Determining the time of death

No single method is reliable on its own, so a forensic pathologist combines several:

• Body temperature (algor mortis) — after death, metabolism stops and the body cools towards the ambient temperature. The initial rate is roughly 1.5 °C per hour, but cooling follows a sigmoid curve and is affected by body mass, clothing, air movement and the surroundings — so it is only useful for roughly the first 24 hours.
• Rigor mortis — muscles stiffen from about 3–4 hours after death, are fully rigid by about 12 hours, and the rigidity passes off by around 36 hours.
• Extent of decomposition — autolysis, then putrefaction by gut bacteria; the abdomen greens and swells with gas, then the tissues liquefy.
• Stage of succession — the community of decomposers on and around a corpse changes in a predictable sequence.
• Forensic entomology — the species and life stage of the insects present.

The biology of rigor mortis: respiration stops, so no ATP is made. Without ATP, myosin heads cannot detach from actin, so the cross-bridges lock and the muscle cannot relax. Meanwhile lysosomes rupture and their enzymes begin to digest the muscle proteins — which is why rigor eventually passes off.

Your turn — time of death from body temperature

Forensic entomology and decomposition

A corpse is a habitat, and it undergoes its own succession. Blowflies (Calliphora) arrive within minutes to hours and lay eggs in the natural orifices and wounds. Eggs hatch into 1st, 2nd and 3rd instar larvae (maggots), which pupate and emerge as adults. Later, beetles and moths arrive to feed on the drier remains.

Because insects are ectotherms, their development rate depends on temperature. Forensic entomologists therefore use accumulated degree days (ADD) — a species needs a fixed number of degree-days above a base temperature to reach each stage:

Decomposition and the carbon cycle (6.2): saprobiotic (saprotrophic) bacteria and fungi secrete extracellular enzymes onto dead organic matter, digesting it and absorbing the soluble products. They respire, returning CO₂ to the atmosphere — without decomposers, carbon and nitrogen would remain locked in dead bodies.

Your turn — forensic entomology

Which method, and when?

DNA profiling and PCR

Only about 2 % of human DNA codes for protein. The rest contains short tandem repeats (STRs / VNTRs) — short sequences repeated over and over. The number of repeats at each locus varies enormously between individuals (though half are shared with each parent). Comparing the repeat numbers at ~10–16 loci gives a profile that is, in practice, unique to an individual apart from identical twins.

The polymerase chain reaction (PCR) amplifies the tiny amounts of DNA found at a crime scene. Each cycle has three steps:

• Denature — 95 °C: the hydrogen bonds break and the two strands separate.
• Anneal — 50–65 °C: short primers bind to the ends of the target sequence.
• Extend — 72 °C: Taq polymerase — a thermostable enzyme from a hot-spring bacterium, so it is not denatured at 95 °C — builds the complementary strand from free nucleotides.

Your turn — PCR amplification

Gel electrophoresis

The amplified DNA is cut by restriction enzymes and separated by gel electrophoresis:

• DNA samples are loaded into wells at one end of an agarose gel, and a voltage is applied.
• DNA has a negatively charged phosphate backbone, so every fragment moves towards the anode (positive electrode).
• The gel is a molecular sieve: short fragments move furthest, long fragments are held back. Fragments are therefore separated by length.
• The DNA is transferred to a membrane, and a radioactive or fluorescent DNA probe complementary to the repeat sequence binds to it, revealing the bands.

Interpreting a profile: a suspect matches only if every band lines up with the crime-scene sample. A child’s bands must each be present in one parent or the other — that is how paternity and relatedness are established, and how the technique is used for plants and animals too.

Reading the gel

Bacteria, viruses, TB and HIV

Bacterium: a living prokaryotic cell — cell wall of murein (peptidoglycan), cell-surface membrane, cytoplasm, 70S ribosomes, circular DNA, often plasmids. It reproduces independently by binary fission. Antibiotics can target it.

Virus: non-cellular — nucleic acid (DNA or RNA) inside a protein capsid, sometimes with a lipid envelope. No cytoplasm, no ribosomes, no metabolism. It can only replicate inside a host cell by taking over the host’s machinery. Antibiotics do not work on viruses.

TB (Mycobacterium tuberculosis): inhaled in droplets; engulfed by macrophages but survives inside them because its waxy wall resists digestion. The immune system walls the bacteria into tubercles (granulomas), and the infection may stay latent for years. If immunity falls, the bacteria break out, destroying lung tissue — causing a persistent cough, blood in the sputum, fever and weight loss.

HIV: a retrovirus. Its gp120 attachment protein binds CD4 receptors on T helper cells. Reverse transcriptase makes DNA from the viral RNA; integrase inserts it into the host genome, where it may lie latent. When active, new viruses bud off and destroy the T helper cell. As T helper numbers collapse, the specific immune response fails and the person develops AIDS: they die of opportunistic infections (such as TB), not of HIV directly.

