Explain how mutations change the DNA sequence and therefore proteins and traits, and how they can be harmful, neutral, or beneficial (Ohio's Learning Standards for Science, Biology, B.H.4 and B.H.5).

What this topic is asking

Ohio standards B.H.4 and B.H.5 say that the molecular structure of DNA determines its information and that the structure of DNA determines the structure of proteins. A mutation is what happens when that structure changes. Ohio's Biology EOC turns this into items on what a mutation is, the main types, how a changed base can change a protein and a trait, and why mutations can be harmful, neutral, or beneficial. This topic is the bridge from molecular genetics to natural selection and adaptation, because mutations are the ultimate source of new alleles.

What a mutation is

Mutations happen because DNA, although copied very accurately, is not copied perfectly, and because outside agents can damage it. A mutation in a body cell affects only that cell and its descendants; a mutation in a cell that makes gametes can be passed to offspring and so enter the next generation.

The main types of point mutation

The EOC focuses on three small-scale (point) mutations.

• Substitution. One base is swapped for another (for example A becomes G). This changes at most one codon, so it may change one amino acid, or none at all if the new codon codes for the same amino acid.
• Insertion. An extra base is added. This can shift the entire reading frame from that point on (a frameshift), changing many codons and amino acids.
• Deletion. A base is removed. Like an insertion, it can shift the reading frame and change many amino acids downstream.

Insertions and deletions often have a bigger effect than substitutions because they can disrupt every codon after the change.

From mutation to protein to trait

The reason a mutation matters is the chain back through protein synthesis. The DNA sequence sets the mRNA codons, which set the amino acids, which set the protein, which affects the trait. So a change in one DNA base can:

1. change a codon,
2. change an amino acid,
3. change how the protein folds or works,
4. change the trait the protein produces.

If the protein is an enzyme, for instance, a mutation might make it work better, worse, or not at all.

Causes: mutagens

Some mutations arise from random copying errors, but their rate is increased by mutagens, agents that damage or alter DNA.

• Radiation, such as ultraviolet (UV) light from the Sun, X-rays, and other ionizing radiation.
• Certain chemicals, such as some compounds in tobacco smoke.

Mutagens that increase the risk of cancer are also called carcinogens, which connects back to the mutation-cancer chain in the cell cycle and mitosis.

Harmful, neutral, or beneficial

A central EOC point is that a mutation is not automatically bad. Its effect depends on how it changes the protein and the situation.

• Harmful. Damages the protein in a way that reduces survival or reproduction (many genetic disorders).
• Neutral. Has no real effect on survival, for example a substitution that does not change the amino acid, or a change in a region that is not used.
• Beneficial. Improves survival or reproduction in the current environment (for example a mutation that lets bacteria resist an antibiotic).

Because mutations create the new alleles that variation is built from, beneficial mutations are the raw material of adaptation. If a mutation helps its carrier survive and reproduce, natural selection can make that allele more common over generations.

Try this

Q1. Name the three types of point mutation and state what each does to the DNA. [3]

• Cue. Substitution swaps one base for another; insertion adds a base; deletion removes a base.

Q2. Explain why mutations are described as the source of new alleles. [2]

• Cue. A mutation changes the DNA sequence, creating a new version (allele) of a gene that did not exist before; other processes only shuffle existing alleles.