The big idea: A genome is all of the DNA of an organism — its complete set of genetic instructions.
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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Genomes vary enormously in size between species. But — and this surprises many students — a bigger genome does not mean a more complex organism.
Some plants and amphibians have genomes larger than a human's, yet are not more complex. Much of the extra DNA does not code for proteins.
| Organism | Genome size | Number of genes |
|---|---|---|
| Bacterium (E. coli) | very small | few thousand |
| Fruit fly | small | ~14 000 |
| Human | large | ~20 000 |
| Some plants & amphibians | huge (bigger than human) | not many more |
- Genome
- All of the genetic material (DNA) of an organism — its complete set of instructions.
- Whole genome sequencing
- Determining the full order of bases (A, T, C, G) across an organism's entire genome.
- 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.
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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How this is tested: On Paper 1A (multiple choice) you may be asked to identify which cell could supply a complete genome, to deduce a conclusion from a table of genome sizes, or to select the evidence for common ancestry (the universal code).
On Paper 1B / Paper 2 the headline question is a Discuss task on the present and future applications of whole genome sequencing — worth several marks.
IB-style question — applications of whole genome sequencing
Whole genome sequencing is now fast and affordable. Discuss the present and possible future applications of whole genome sequencing. [3]
How to build a full-mark 'Discuss'
- Give a present-day use. Sequencing a patient's genome can diagnose inherited diseases or identify mutations, so treatment can be tailored to the individual (personalised medicine).
- Give a second, different use. Comparing genomes lets biologists work out how species are related and build accurate evolutionary (phylogenetic) trees.
- Add a future possibility AND a balancing point. In future, cheap sequencing could screen newborns for risk of many diseases — but this raises concerns about privacy and how genetic data is used. (A 'Discuss' answer needs benefits and a limitation/concern.)
- Answer the command term. State a clear overall judgement: whole genome sequencing already supports diagnosis and evolutionary studies, and will likely expand into prediction and prevention, provided ethical safeguards keep pace.
Final answer
Present uses include diagnosing genetic disease and personalising treatment, and comparing genomes to map evolutionary relationships; future uses include predictive screening — balanced against privacy and ethical concerns about genetic data.
What 'Discuss' is looking for: Two or more distinct applications, at least one future use, and a limitation or ethical concern — finishing with a clear overall judgement. Listing three benefits with no balancing point usually caps the marks.