In this explainer, we will learn how to define what a mutation is, recognize various types of mutations, and state some potential impacts of mutations.
Mutations are changes in an organism’s genome. For example, a rare mutation in the TYR gene can cause albinism in humans, where the body does not make any pigment in their skin, hair, and eyes. This can result in a person having very pale skin and very white hair. Mutations are incredibly important as they create genetic diversity. They drive evolution and allow species to adapt to changing environments and conditions. Two mutation types exist: genetic and chromosomal mutations.
Let’s take a look at genetic mutations first.
Definition: Genetic Mutation
A genetic mutation is a change in the base sequence of a DNA molecule.
DNA is a long molecule with two sugar–phosphate strands the wind around each other to form a twisting ladder shape called a helix. Inside the helix, attached to the sugar–phosphate background, are four DNA bases, adenine (A), guanine (G), cytosine (C), and thymine (T). Its structure is shown in Figure 1. Within DNA molecules, the order and number of the bases encodes genetic information to create characteristics and traits.
DNA sequences that encode information for creating characteristics are called “genes.” A gene is a sequence of DNA that codes for a particular protein, which in turn will produce a particular characteristic or version of a characteristic. For example, our genes are responsible for determining our eye color, natural hair color, and blood type and even play a role in how tall we can grow!
A gene is a sequence of DNA that contains the information needed to produce a certain characteristic (e.g., eye color).
So, how does a sequence of DNA become a protein? Frist, the cell converts DNA sequences into a similar molecular sequence called RNA. The RNA sequence is then “read” by special organelles called ribosomes. Ribosomes translate the RNA into specific protein products that create observable characteristics in the organism. A basic outline of this process is shown in Figure 2.
It should be noted that gene sequences are translated into protein by “reading” the gene sequence in groups of three bases. Every three DNA bases correspond to a specific amino acid. This way of dividing nucleotides into consecutive triplets is called the sequence’s “reading frame.”
When a sequence of mRNA is translated into a protein, a ribosome matches every three mRNA bases to their corresponding amino acid. This allows the cell to create and reproduce a diversity of unique and specific proteins. The reading frame is very important for building the correct protein and expressing its corresponding trait. A codon wheel, such as the one shown below in Figure 3, can be used to determine what amino acid is matched to a sequence of three mRNA nucleotides. For instance, reading to , the sequence AGC will give the amino acid serine (ser).
Not all of an organism’s DNA are genes, some DNA has been considered “junk DNA” because it does not encode any information for observable traits! Other DNA sequences provide a location for regulatory proteins or contain repetitive sequences that prevent degradation of the DNA molecule. Even genes have DNA sequences that are removed after transcription into RNA. These removed sequences are called “introns” and are not translated into a protein. Meanwhile, expressed gene sequences are called “exons.”
Genetic mutations are changes in an organism’s DNA base sequence. Genetic mutations arise from DNA copying mistakes or exposure to outside environmental conditions, like UV radiation or harmful chemicals. Mutations can alter an organism’s gene sequences, which encode protein synthesis instructions. If an organism’s gene sequence mutates, its encoded protein sequence may change. As proteins influence an organism’s traits, mutations can ultimately alter an organism’s phenotype (an organism’s physical characteristics).
The phenotype is the observable traits of an organism and is determined by its genotype.
Mutations can occur anywhere in the genome. Suppose a DNA change occurs outside a gene sequence in junk DNA. In that case, the mutation likely has little or no phenotypic effect, as the mutation does not alter a protein-coding gene. Such mutation is “neutral,” as the organism has no phenotypic change. However, if the mutation occurs in a gene sequence, the mutation may affect the organism’s phenotype.
Suppose a mutation alters a gene sequence so that its corresponding protein is harmfully altered. In that case, the organism will be harmed by the mutated phenotype. For instance, mutations in the insulin protein’s receptor-binding domain can lead to diabetes, as the insulin receptors no longer function properly.
