Genome diversity at Higher Level: The same statement as SL. At HL, sequence differences between species are the basis of cladograms and the molecular clock, so be sure that more differences means more distant relatives.
Practise this as you read
- Outline two ways eukaryote genomes differ.
- Compare variation within and between species.
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The big idea: Genomes differ in their base sequence, and they differ far more between species than within one species.
The points to remember
- Every eukaryote's genome has its own base sequence.
- Within one species, genomes are almost the same: a few SNPs differ.
- Between species, the base sequences differ much more.
- The more closely related two species are, the fewer differences they have.
Remember it as: Within a species: a few letters. Between species: many.
Real example: dogs, wolves and cats: Dogs descend from wolves, and the two differ in very few bases. Dogs and cats belong to different families of carnivores and differ in far more.
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The big idea: Genomes also differ in size: genome size is the total amount of DNA, counted in base pairs.
The points to remember
- Genome size = the total amount of DNA, measured in base pairs.
- Eukaryote genome sizes range from about 12 million base pairs (baker's yeast) to about 149 billion (the plant Paris japonica).
- Much of a large genome is DNA that does not code for proteins.
- So genomes vary in two ways: in size and in base sequence.
Size is not the number of genes: Genome size counts all the DNA, coding or not. A big genome does not mean many more genes.
How it is tested: Paper 1A can ask in which two ways genomes vary, or compare differences within and between species.
IB-style questionOutline[2 marks]
Outline two ways in which the genomes of eukaryotes differ from each other.
Model answer plan
See the answer broken into clear points, with guidance on how each point earns marks.
Use the comparison word: Write 'much larger between species than within a species', not just 'they are different'.