What is the genome?

The genome of an organism is the entire genetic material of that organism — all of its DNA.

• The genome is stored as chromosomes inside the nucleus of a cell.
• Understanding the whole human genome helps scientists find genes linked to inherited diseases, trace human ancestry and develop new medical treatments.

Zoom out to zoom in: the genome is made of chromosomes → each chromosome is one very long molecule of DNA → sections of that DNA are genes. Keep this ladder in mind for the whole lesson.

Chromosomes come in pairs

Chromosomes are genetic structures found in the nucleus of cells. In body cells they occur in functional pairs. Human body cells have 23 pairs (46 chromosomes in total).

Watch out: chromosomes occur in pairs in ordinary body cells, but not in gametes (sperm and egg) or in bacteria.

How many chromosomes?

The structure of DNA

A chromosome is a long, coiled molecule of DNA. DNA is a double helix — two strands twisted together like a spiral ladder.

• The backbone of each strand is made of alternating phosphate and sugar (deoxyribose) units.
• The two strands are linked by bases: A, T, C and G.
• Base-pairing rules: A always pairs with T, and C always pairs with G.
• Because the base order is unique to each person, DNA gives everyone (except identical twins) a unique genetic fingerprint.

Nail the pairs: A–T and C–G only. If a strand reads ATCG, the matching strand reads TAGC.

Sort each fact about DNA

Tap a fact, then tap where it belongs.

Your turn — base pairing

Genes and alleles

A gene is a section of a chromosome — a short length of DNA — that acts as a functional unit controlling a characteristic (for example eye colour).

An allele is a different form of the same gene. For example, the eye-colour gene has alleles for "brown" and for "blue".

Don't mix them up: a gene controls a characteristic; an allele is a specific version of that gene. Everyone has the eye-colour gene, but not everyone has the same alleles.

Gene or allele?

How DNA codes for proteins

The order of bases in a gene is a code. This code controls the order in which amino acids are joined together to build a protein.

• Each group of three bases (a triplet) codes for one amino acid.
• Amino acids are joined in the correct order to form a protein.
• Different genes code for different proteins (e.g. enzymes, keratin, haemoglobin), which give the organism its features.

CCEA note: you need the base-triplet idea — 3 bases → 1 amino acid → a chain of amino acids makes a protein. You are not required to know the detailed steps of transcription and translation.

Your turn — triplets to amino acids

Why the base order matters

Because the sequence of bases is different in almost every individual, DNA is unique to a person. This is why DNA can be used in:

• Forensic science — matching a DNA sample to a suspect.
• Paternity testing and tracing family relationships.
• Medical research — reading the genome to find genes linked to disease.

Key link: a change in even one base (a mutation) can change an amino acid, which can change a protein — and so change a characteristic. This is the foundation for variation, which you'll meet in later lessons.

Match each term to its meaning

Tap a term on the left, then its matching meaning on the right.

What is the genome?

Read the other strand

The big ideas to know

You've covered the CCEA genome, chromosomes, DNA and genes content. Press Finish to see your score.