On the other hand, a mutation can also change a gene sequence to beneficially change its corresponding protein. For example, some people have a mutation in the CCR-5 gene that changes the receptor that HIV uses to get into immune cells, giving them resistance to HIV infections. The organism will then benefit from the mutation. Therefore, mutations may have neutral, harmful, or beneficial effects.
There are several genetic mutation types. Genetic mutation types include substitution, deletion, and insertion mutations.
In a substitution mutation, one DNA base is exchanged for another DNA base at a specific genomic location (a locus). A substitution mutation is depicted in Figure 4. If a substitution mutation occurs in a gene, several different outcomes may occur. If the base substitution does not change the final amino acid sequence encoded (as multiple codons code for some amino acids), the mutation is neutral. Consequently, no observable change occurs.
However, if a substitution within a gene exchanges a DNA base for another so that it encodes a different amino acid in the final protein, such substitution may have either beneficial or harmful phenotypic effects. The base substitution may even encode a stop codon, causing protein translation to stop prematurely, which can produce a nonfunctional protein. This usually has serious detrimental effects on the organism.
Key Term: Substitution Mutations
In substitution mutations, a DNA base is swapped for another DNA base at a specific position in an organism’s DNA sequence.
In insertion mutations, one or more new base pairs are inserted at a DNA locus. This process is shown in Figure 5.
Conversely, in deletion mutations, one or more existing DNA base pairs are lost at a DNA locus, as shown in Figure 6.
Key Term: Insertion Mutation
In an insertion mutation, one or more new bases are added to a location with an organism’s DNA sequence.
Key Term: Deletion Mutation
In a deletion mutation, one or more existing base pairs are lost (or deleted) from an organism’s DNA sequence.
Suppose base pairs are inserted into or deleted from a gene’s reading frame. In that case, this may have severe consequences for the organism. When a number of base pairs other than three or multiples of three are inserted into a gene sequence, the reading frame shifts and results in an entirely different protein sequence. This sequence is likely random and introduces a premature stop codon that results in a dysfunctional protein product that harms the organism. Additionally, this process can introduce a frameshift that changes all amino acids after the change in the DNA code that can harmfully change an organism’s characteristics. This process is shown in Figure 7.
Example 1: Understanding Different DNA Mutation Types
Which of the following is not a type of genetic mutation?
A genetic mutation is a base change in a DNA sequence. There are several genetic mutation types. If one or more bases are lost from the original DNA sequence at a specific location in the genome, this is said to be a “deletion mutation,” making a deletion incorrect (type 1).
Type 1: A deletion mutation in DNA sequence
Another genetic mutation type is an insertion mutation. In insertion mutations, one or more new nucleotides are inserted into the DNA sequence at a specific genomic location (type 2). Therefore, an insertion is incorrect.
Type 2: An insertion mutation in a DNA sequence
Finally, another genetic mutation is the substitution mutation. In substitution mutations, one nucleotide base is swapped for another nucleotide base in a DNA sequence (type 3), making substitutions incorrect.
Type 3: Substitution mutations in a DNA sequence
Cells undergo differentiation by expressing genes that characterize specific cell types. For example, a stem cell (a nondifferentiated cell) will differentiate into a nerve or muscle cell by expressing different genes. However, a cell’s DNA sequence does not change during differentiation and therefore is not a mutation type. Thus, B is correct: differentiation is not a type of genetic mutation.
Example 2: Determining Which Mutation Types Cause Frameshift Mutations
Which of the following types of mutations can cause a frameshift in a DNA sequence?
- Insertion and deletion
- Deletion only
- Insertion and inversion
- Substitution and saturation
- Substitution only
Gene sequences consist of nucleotide bases. Within the gene, every three nucleotide bases are a codon. When ribosomes translate the gene into protein, every codon codes for a specific amino acid. A gene’s reading frame is how a gene’s bases are grouped into codons, and so a “frameshift” refers to a change that results in a large portion of the reading frame being read in a different way. Frameshift mutations will change the number of bases within the reading frame to a number that cannot be divided by 3.
