The genome & DNA profiling

The big idea: The genome is the whole of an organism's genetic information — every base of all its DNA added together.

To picture it, zoom out step by step. A single base (one letter: A, T, C or G) is part of a gene; many genes lie along one chromosome; and all the chromosomes together make up the genome.

So the genome is the outermost, all-containing level — not one gene, and not one chromosome, but the complete set of DNA.

Genome

The whole of the genetic information of an organism — all of its DNA, including every gene and the non-coding DNA between genes.

Gene

A length of DNA that codes for one product (such as a protein). A genome contains thousands of genes.

Chromosome

One long DNA molecule that carries many genes. A human body cell has 46 chromosomes.

Base

One of the four DNA letters — adenine (A), thymine (T), cytosine (C) or guanine (G) — the smallest unit of genetic information.

DNA profiling

A technique that reads the highly variable repeated regions of the genome to identify an individual or test how closely two individuals are related.

Genome = the WHOLE lot: If a question asks for the term for all of an organism's genetic information, the answer is the genome.

Do not write 'gene' (just one length of DNA) or 'chromosome' (just one DNA molecule) — the genome is everything.

Every nucleated body cell of an organism carries a complete copy of the whole genome — a skin cell holds exactly the same genome as a liver cell.

That is why a single cell from almost anywhere in the body can supply a full copy of the genome for analysis, and why a tiny sample (a drop of blood, a hair root, a cheek swab) is enough for DNA profiling.

Why profiling looks at the VARIABLE regions: Most of the genome is almost identical from person to person — that is why we are all human. So the coding genes are no use for telling individuals apart.

Instead, DNA profiling looks at short stretches of non-coding DNA that are repeated over and over — called short tandem repeats. The number of repeats at each place varies a lot between individuals.

By counting the repeats at several places in the genome, you build a pattern — a DNA profile — that is essentially unique to each person (identical twins aside).

What a profile can show: Because half of your DNA comes from each parent, your profile shares repeat patterns with your relatives.

So comparing profiles can:

• match a sample to the individual it came from (forensics);

• test whether two people are related (for example a parentage test);

• the more repeat patterns two profiles share, the more closely related the two individuals are.

A memory hook: The genome is what makes us the same (mostly identical DNA). DNA profiling zooms in on the few variable repeat regions that make each of us different.

How this is tested: The headline skill here is a 1-mark recall: name the term for the whole of an organism's genetic information — the answer is the genome.

A common 1-mark identify question asks which cell could supply a complete copy of the genome — any nucleated body cell works, because every cell holds the whole genome.

On Paper 1B (data) this topic also appears as a genome-size table: you are given the genome sizes of several very different organisms and asked to deduce whether genome size matches complexity. The trap answer is 'yes' — the correct answer is no.

✓ Why these score: Part (a) needs the exact term — genome — not a description.

Part (b) scores by naming a nucleated cell (a red blood cell has no nucleus, so it would not work) AND giving the reason: every nucleated cell carries the whole genome.