• PCR (Polymerase Chain Reaction) — an in-vitro way of photocopying tiny amounts of DNA into billions of identical copies, using primers, Taq polymerase, free nucleotides, and a buffer, cycled through three temperature stages.
• Gel electrophoresis — separates DNA fragments by size using an electric field; smaller fragments travel further towards the anode, producing a "barcode" of bands.
• DNA profiling — comparing the band patterns (based on VNTR/short tandem repeat regions) between people to link suspects to crime scenes or confirm parentage.
• Time of death (TOD) estimation — uses five lines of evidence: decomposition stage, ecological succession, forensic entomology (insects), body temperature (algor mortis), and muscle stiffening (rigor mortis).

Think of PCR as a molecular photocopier. You start with just one molecule of DNA — maybe from a single flake of skin left at a crime scene — and within a few hours, a machine can turn it into billions of identical copies. Without enough DNA, you can't run gel electrophoresis or get a usable profile, so PCR is the essential first step that makes everything else possible.

Why do we need a "photocopier" at all? Because real crime-scene DNA samples are usually microscopic — a hair root, a smear of saliva, a trace of blood. There simply isn't enough of it to analyse directly. PCR solves this by exploiting the same basic chemistry your cells use to copy DNA before dividing, except it's done in a test tube (in vitro) rather than inside a living cell.

What every PCR reaction needs

The three stages of one PCR cycle

Each cycle is run automatically by a machine called a thermal cycler, which controls the exact temperature and timing for each stage:

After PCR is finished, the amplified DNA is treated with restriction endonuclease enzymes (which chop it into fragments of different lengths at specific cut sites) and often given a fluorescent tag so the fragments can be seen glowing under UV light later. This prepares the sample for the next step: gel electrophoresis.

Now that you have billions of copies of DNA fragments (of various lengths, thanks to the restriction enzymes cutting them up), you need a way to actually see the pattern of fragment sizes. That's what gel electrophoresis does — it's essentially a sorting machine that separates DNA fragments by size, using electricity.

Here's the key physical fact that makes this whole technique work: DNA is negatively charged because of the phosphate groups in its backbone. If you place DNA in an electric field, it will always move towards the positive end (the anode) — just like a negative charge is naturally attracted to a positive one.

Step-by-step process

1. An agarose gel plate is made, with wells cut into one end.
2. The gel is submerged in a tank of electrolyte solution (a salt solution that conducts electricity).
3. DNA samples are pipetted into the wells — importantly, a DNA standard (a sample with known fragment sizes) is always loaded into the first well, so results can be compared against a known reference.
4. Electrodes are connected — negative at the wells end, positive (anode) at the far end.
5. Current is passed through. DNA fragments migrate towards the anode. Smaller/lighter fragments move faster and travel further because they can slip through the tiny pores in the gel more easily than large fragments.
6. Probes (single-stranded DNA sequences complementary to regions of interest) are added — these carry either a radioactive label (visualised via X-ray, producing dark bands) or a fluorescent dye (visualised under UV light, producing glowing bands).

Once gel electrophoresis has produced a pattern of bands, you have a DNA profile — essentially a unique "barcode" for that individual. But why is everyone's barcode different, when we're all supposedly built from "the same" DNA code (99.9% identical between any two humans)?

The answer lies in regions of DNA called VNTRs (Variable Number Tandem Repeats) — also called short tandem repeats or micro-/mini-satellites. These are stretches of non-coding, repeated DNA sequences. Crucially, different people have different numbers of repeats in these regions. Restriction endonucleases always cut DNA at specific points around these repeat regions, so a person with more repeats will produce a longer fragment there, and a person with fewer repeats will produce a shorter one. That's what creates the unique banding pattern.

Use 1 — Paternity testing

A child inherits exactly half their DNA from each biological parent. So in a paternity test, scientists compare the child's DNA profile against several candidate fathers. Whichever candidate shares the most matching bands with the child (accounting for the bands already explained by the mother) is the most likely biological father.

