Genetic Mutations: Change, Change, Change

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A bite by a radioactive spider turned Peter Piper into Spiderman. Dr Bruce Branner turned into The Incredible Hulk when he was exposed to gamma radiation. In the X-Men, mutants inherit a mutant gene that gives them extraordinary powers.

Mutations have been the subject for many comic books and movies, but you don’t need a freak accident to get mutations. As a matter of fact, mutations occur in all of us. The process is actually quite simple. And though they don’t give us the same superpowers as our favourite superheroes, mutations do have other superpowers - the diversity of life.

This post explains what mutations are, the main types, and how mutations may affect an organism. The next post will look at how mutations can lead to genetic diseases or help organisms to adapt to their environment.

But before we can look at mutations, we first need to understand our genetic information.

Setting the Scene

The genome is the inherited genetic material found in all living cells. It provides all the information to make an organism and enable it to function during its lifetime. The genome consists of long double-stranded molecules called DNA (deoxyribonucleic acid), and each DNA molecule is packaged into structures called chromosomes. Every species will have a characteristic number of chromosomes, for example, all the cells in our bodies (expect for sperm and eggs) have 46 chromosomes (or 23 pairs).

DNA is made up of a four-letter code called nucleotides. The four nucleotides are adenine (A), thymine (T), cytosine (C) and guanine (G). As 0s and 1s encode the information in a computer file, these four nucleotides encode the genetic information that make up the living cell and control all of its activities. This genetic code is universal. So, you can take a piece of DNA from a human cell, insert it into bacterial DNA, and the bacteria will be able to read and translate the information as well as copy it. Pretty cool!

But DNA does not do the work itself. Something else is needed to follow its instructions. Imagine DNA as the blueprint of life, similar to an architect’s blueprint. The DNA blueprint has all the detail needed to build the cell, but its construction and functioning is carried out mostly by proteins.

And one last thing. Genes are segments of the DNA that encode the information to make a particular product; either a protein or an RNA (ribonucleic acid) molecule.

What is a Mutation?

A mutation is a change in the DNA. Any change.

The mutation can be small, changing a single nucleotide, or it can be big, for example, insertion or deletion of larger segments of the DNA molecule. These changes can lead to disease or disrupt normal development.

How do Mutations Happen?

Many mutations are spontaneous. They happen during normal cellular events or through random interactions with the environment. For example, errors are sometimes made while making copies of the DNA (during DNA replication). Every organism has a background level of spontaneous mutations, and though relatively rare, these mutations are a source of genetic variation in populations.

Mutations can also be caused by certain physical or chemical agents called mutagens. Ultraviolet (UV) light and chemicals in cigarette smoke are examples of mutagens.

Mutations that occur in the gametes (the egg or sperm) can be passed down through the generations. But if the mutation disrupts a gene that is essential for the organism to survive, then the mutation is lethal.

Point Mutations

Mutations that are caused by a single change are called point mutations. Point mutations are usually categorised as:

·       Nucleotide substitutions, for example, replacing T with an A

·       Nucleotide insertions when a single nucleotide is added, or

·       Nucleotide deletions when a single nucleotide is removed.

These different point mutations are illustrated in Figures 1A – 1C below.

Nucleotide substitution

Nucleotide substitution

Figure 1A: Nucleotide substitutions. A nucleotide in the original DNA molecule is replaced with another

Copyright © MondoScience

Nucleotide insertion

Nucleotide insertion

Figure 1B: Nucleotide insertions. A single nucleotide is inserted into the original DNA molecule. Copyright © MondoScience

Nucleotide deletion

Nucleotide deletion

Figure 1C: Nucleotide deletion. A nucleotide is deleted from the original DNA molecule. Copyright © MondoScience

The effect of a point mutation to a cell differs. It will depend on how the new change in the gene affects the final protein it encodes. Point mutations can therefore lead to a variety of outcomes such as:

  • No change to the protein. The protein functions as normal as if the mutation never happened. This is known as a silent mutation

  • A change to the protein that has little or no effect on the protein functioning as normal

  • Change in the protein that prevents it from functioning properly; a loss of function mutation

  • A change so the protein now has a new function; a gain of function mutation, or

  • No protein made at all

Mutations involving nucleotides deletions or insertions nearly always have a more disastrous effect. In these cases, the final protein will probably not function at all. If that protein is essential for the cell to function, then these mutations are lethal.

Chromosome Mutations

Mutations can also be over large segments of DNA affecting its structure. Some of these large mutations are illustrated in Figures 2A – 2D below. Changes in chromosome structure affect many genes and the proteins that they encode. Chromosomal mutations include:

  • Translocations – when part of one chromosome has broken off and attached to another

  • Inversions – part of the chromosome is broken off, rotated 180° and then reinserted back into the chromosome

  • Duplications – part of the chromosome is copied resulting in extra copies of the genetic material

  • Deletions – loss of genetic material with all of the genes in the area lost

Chromosome Translocation, Inversion, Duplication, Deletion

Chromosome Translocation, Inversion, Duplication, Deletion

As with point mutations, some of these structural changes have little effect, while others can lead to disease.

In summary, mutations are changes in the DNA. They can occur spontaneously or may be induced by mutagens. All of these changes have different consequences to the cell.

The next post will look at how mutations lead to genetic diseases and adaptation.

After all, mutations are the main reason for the diversity in organisms. And without mutations, evolution would not happen.

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