IB Biology Sub-topic D1.3 Notes

Mutations

Just like the generation of new alleles, the process of evolution is often dependent on mutations. These are defined as changes in the base sequence of DNA. Most of the time they are not advantageous, but neutral, harmful, or lethal.

There are two main types of mutations you need to be aware of:

1. A substitution mutation replaces one nucleotide with one another. Due to the degeneracy of the genetic code, this may produce a codon for the same amino acid, having no effect, or produce a codon for a different amino acid.
2. A frameshift mutation refers to a mutation that shifts an entire part of the amino acid sequence by one or two nucleotides. This changes all the codons in that part, resulting in a different polypeptide.
   1. An insertion alters the DNA sequence by adding nucleotides to the gene.
   2. A deletion similarly will change the DNA sequence by removing nucleotides from the gene.

You can thus appreciate that mutations are more likely to cause harm than good. Since there is no cellular system in place to deliberately alter DNA sequences and change traits, mutations are mostly completely random.

However, some bases are more prone to mutation than others, meaning they are more likely to mutate. If this occurs, there are certain risks involved depending on the cell type involved:

1. Germ cells - mutations in germ cells can be passed on to offspring and inherited, resulting in genetic diseases for children even if not present in the adult.
2. Somatic cells - mutations in somatic cells can result in improperly functioning somatic cells, requiring medical treatment to compensate for. If the cell cycle is affected, then this can lead to cancer, which can be very treatable or terminal depending on the type.

Mutagens

Mutagens are radiation or chemicals that increase the rate of mutation. The three types you need to remember are:

1. α, β, or γ radiation
2. UV light
3. Certain chemicals, such as the ones found in cigarettes.

Therefore, large scale releases of radiation increase cancer rates. You need to be aware of two examples:

1. Hiroshima bombings killed 150,000-200,000 individuals in the blasts. However, survivors developed 17,448 tumours between them.
2. Similarly, the nuclear meltdown of Chernobyl produced 6 tonnes of uranium particles, killing flora and fauna but more notably increasing the incidence of thyroid cancer markedly in survivors.