4.4: When DNA Changes- Mutations

What is a Mutation?

A mutation is any change in the nucleotide sequence of DNA. The word often conjures images of science fiction monsters or superheroes with incredible abilities. In reality, mutations are extremely common. Every one of us carries dozens or even hundreds of mutations, and the majority of them have not noticeable effect.

Mutations can occur spontaneously during DNA replication like a kind of copying error, or be triggered by environmental agents called mutagens—things like UV radiation from the sun, certain chemicals, or high-energy radiation.

Types of Mutations

Point mutations involve a change in a single nucleotide. The effect depends on what the change does to the resulting protein:

• A silent mutation changes a codon but codes for the same amino acid with no change to the protein.
• A missense mutation changes the codon so a different amino acid is inserted which may make the protein work differently or not at all.
• A nonsense mutation creates a premature stop codon, shortening the protein.

Frameshift mutations happen when a nucleotide is inserted or deleted, shifting the entire reading frame. Because the cell reads codons in groups of three, even one extra or missing nucleotide scrambles every codon that follows. These mutations often have dramatic effects.

Chromosomal alterations affect entire chromosomes. Segments can be deleted, duplicated, inverted, or translocated to a different chromosome. Down syndrome (trisomy 21), for example, results from having three copies of chromosome 21 instead of two.

Mutations and Evolution

From an evolutionary perspective, mutations are essential—they are the raw material of evolution. Most are neutral or harmful in the short term, but occasionally a mutation produces a slight advantage in a particular environment and spreads through a population over generations. The immense diversity of life on Earth ultimately traces back to accumulated mutations over billions of years.

Germline vs. Somatic Mutations

Germline mutations occur in sperm or egg cells. Because gametes are passed on to offspring, germline mutations are heritable and can be transmitted to children and grandchildren.

Somatic mutations occur in other body cells. They affect only the individual and cannot be passed to offspring. However, somatic mutations can accumulate over a lifetime and are a major cause of cancer.

A Built-in Edit Button for Evolution

If DNA is a blueprint, RNA is the messenger that carries those instructions to the rest of the cell. Normally, that message is copied faithfully. But octopuses appear to break this rule.

Researchers have discovered that octopuses can edit their own RNA on the fly, effectively rewriting the message before it’s used. This gives individual octopuses a way to adjust traits, like tolerance to cold water, without waiting for a DNA mutation to spread through the population over generations. Some evidence suggests this editing ability may be linked to octopus intelligence as well. Nautiluses, close relatives that lack this RNA-editing trick, are notably less clever.

But this flexibility comes at a cost. To protect their RNA-editing system, octopus DNA is unusually resistant to mutation—the raw material evolution normally relies on. So while an individual octopus can fine-tune itself in real time, the species as a whole barely changes from one generation to the next.

This is a vivid example of the trade-offs you’ve been learning about: mutation drives long-term evolutionary change, but it also introduces risk. Octopuses seem to have found a workaround by sacrificing some of that mutation rate in exchange for a built-in editing tool that lets individuals adapt within their own lifetime. It’s a reminder that the central dogma isn’t always a one-way street, and that evolution can solve the same problem—adapting to a changing environment—in more than way.

Source: Scharping, N. (2018, January 1). Octopuses Can Stray From Their DNA. Discover, 39(1), 38–39.