Bacterium or virus?

Tap a feature, then tap where it belongs.

Barriers and the non-specific response

Routes of entry (6.11): the respiratory tract (inhalation), the digestive system (contaminated food and water), broken skin, the reproductive tract, and via a vector.

Barriers: intact skin (a physical barrier plus a dry, slightly acidic surface); stomach acid at about pH 2, which denatures the enzymes of most swallowed pathogens; mucus and cilia in the airways; and the gut and skin flora — harmless bacteria that out-compete pathogens for space and nutrients.

Non-specific responses (6.7):

• Inflammation — mast cells release histamine: vasodilation and increased capillary permeability bring more blood, plasma and white cells to the site (redness, heat, swelling).
• Lysozyme — an enzyme in tears, saliva and mucus that hydrolyses the murein in bacterial cell walls, bursting them.
• Interferon — released by virus-infected cells; it inhibits viral protein synthesis in neighbouring cells and activates the immune response.
• Phagocytosis — a phagocyte is attracted by chemicals from the pathogen, engulfs it into a phagosome, which fuses with a lysosome. Hydrolytic enzymes digest the pathogen, and the phagocyte then displays the antigens on its surface, becoming an antigen-presenting cell.

The specific immune response

An antigen is a molecule (usually a protein or glycoprotein on a pathogen’s surface) that triggers an immune response. An antibody is a Y-shaped immunoglobulin with two variable regions whose shape is complementary to one specific antigen, plus a constant region.

Cell-mediated response (T cells): an antigen-presenting cell displays the antigen. The T helper cell with the complementary receptor binds and is activated; it divides by mitosis and releases cytokines that stimulate phagocytes, T killer cells and B cells. T killer cells destroy infected body cells by releasing perforin. T memory cells remain.

Humoral response (B cells): the B cell with the complementary antibody binds the antigen and, stimulated by T helper cytokines, undergoes clonal selection and expansion. It differentiates into:

• B effector / plasma cells — secrete thousands of antibodies per second; they agglutinate pathogens and mark them for phagocytosis. Short-lived.
• B memory cells — long-lived. On re-infection the secondary response is faster, stronger and longer-lasting, so you show no symptoms. That is immunity.

Match the immune cell to its job

Tap a job on the left, then the cell that does it.

Types of immunity, splicing and the evolutionary race

Four types of immunity (6.12):

• Natural active — you catch the disease and make your own antibodies and memory cells. Long-lasting.
• Artificial active — vaccination with a dead, attenuated or subunit antigen: you make your own memory cells without the illness. Long-lasting. Herd immunity then protects the unvaccinated.
• Natural passive — antibodies received across the placenta or in breast milk. Immediate, but short-lived (no memory cells).
• Artificial passive — an injection of ready-made antibodies (e.g. antivenom, tetanus). Immediate but short-lived.

One gene, many proteins (6.10): a eukaryotic gene contains coding exons and non-coding introns. The primary mRNA transcript is edited in the nucleus: the introns are spliced out. Because exons can be joined in different combinations (alternative splicing), one gene can give rise to several different polypeptides.

The evolutionary race (6.13): pathogens evolve evasion mechanisms — HIV changes its surface antigens with each replication (antigenic variation), TB hides inside macrophages, and some bacteria have capsules that resist phagocytosis. Hosts respond with new defences. Neither side ever wins for long.

Antibiotics, resistance and hospital infections

Bacteriostatic antibiotics inhibit growth and reproduction (e.g. by blocking protein synthesis at the 70S ribosome), leaving the immune system to finish the job. Bactericidal antibiotics kill the bacteria outright (e.g. penicillin, which inhibits the enzyme that cross-links murein, so the wall cannot be built and the cell bursts by osmosis).

Resistance evolves by natural selection: a chance mutation gives one bacterium resistance (e.g. it makes β-lactamase, which hydrolyses penicillin). The antibiotic is the selection pressure: susceptible bacteria die, the resistant one survives, reproduces and passes the allele on — including horizontally, by plasmid transfer (conjugation), even to other species.

Hospital-acquired infections (6.15): hospitals concentrate vulnerable patients, invasive procedures and heavy antibiotic use, which selects strongly for resistant strains such as MRSA and C. difficile. The codes of practice follow directly: hand hygiene, isolation of infected patients, thorough cleaning, and antibiotic stewardship — narrow-spectrum drugs only where needed, complete the full course, and never prescribe antibiotics for viral infections.

Your turn — zone of inhibition

Why does antibiotic resistance spread so fast?

The big ideas to know

You have covered the whole of SNAB Topic 6 — forensics included. Press Finish to see your score.