Deletion mutations are mutations that remove a base from a gene. This will alter the reading frame and is likely to generate different codons and, therefore, a new protein sequence. An example of a deletion mutation is shown below. As you can see, removing just one base changes the entire amino acid sequence, and the number of nucleotides is no longer divisible by three.
Substitution mutations do not change a sequence’s reading frame, as original nucleotides are simply swapped for new nucleotides. The codon order does not shift, and no frameshift mutation occurs, making D and E false.
Inversions are large chromosomal mutations where a chromosomal segment breaks away from its parent chromosome and reattaches in the reversed orientation. Though the inverted chromosome may lose nucleotides from an inversion event, for a frameshift to occur, the nucleotide number of a gene sequence (coding DNA) must change. As most genomic DNA is noncoding, the chance of a chromosome reattaching at a gene sequence is tiny, making C false.
However, in addition to deletions, mutations that add bases to a particular location within a gene sequence can also modify the sequence’s reading frame, generating an entirely new protein product. These mutations are called insertions, as new bases are “inserted” into the original DNA strand. An example of this is shown below. As you can see, the insertion of one base early in the reading frame changes the way the entire sequence is translated into an amino acid sequence.
As both insertions and deletions can cause frameshift mutations, the correct answer is option A: insertions and deletions.
Mutations may be much larger than small genetic mutations and can involve whole chromosomes and chromosome segments. As chromosomes have many genes, chromosomal mutations alter large portions of the chromosomes’ structure, consequently mutating many genes by either deleting or duplicating genes. The collective effect of these mutations usually harms the organism.
Several chromosomal mutation types exist.
In chromosomal duplications, an organism carries one or more extra copies of one or more chromosomes or chromosome segments. A large amount of extra genetic material may be harmful but can give evolutionary advantages in plants. For example, polyploid plants may have increased leaf and flower size. Organisms with more than two chromosome sets are said to be polyploid. Many crops are even bred for polyploidy.
Key Term: Chromosomal Duplication
In a chromosomal duplication, an organism carries an extra chromosome(s) or chromosomal segment(s).
In chromosomal deletions, one or more whole chromosomes or chromosome segments are lost or deleted. Similar to chromosome duplications, this large change in genetic material amount has significant phenotypic consequences. For example, individuals with trisomy 21 have three copies of chromosome 21. This extra chromosome 21 copy causes a distinct short stature, shorter lifespan, and intellectual disability.
Key Term: Chromosomal Deletion
In a chromosomal deletion, a chromosomal segment or whole chromosome is lost from an organism’s genome.
In an inversion, a chromosome segment breaks off from the rest of the chromosome and reattaches in the reversed direction. Chromosomal inversions rearrange the chromosome. In humans, chromosomal inversions can cause fertility problems but typically carry no other harmful effects. For example, inversions on chromosome 9 can cause fertility problems.
Key Term: Chromosomal Inversion
Chromosome inversions are chromosomal rearrangements where a chromosome segment is reversed end to end.
Chromosomal deletions, duplications, and inversions are shown in Figure 8.
Example 3: Describing Cells with Three or More Chromosome Copies
In a normal human cell, there are two copies of each chromosome. Chromosomal mutations can duplicate entire chromosomes. What term is given to a human cell that has more than two copies of a chromosome?
Each normal human body cell contains 23 chromosome pairs, with 46 total chromosomes. A cell with two copies of each of its chromosomes is diploid, and a cell with only one copy of each of its chromosomes is haploid.
In humans, body cells are diploid and gametes (sperm and egg cells) are haploid.
Cells with three or more copies of each chromosome are polyploid. Very few human cell types are polyploid, but many plant cells are. As the question describes a cell with three or more copies of each chromosome, the cell is polyploid.
