Molecular Biology of the Gene

Mutations

Several types of mutations can lead to the formation and expression of new traits.
A mutation is a change in the DNA sequence of an organism. Some mutations have no effect at all (such as those that occur in some noncoding regions (those that do not produce proteins or traits), or those that do not change the amino acid sequence), while others can cause serious and debilitating diseases. Mutations can occur in both prokaryotic and eukaryotic cells and can arise in two different ways: by failures in the error-checking mechanisms of the cell or by changes introduced by the environment (such as radiation exposure). Somatic mutations are those that happen within an organism's body cells during mitosis. The changes in the DNA code get passed from one cell to another when the cells reproduce. However, this type of mutation will only affect the organism itself; it does not get passed on to offspring. Germ-line mutations are mutations that occur in gamete (reproductive) cells and result in gametes that have defective genes. Because the gametes are used for reproduction in a sexually reproducing species, these mutations can get passed on to the offspring.

Effects of Mutations

A mutation may cause a loss of function, a gain of function, a reversion to a previous state, a change only under certain conditions, or no change at all.
There are many ways mutations can affect an organism. A silent mutation has no effect on protein function. It may not change the protein that is coded, or it may lie in noncoding regions of the DNA strand. A mutation may instead cause a loss of function; that is, the mutation may cause a gene to not be expressed at all. This changes the phenotype of the organism—its observable characteristics. An example of this type of mutation is albinism, a condition that causes the absence of the skin pigment melanin. Finally, a mutation may be conditional; that is, it may only be activated under certain circumstances. Although this is more common in unicellular organisms than in complex, multicellular organisms, it can happen in both. For example, sickle-cell disease arises when an individual has two copies of the gene for sickle-shaped cells. When an individual has only one copy of the gene, they produce both normal and sickle-shaped cells. They do not necessarily express symptoms of the disease, but are carriers for it. This means they can pass the disease onto their children.

Impact of Mutations on Phenotype

Type of Mutation Based on Phenotypic Effect Impact on the Organism
Silent Does not usually change protein function; happens in areas of DNA that do not code for proteins
Loss of function Affects protein function; can cause genes to not be expressed at all; recessive in diploid organisms; changes the phenotype of the organism
Gain of function New protein leads to altered function; usually a dominant trait; common in some types of cancer because the mutant allele is not prevented from functioning
Conditional Affects phenotype only under certain conditions

Mutations can have many different effects on the phenotypes of organisms.

Types of Mutations

Point Mutations

Point mutations involve the change of one or a few nucleotide bases due to addition, subtraction, or replacement of the base or bases.
The main type of mutation that occurs in DNA is called a point mutation. This involves a change in either one or a few of the nucleotide bases in the genetic sequence, from the addition, subtraction, or replacement of the base or bases. These can fall into one of two categories:
  • Transition—one base is replaced with another of the same kind (purine for purine or pyrimidine for pyrimidine).
  • Transversion—one base is replaced with another of the opposite kind (purine for pyrimidine or pyrimidine for purine).

Within these two main categories, several different outcomes can occur:

  • Missense mutation—This mutation changes the genetic information (codon) so that one amino acid is substituted for another in a given protein. It may result in a defective protein but often does not impact the protein's function if the substituted amino acid is similar enough in chemical structure and function to the original amino acid.
A missense mutation causes a single amino acid to be changed to a different amino acid. This can affect the function of the protein produced, although if the substituted amino acid is similar enough to the original one, protein function may not be impaired.
  • Nonsense mutation—This occurs when a nucleotide substitution happens that results in a STOP codon being produced. The STOP codon is a sequence of DNA bases that tells cellular machinery to end the message that makes a certain protein. This produces shorter proteins because the translation of the mRNA strand is terminated too early.
A nonsense mutation encodes a STOP codon where none should appear, truncating the protein.
  • Frame-shift mutation—This happens when one or two nucleotides are inserted or deleted from the genome. This is called a frame shift because the triplet code that is used to produce an amino acid is shifted to the left or right, resulting in a different amino acid being coded. Ultimately, this can result in an entirely new sequence of amino acids; thus, new proteins are formed.
The reading of the mutated DNA happens codon by codon (every three bases). If a particular nucleotide base is missing, the resulting protein will be altered, from that position onwards.

Chromosomal Mutations

Chromosomal mutations happen when pieces of a chromosome break off, get mixed up, or rejoin the strand in another location.
In addition to the point mutations that can occur at a single nucleotide, there are also mutations that can affect entire chromosomes. Pieces of chromosomes can break off, get mixed up, and even rejoin in different areas. These types of mutations can cause significant changes in the expression of phenotypes. There are four types of chromosomal mutations:
  • Deletion—The loss of a piece of a chromosome, such as when part of a chromosome breaks off and the two exposed ends rejoin.
  • Duplication—Part of the chromosome is doubled. This usually results from an unequal crossover, resulting in one homologous chromosome having a deletion mutation while the other has a duplication mutation.
  • Inversion—A segment of DNA breaks off a chromosome, flips around, and reattaches in the opposite direction.
  • Translocation—A segment of DNA breaks off and then attaches to a different chromosome.
Mutations, or changes in DNA, such as those seen here can result in dramatic changes in the proteins being made. Deletions cause the loss of one or more nucleotides. Duplications copy certain nucleotides, and inversions change the locations of certain nucleotides. These can all result in the expression of new traits.

Costs and Benefits of Mutations

Mutations can have harmful or beneficial effects on organisms. Mutations are essential to evolution.
Mutations can cause altered proteins to be made that, in turn, create different expressions of traits. Often, mutations lead to problems that harm the organism or even result in its death. This can happen whether the mutation causes a loss of function or a gain of function. For example, Tay-Sachs disease is a rare but fatal disorder caused by absence of the enzyme hexosaminidase A. Many cancers are caused by a gain or loss of function arising from mutations that cause cells to grow and reproduce unchecked. When such mutations arise, the resulting effects are harmful to the organism.

Sometimes, however, new traits that arise from genetic mutations are beneficial. For example, most mammals lose the ability to produce lactase (the enzyme that breaks down the milk sugar lactose) shortly after being weaned. Many humans, however, have a mutation that causes the production of lactase to persist throughout their lifetime. This lactase production allows humans to drink milk from infancy to adulthood, which provides them with nutrients such as vitamin A as well as healthy fats and proteins.

A trait may allow an individual to survive longer and produce more offspring than other individuals. When this happens, the favorable trait (from a genetic mutation) is passed on. This leads to an increase in genetic variation of an organism's population: some individuals have the trait and others do not. If the trait confers a significant advantage for survival and reproduction, more individuals who have the trait may be stronger or survive longer versus individuals who do not have the trait. Over time, most members of the species may inherit the trait. This process is known as natural selection. For example, after the Industrial Revolution, a mutation occurred in the wing color of peppered moths. This mutation changed the color of the moth's wings from white and black to dark gray and black. The darker moths could blend in better with their surroundings, which had darkened due to the pollution produced by new industries. Over time, the darker moth came to predominate in urban areas, while the lighter moth was still common in rural, less polluted areas. Thus, mutations are essential for evolution to occur.