The big idea: A genome is all of the DNA of an organism — its complete set of genetic instructions.
Reading a genome
- Reading that entire sequence, base by base, is called whole genome sequencing.
- Once we can read a genome, we can compare organisms, study disease and even trace how living things are related.
From a single base up to a whole genome
- Base — one letter of the code (A, T, C or G)
- Gene — a length of DNA that codes for one product (e.g. a protein)
- Chromosome — one very long DNA molecule carrying many genes
- Genome — every chromosome together: the organism's complete DNA
The nested scale: a base sits inside a gene, which sits inside a chromosome, which together make up the genome — all the DNA of the organism.
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Genome vs gene: A gene is one instruction.
A genome is the whole instruction book — all the genes plus the DNA in between.
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- Genome
- All of the genetic material (DNA) of an organism — its complete set of instructions.
- Universal genetic code
- The same DNA bases code for the same amino acids in (almost) every living thing.
- Common ancestor
- An ancestral organism from which different species have descended.
Why the shared code is powerful evidence: The genetic code is almost universal — a given codon means the same amino acid in a bacterium, an oak tree and a human.
The simplest explanation is that all life descended from a common ancestor and inherited the same code. This shared, universal code is strong evidence for common ancestry.
So a complete genome does not need the whole organism:
Where to find a whole genome: A complete copy of an organism's genome sits in the nucleus of (almost) any one of its body cells — for example a single white blood cell.
You do not need the whole organism, and you do not need a sex cell — a gamete carries only half the genome.
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The big idea: Members of one species share most of their genome, but small differences in the base sequence make each individual unique.
The points to remember
- The genome is all the genetic information of an organism.
- Individuals of one species share most of their genome: the same genes in the same order.
- Here and there one base differs: a single-nucleotide polymorphism (SNP).
- SNPs give the diversity within a species, alongside variation in traits.
- So genomes show unity (shared) and diversity (SNPs) at the same time.
Remember it as: Same book, a few letters changed.
Real example: you and anyone else: Any two people share about 99.9% of their DNA base sequence. The rest, a few million SNPs spread across 3 billion bases, helps explain differences such as how people respond to some medicines.
How it is tested: Paper 1A asks which term means all the genetic information of an organism, or which cell would give a complete copy of it.
Which term describes all of the genetic information of an organism: gene, genome, allele or gene pool?
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Genome is not gene pool: A gene pool belongs to a population; a genome belongs to one organism.