A mutation is a rare change in a gene's DNA sequence that can result in a defective cellular protein. This article covers the basics of genetic mutation.

What is a Mutation?

Genes are units of inheritance that exist within chromosomes. Each chromosome is a double-stranded molecule of DNA. A mutation is an alteration of a gene’s DNA sequence that can result in changes to the protein that the mutated gene codes for.

To better understand the relationship between mutations, genes, DNA and proteins, it is helpful to first review what DNA is and how it works.

Nucleic Acids & Nucleotides

Deoxyribonucleic acid (DNA) is the genetic material of cells. Nucleic acids are polymers (big molecules) composed of monomers (little molecules) called nucleotides. Nucleotide monomers consist of 3 portions:

• a pentose sugar
• one or more phosphate groups
• one of five cyclic nitrogenous bases

The Twisted “Ladder” of Nucleic Acid

The nucleotides of DNA are linked together by covalent bonds between the phosphate of one nucleotide and the sugar of next, creating a phosphate-sugar backbone with the nitrogenous bases extending from this backbone like the teeth of a comb.

In all living things, DNA exists as a double-stranded molecule with the bases from each strand attracted to each other in hydrogen bonds. The hydrogen bonding also twists the phosphate-deoxyribose backbones into a twisted helix shape.

Nucleotide Bases & the Genetic Code

The nucleotides of DNA each contain one of four possible nitrogenous bases:

• Adenine (A)
• Cytosine (C)
• Guanine (G)
• Thymine (T)

The specific base is the only thing that makes one nucleotide differ from another. When nucleotides exist together in a nucleic acid, such as the DNA of our genome, the sequence of these bases is actually the genetic code making each of us unique.

Replication: Copying DNA

Most of the cells in our bodies are frequently dividing, either mitotic division (mitosis) that provides more cells for growth, repair and replacement, or meiotic cell division (meiosis) that forms gametes (sperm and ova sex cells). When cells divide, a new copy of DNA must be made, so that each new cell has a complete set of genetic instructions.

Transcription & Translation: Building Proteins Based on the DNA Blueprint

Transcription

The information in DNA is also copied during transcription, the process in which genetic information is transferred from DNA to another type of nucleic acid, ribonucleic acid (RNA). It is RNA’s job to transport the genetic information out of the cell’s nucleus and use it as instructions for building proteins.

Proteins are large organic molecules that are absolutely vital to the structure and function of our cells. Like nucleic acids, proteins (also called polypeptides) are polymers made of monomer subunits. The subunits of proteins are called amino acids.

Translation

Translation is the process in which the RNA blueprint is read and the information used to build a protein molecule. Each group of three nucleotides in our mRNA blueprint is called a codon and encodes for one amino acid. In other words, our cells read the nucleic acid triplet code and build proteins based on the 3-nucleotide ‘words.’

DNA, Genes and Mutations

Very rarely, when a new DNA molecule is being built, the wrong nucleotide base is inserted. This is a mutation. This type of mistake may result in a three-base codon coding for the wrong amino acid during translation. Using the wrong amino acid to build a protein could then result in a defective protein molecule that doesn’t work properly.

Mutations are passed on from one generation to the next only if this error occurs in cells that ultimately give rise to sperm and eggs. If a mutation happens in somatic cells (cells of the body that are not gametes), the mutation will be limited to the organism that it occurred in.

More Information on DNA & Genetics

For additional resources on genetics and cell biology, see the science website Science Prof Online.