested pathogen on its surface.
Plasma cell	A differentiated B cell that secretes antibodies.
Opportunistic infection	Infection that takes advantage of a weakened immune system.
Narrow-spectrum antibiotic	An antibiotic active against a narrow range of bacteria.
Hospital acquired infection	An infection contracted by a patient while in hospital.

Section 27 of 27Exam technique

1

Calling HIV the "HIV virus" or treating HIV and AIDS as the same — HIV is the virus, AIDS the disease.

2

Saying antibiotics work against viruses — viruses lack cell walls, ribosomes and enzymes for antibiotics to target.

3

Saying succession on a dead body is the same as ecological succession — early arrivals remain on a body.

4

Saying time of death can be pinpointed exactly — it is only an estimate.

5

Saying passive immunity produces memory cells — antibodies are received, not made, so there are none.