Variation and Mutation
Causes of Mutations
Mutations are changes to the nucleotide sequence found in DNA.
The two types of mutations are:
Somatic: Mutations in body cells and aren't passed onto offspring
Germline: Mutations that occur in gametes (ova and sperm), and are passed onto offspring if those gametes are fertilised
Causes of Mutations
There are a number of causes of mutations.
1. Chemical mutagens
2. Physical mutagens
3. Biological agents
4. Errors in DNA replication
5. Errors in cell division
Chemical mutagens are various chemicals that can cause damage to DNA. There are a few examples which are good to remember, in case you're asked about mutagens in the exam!
Ethidium bromide: Disrupts the packing of DNA by slipping in between nitrogenous bases.
Mustard gas: Guanine in DNA is replaced by other bases.
Nitric acid: Adenine in DNA is deaminated, so it behaves like guanine.
Physical mutagens are various types of radiation that can cause damage to DNA. They often affect the nitrogenous bases of DNA, causing distortions in the double helix structure of DNA.
UV light: Causes structural distortion by cross-linking neighbouring nucleotides.
X-rays: Causes gene and chromosome mutations.
Nuclear radiation: Breaks DNA strands entirely.
Biological agents are bacteria or viruses which can cause damage to DNA, through processes such as horizontal gene transfer.
An example is agrobacterium causing crown gall disease in plant stems. Agrobacterium inserts a Ti plasmid into the cell of the host plant, and this plasmid contains genes that code for particular enzymes to cut the host plant's DNA and integrate segments of the Ti plasmid into it, thus being modified by horizontal gene transfer.
Errors in DNA Replication
Spontaneous mutations can occur during the synthesis phase (S phase) of the cell cycle when DNA is exposed during replication and is far more vulnerable to damage.
Different nucleotides may be paired, going against the complementary base pairing rule due to a spontaneous chemical change.
Repair mechanisms can fix the mistakes during the G2 phase of the cell cycle, but on rare occasions, this does not work.
Errors in Cell Division
As mentioned earlier, mutations can occur in both somatic cells during mitosis or germline cells during meiosis.
Mutations can result from a number of events, such as an unequal crossing over during meiosis. If non-sister chromatids misalign during crossing over, one gamete may gain extra nucleotides while another may lose nucleotides (structural mutations).
Larger-scale mutations have the chance of occurring during anaphase in both mitosis and meiosis (I and II) when homologous chromosomes don't separate at the centromere.