Mutations are alterations of the amino acid codes within our DNA. You may associate mutations with disease; however, mutations are responsible for species survival, natural selection, and genetic diversity within a species. While mutations may help us in the grand scheme of things; they are also responsible for a majority of diseases that plague humans and animals alike. So, how do point mutations occur? Let's delve deeper to find out.
Point mutation definition
In humans, most point mutations do not cause any adverse complications; however, some point mutations are responsible for terrible diseases.
A point mutation occurs when one specific nucleotide base pair is added, deleted, or changed within a genome.
The trillions of cells within your body experience point mutations each day.
These genetic chances are due to random copying errors within your DNA as your cells divide or as your cells become exposed to environmental toxins such as UV rays.
Point mutations during DNA replication
Point mutations occur during DNA replication when a cell is dividing. DNA replication is the process by which a dividing cell copies its DNA genome in order for the new daughter cell to have a complete DNA genome of its own. In eukaryotic cells, DNA replication occurs in the cell's nucleus during interphase.
Let's take a closer look at the process of DNA replication.
The first step of DNA replication is separating the double helix into two single strands. This job is done by the enzyme helicase. Helicase separates the DNA strands by breaking the hydrogen bonds that hold the base pairs together. As helicase pulls the DNA strands apart, SSB (single-stranded binding proteins) proteins bind to each DNA strand to prevent them from rejoining.
Meanwhile, another enzyme called topoisomerase binds to each DNA strand to prevent them from coiling, as the DNA strands need to be lined up perfectly for successful replication. Once the strands are nice and separated, the enzyme primase places primers at the 3' end of each DNA strand so that DNA polymerase will know where to start copying.
DNA polymerase is responsible for creating the 2 new complement strands of DNA that will bind to each original strand, resulting in the formation of two double helices. DNA polymerase does this by matching DNA bases.
Point mutations arise if DNA polymerase inserts, changes, or deletes a base pair within the DNA while it is forming the new complement strand.
Usually, point mutations occur primarily in germ cells; however, point mutations can also occur in somatic cells. Point mutations in somatic cells can give rise to cancer within an individual, while point mutations in germ cells can give rise to hereditary diseases.
The majority of cells within your body are somatic cells. Somatic cells are diploid and divide via mitosis. These cells are responsible for many functions within your body such as breathing, maintaining heart rate, and digestion. Point mutations in these cells may not affect a person's offspring. Germ cells on the other hand create gametes which are reproductive cells and divide via meiosis. Female germ cells are called eggs, while male germ cells are called sperm. Point mutations in these cells will be passed down through generations.
Point mutations usually occur during DNA replication. During DNA replication, your double-stranded DNA is separated into two single-stranded pieces that serve as templates for the complementary strands. During the replication process, a single base may be deleted, changed, or added which can change the amino acid that the affected nucleotide codes for.
Point mutation example
Point mutations occur in a wide range of our cells and most of them are harmless; however, some of them do cause disease.
For example, in sickle cell disease, a single-point mutation in the beta-hemoglobin gene converts a GAG codon and turns it into a GUG codon. GAG is responsible for encoding glutamic acid; while GUG encodes valine. The replacement of glutamic acid for valine changes the shape of the hemoglobin protein found in blood cells, which causes them to take a sickle shape and stick together.
Types of point mutations
Point mutations come in many varieties based on the changes they make to the affected DNA or RNA strand.
DNA is made up of five different nucleotides: cytosine (C), guanine (G), adenine (A), and thymine (T). RNA on the other hand is made up of cytosine (C), guanine (G), adenine (A), and uracil (U). Within DNA and RNA, each nucleotide base is arranged in groups of three, known as codons. Each codon encodes a specific nucleic acid that is responsible for carrying out important functions.
A substitution mutation occurs when one base pair is substituted for another. This could be replacing cytosine with guanine. Replacing one base pair opens the door for many types of changes. There are three types of substitution mutations: nonsense, missense, and silent.
A nonsense mutation occurs when the substitution of a single base pair creates a stop codon instead of a codon that produces an amino acid. 1
The creation of the stop codon prevents the entire downstream strand of DNA from being read and coded into amino acids.
Within DNA stop codons are TAG, TAA, or TGA while in RNA there are UAG, UAA, or UGA.
Usually, these stop codons are found at the end of the DNA or RNA sequence; however, a substitution mutation causing one of these stop codons to appear in the beginning or middle of the DNA will prematurely terminate the amino acid sequence, resulting in the production of the wrong protein.
Unlike a nonsense mutation, a silent mutation basically has no effect on the amino acid sequence.1
A silent mutation occurs when a substitution produces a codon that codes for the same amino acid as the original. Silent mutations occur because multiple codons can code for the same amino acid.
For example, a silent mutation in an AAG codon where the G is substituted for an A will produce AAA. Since AAG and AAA both code for lysine, the amino acid sequence, and the subsequent protein will not be changed.
Point mutation vs frameshift mutation
As you know from the previous sections above, a point mutation results when a single base pair is substituted for another base pair. What happens when an extra base pair is added? This phenomenon is known as an insertion mutation.
An insertion mutation occurs when an extra base pair is added to DNA, while a deletion mutation occurs when a base pair is deleted from a DNA sequence.1
Insertion and deletion mutations are special types of point mutations that can hugely affect the DNA strand and amino acid sequence.
Insertion and deletion mutations that change all codons within the DNA strand as each base pair is moved forward or backward are called frameshift mutations.1
For example, a DNA sequence of ATG CCT TTT with an insertion mutation that adds an extra A to the beginning of the sequence will be AAT GCC TTT T. This single insertion mutation completely changes the codons within the sequence and will thus change the amino acids that are encoded. Similarly, if the first A in the initial sequence was deleted, the sequence would also be changed.
Missense point mutation
Missense mutations are another type of point mutation that occurs when one base pair substitution generates a codon that codes for a different amino acid.1
Unlike a silent mutation where another codon that codes the same amino acid is generated, a missense mutation completely changes the amino acid produced.
For example, in the case of sickle cell disease, a missense mutation in the DNA of the hemoglobin gene causes GAG to become GUG. Instead of the normal GAG which encodes glutamic acid, the codon becomes GUG which now encodes valine. Due to this change in amino acids, the hemoglobin protein becomes misshaped and sticky resulting in sickle cell disease.
In biology, a missense mutation is considered conservative if the replaced amino acid has similar functions to the original.1
In contrast, a non-conservative missense mutation results when the replaced amino acid has different functions than the original.1
In the case of sickle cell disease, the missense mutation is non-conservative.
Now you have learned about the different types of point mutations and have a better understanding of how a point mutation can change the sequence and structure of our DNA and proteins.
Point Mutations - Key takeaways
- A non-conservative missense mutation results when the replaced amino acid has different functions than the original.
- A missense mutation is considered conservative if the replaced amino acid has similar functions to the original.
- An insertion mutation occurs when an extra base pair is added to DNA, while a deletion mutation occurs when a base pair is deleted from a DNA sequence
- Insertion and deletion mutations that change all codons within the DNA strand as each base pair is moved forward or backward are called frameshift mutations.
- A nonsense mutation occurs when the substitution of a single base pair creates a stop codon instead of a codon that produces an amino acid.
References
- Editors, B. B., By:, & Editors, B., Point Mutation: Definition, Types, Examples, 05 Oct. 2019.
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