Use 2 — Linking suspects to a crime scene

DNA found at a crime scene (blood, hair, skin cells) is profiled and compared against DNA profiles taken from suspects. The suspect whose profile most closely matches the crime-scene sample is the most likely to have been present.

Use 3 — Breeding programmes

DNA profiling isn't just for solving crimes — it's also used in selective/captive breeding programmes for endangered animals or plant cultivation. By comparing DNA profiles, breeders can identify which individuals are most genetically different from each other and deliberately pair those individuals to breed.

This matters because breeding closely related individuals — inbreeding — causes real problems:

• Harmful recessive alleles that would normally be masked by a healthy dominant allele can build up and become expressed, since related individuals are more likely to both carry the same rare recessive allele.
• It shrinks the gene pool of a population, reducing genetic diversity and therefore reducing the population's ability to adapt to environmental change (less raw material for natural selection to act on).

Forensics is the application of science to criminal investigations. One of the most important — and trickiest — questions forensic scientists are asked is: "When did this person die?" Since nobody was there with a stopwatch, scientists have to piece together an estimate from five separate lines of physical evidence. No single method gives an exact answer — that's why it's always called an estimate, not a precise time.

4.1 Extent of decomposition

As soon as someone dies, decomposers (bacteria and fungi) start breaking down the body — their enzymes digest biological molecules in the dead tissue. This produces a fairly predictable visual timeline:

4.2 Stage of succession

You may already know "succession" from ecology — it's the change in the community of organisms living in a habitat over time (like a pond gradually becoming a woodland). Here, the "habitat" is unusual: it's the dead body itself.

Above ground, the typical order of colonisation is:

1. Bacteria — present in/on the body almost immediately after death.
2. Flies lay eggs as soon as tissue decomposition creates favourable (moist) conditions; larvae hatch and feed.
3. Beetles establish once more soft tissue has been broken down by the fly larvae.
4. As tissue dries out, flies leave (they prefer moisture) — but beetles remain, since they can decompose dry tissue too.
5. Once all tissue is gone, most organisms leave the body.

This sequence will look different depending on where the body is — buried in soil, buried in a coffin, or underwater — because these environments dramatically change insect accessibility and oxygen availability.

4.3 Forensic entomology (insect evidence)

Because different insect species colonise a body at different, fairly predictable times after death, and progress through their life cycles at known rates, insects act like a natural clock.

Factors that can speed up or slow down insect life-cycle progression — and therefore throw off the estimate — include: drugs present in the body, humidity, oxygen availability, and temperature.

4.4 Body temperature (Algor Mortis)

While alive, respiration and other metabolic reactions continuously generate heat, which is what keeps your body at a steady ~37°C. The moment someone dies, metabolism stops — no more heat is generated, so the body begins cooling towards the temperature of its surroundings. This cooling process is called algor mortis.

This rate is affected by: air temperature, surface area : volume ratio of the body, presence of clothing, and percentage body fat (fat insulates, slowing heat loss).

4.5 Degree of muscle contraction (Rigor Mortis)

This is the trickiest mechanism, so let's build it up step by step, because the underlying biology is genuinely satisfying once it clicks.

Why does ATP matter so much here? Muscle contraction works because myosin heads bind to actin filaments and pull (the "bending" motion), shortening the muscle. To let go of actin and reset for another contraction (or simply relax), the myosin head needs to bind a fresh molecule of ATP. Normally this cycle keeps happening constantly. But after death, ATP production has stopped (step 6-7 above), so the myosin heads get physically stuck bound to actin — the muscle can't relax, and stays locked in whatever contracted state it was in. That's rigor mortis.

The rate of rigor mortis onset is affected by the level of muscle development (more muscle mass = generally slower to fully stiffen) and surrounding temperature — higher temperatures speed up the chemical reactions involved, so rigor mortis develops faster in warm conditions.