Forensics

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Forensics

Big idea: Every person's DNA is unique (except identical twins), so scientists can copy it (PCR), sort it by size (gel electrophoresis), and read the pattern like a barcode — to identify who was at a crime scene, and a decomposing body's chemistry and insect visitors can be read like a clock to estimate when someone died.

Summary — What This Chapter Covers

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).

1. The Polymerase Chain Reaction (PCR)

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

Ingredient	Its Job
DNA (or RNA)	The template — the sequence you actually want copied.
Primers	Short single strands of DNA complementary to the 3' end of the target region. They act like a "start here" flag, telling DNA polymerase exactly where to begin building.
Taq polymerase	The enzyme that builds new DNA strands. It comes from Thermus aquaticus, a bacterium that lives in hot springs — which is exactly why it's useful here (see below).
Free nucleotides	The raw "letters" (A, T, C, G) used to build the new strands.
Buffer solution	Keeps the pH at the optimum level so the enzymes work properly.

Why Taq polymerase specifically? Normal human DNA polymerase would denature (fall apart) the instant it hit 95°C. Taq polymerase comes from bacteria that live in near-boiling hot springs, so it's built to survive high heat without breaking down. This is the whole reason PCR is even possible as an automated, repeatable cycle — you can blast the reaction with heat over and over and the enzyme just keeps working.