A mutation is a rare change in a gene’s DNA sequence. The result of these genetic mistakes can range from no effect on the organism to the production a defective cellular protein that may have disastrous implications.
Deoxyribonucleic acid (DNA), is the genetic material of cells; the organic molecule of genes and the chromosomes that genes are part of. Nucleic acids are polymers (big molecules) built from monomers (little molecules) called nucleotides. In order to understand what a mutation is, it helps to know a little about the structure of DNA’s building blocks, nucleotide.
DNA’s nucleotide monomers consist of 3 portions:
1. a pentose sugar
2. one or more phosphate groups
3. one of four cyclic nitrogenous bases
* Nucleotide Bases & the Genetic Code *
The nucleotides of DNA each contain one of four possible nitrogenous bases:
Purines (double-ring bases):
* Adenine (A)
* Guanine (G)
Pyrimidines (single-ring bases):
* Cytosine (C)
* 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.
* DNA Replication: Copying DNA *
Most of the cells in our bodies are frequently dividing, creating new cells. When cells divide, a new copy of DNA must be made, so that each new cell has a complete set of genetic instructions.
* Translation: Making Proteins *
Translation is the process in which the nucleic acid blueprint is read and the information used to build a protein molecule. Each group of three nucleotides in the genetic 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 and Mutations *
Very rarely, when a new DNA molecule is being built, the wrong nucleotide base is inserted. This is a mutation. Some mutations are large-scale and can involved large chromosomal sections or entire genes. Other mutations are considered small-scale, only affecting one or a few nucleotides. Small mutations may or may not cause a problem with the DNA blueprint being translated. The following are different types of small-scale mutations:
* Point Mutations *
Point mutations involve the exchange a single nucleotide for another; most typically a purine for a purine (A switch with G) or a pyrimidine for a pyrimidine, (C switched with T). A similar, but less common mistake is called a transversion, when a purine is used instead of a pyrimidine or a pyrimidine instead of a purine (C/T A/G).
Some sections of DNA code for the building of a protein molecule (translation). Point mutations that occur within the protein coding region of a gene may be classified as one of three kinds, depending upon what the mutated codon codes for:
1. Silent mutations: Even with the wrong base, the codon still codes for the same amino acid.
2. Missense mutations: The erroneous codon codes for a different amino acid than it should.
3. Nonsense mutations: The erroneous codon codes for a stop and can terminate the protein before it is completed.
* Insertions, Deletions & Frameshift Mutations *
This type of genetic mistake is when one or more extra nucleotides are added into, or deleted from, the DNA. Insertions and deletions typically have a disastrous effect on the protein that is coded for by the gene, and can significantly alter the gene product (protein).
Because mRNA (the blueprint of DNA that is read to build a protein) is read in a series of three nucleotides (the nucleotide triplet) insertions and deletions can be frameshift mutations whenever the number of nucleotides added or deleted are not a multiple of three. The result of a frameshift mutation is that all bases downstream from the mutation will be improperly grouped into codons. Unless this mistake occus very near the end of the gene, it will almost certainly result in a nonfunctional protein.
* Sources *
Brown, Bryson (2007) Evolution: A Historical Perspective. Greenwood Press.
Campbell & Reece (2005) Biology, 7th Edition. Pearson