Example 4: Illustrating a Chromosomal Inversion Mutation
The diagram provided shows a simplified outline of the different types of chromosomal mutations that can occur. Which diagram (1, 2, or 3) demonstrates an inversion mutation?
Several chromosomal mutation types exist. Organisms with chromosomal duplications carry an extra copy (or copies) of a chromosome (or chromosomes) or chromosome segments. In contrast, organisms with chromosomal deletions have chromosomes or one or more chromosome segments that are missing.
The third chromosome mutation type is an inversion: a chromosome segment breaks away from its parent chromosome and reorients itself in the opposite direction. Reoriented, it then reattaches back onto the parent chromosome.
Both mutations 1 and 3 are chromosomal deletions, as chromosome segments are missing compared to the original chromosome.
However, in chromosome 2, the blue and gray segments are reversed between the parent and mutated chromosomes. This chromosome segment most likely broke off from the original chromosome and reattached to the chromosome in the opposite direction.
Therefore, mutation 2 is an inversion mutation.
Mutations arise through different mechanisms. Spontaneous mutations occur randomly and arise from natural biological processes like DNA replication errors in sperm and egg production. Spontaneous mutations typically affect all of an organism’s cells, including their reproductive cells, and will be passed on to its offspring.
Definition: Spontaneous Mutations
Spontaneous mutations are random mutations that occur from DNA replication errors during gamete formation. They typically are found in all of an organism’s cells.
On the other hand, induced (or acquired) mutations may arise from certain environmental substances and conditions that mutate DNA. Such conditions and substances are considered “mutagenic,” as they generate many acquired mutations in organisms. These induced mutations exist only in some body cells.
Definition: Induced (or Acquired) Mutations
Induced mutations are mutations that arise from exposure to environmental substances or conditions. They occur only in some of an organism’s cells and are not inherited.
Mutagenic substances and conditions include UV radiation, harmful chemicals, and carcinogens (substances that cause cancer). Acquired mutations can be incredibly destructive. If an acquired mutation occurs in a cell cycle gene, it may cause cancer. Consequently, mutagenic substances are incredibly hazardous.
Example 5: Describing the Inheritance Pattern of Somatic Mutations
True or False: If a mutation occurs in a normal body (somatic) cell of an organism, that mutation will be passed on to that organism’s offspring.
Inherited mutations are passed on from parents to offspring and occur spontaneously in germline cells (sperm and eggs) from DNA replication errors. When a mutation occurs in a gamete cell (a sperm or egg cell) that later develops into an organism, all its descendent cells will carry the mutation.
This directly contrasts with acquired mutations that occur in some body (somatic) cells. Acquired mutations are induced from exposure to certain environmental conditions and substances (like radiation and harmful chemicals) that mutate DNA. Only some somatic cells carry the acquired mutation after exposure to a mutagenic agent. Additionally, the mutation will not be passed on to an organism’s offspring.
As the question describes a mutation in a somatic cell, the mutation is acquired. It will not be passed on to the organism’s offspring.
Therefore, the statement is false.
- Mutations are changes to an organism’s genetic material and drive evolutionary change.
- Genetic mutations are base changes in a DNA molecule.
- Genetic mutations include insertion (addition of new DNA bases), deletion (removal of existing DNA bases), and substitution (swapping of an existing DNA base with a new DNA base) mutations.
- Insertion and deletion mutations can alter a gene’s reading frame, significantly changing the protein it encodes.
- Chromosomal mutations include duplications (extra copies of whole chromosomes or chromosome segments), deletions (missing copies of whole chromosomes or chromosome segments), and inversions (chromosome segments attached in the opposite orientation).
- Mutations may arise spontaneously and randomly in germ cells or may be induced from environmental conditions or substances in body cells.
- Mutations can have neutral (no effect), beneficial, or harmful effects to an organism’s phenotype (physical characteristics).