Genetic Variation in Bacteria
- Bacteria are prokaryotic cells.
- When it comes to IBDP Biology, they have no true nucleus.
- However, they do contain DNA, which has the same molecular structure as eukaryotic DNA.
- The mechanism of its replication is very similar to how it replicates in our cells.
- Bacteria reproduce asexually by binary fission.
- In most cases the DNA is replicated producing two identical strands.
- Therefore, the daughter cells are clones – they are genetically identical to each other and to the original parent cell.
- However, there is great genetic variety in bacteria, even within a species.
Development of genetic variety
1. Vertical gene transmission - Mutations
- During DNA replication, errors may occur, resulting in a change to the genetic code.
- This is very rare, but its occurrence will be high in bacteria with massive populations.
- This results in a structural change to the protein that a gene codes for.
- In most cases, this structural change results in a non-functioning protein.
- Most proteins are enzymes, and control metabolic reactions.
- A non-functioning enzyme may result in a block in a pathway.
- This is normally fatal and results in cell death.
- Occasionally, the cell survives to pass on this allele.
- Sometimes, mutations cause only a slight change to the protein – neutral mutations.
- The bacteria reproduce, passing on the allele.
- However, if the environment changes, there is a chance that some neutral mutations may become advantageous.
- When it comes to IBDP Biology, the frequency of the allele increases if it provides a survival advantage.
2. Horizontal gene transmission – Conjugation
- Bacteria tend to form colonies.
- They attach to one another due to slime capsules and pili, which are protein fibres extending from the cells.
- When it comes to IBDP Biology, where cell membranes come in close contact, they may fuse, forming a conjugation tube between the two bacteria.
- Their cytoplasms meet and substances can pass freely between the two cells.
- Small portions of DNA can be passed from one cell to another.
- These often pass as plasmids – small loops of DNA separate from the main bacterial DNA loop.
- There is evidence that bacteria of different species can exchange DNA by conjugation.
That's the end of this topic.