6. Inheritance, Variation and Evolution
DNA, genes and variation
DNA is the chemical that carries the instructions for building and running an organism. It is packed into chromosomes in the nucleus, divided into genes, and the whole set is called the genome.
What dna, genes and variation covers
Every subtopic below has its own questions, mark schemes and notes.
4.6.1.4DNA, genes and the genome
DNA is the chemical that carries the instructions for building and running an organism. It is packed into chromosomes in the nucleus, divided into genes, and the whole set is called the genome.
4.6.1.5DNA structure, protein synthesis and mutations
Zooming in on DNA: it is a chain of nucleotides whose bases pair up in a fixed way, and every three bases code for one amino acid. That code is copied and carried to a ribosome to build a protein, and a change to the code is a mutation. Only the nucleotide structure and the triplet code are needed at Foundation tier; the sections marked Higher tier only are examined on the Higher paper.
4.6.2.1Variation
Individuals of the same species differ from one another. Those differences come from the genes they inherited, from the conditions they grew up in, or from both together, and every genetic variant traces back to a mutation.
Key terms
The definitions examiners expect, in the wording that earns the mark.
Where marks get lost
The mistakes that come up again and again in this topic.
- Size order is base, then gene, then chromosome, then genome. A question asking which contains the most DNA is asking for the genome.
- A gene codes for a protein; it is not itself a protein and it does not do the protein's job.
- The genome is the whole of an organism's genetic material, not just its genes and not just one chromosome.
- Say 'a small section of DNA on a chromosome' when defining a gene. 'A bit of DNA' will not earn the mark.
- For the migration question, the marks are for comparing small differences between genomes, not for saying that everyone is different.
- Three bases code for one amino acid, so multiply the number of amino acids by three to get the number of bases. Dividing by three is the classic slip.
- A nucleotide is a phosphate group, a sugar and a base. Answering 'a sugar and a base' loses the mark.
- It is the sugar and phosphate groups that form the backbone, not the bases.
- Do not say a mutation always changes the protein. Most mutations have little or no effect; only a few seriously affect it.
- When explaining why an altered enzyme stops working, the marks are for the change in shape of the active site meaning the substrate can no longer bind.
- Insertions and deletions have knock-on effects on every triplet after them; a substitution affects at most one amino acid. Questions often test that difference.
- mRNA carries the code out of the nucleus because the DNA itself is too big to leave. Saying the DNA travels to the ribosome is wrong.
- Variation means differences within one species. Differences between a dog and a daisy are not variation.
- Identical twins are the exception to 'no two individuals are genetically identical', and they still differ because of their environment.
- Most characteristics are caused by genes and environment together; answering 'genetic' alone for something like height loses marks.
- All genetic variants come from mutations, but most mutations change nothing about the phenotype. Both halves of that sentence earn marks.
- A new phenotype only spreads quickly if it makes the individual better suited to the environment it is in.
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