Any permanent alteration in the DNA sequence is termed as mutation. Mutation is derived from the Latin word ‘mutacio’ which means “change”.
Mutations in DNA are of two types depending on the magnitude: Gene mutation and chromosome mutation. Mutations affecting only one or few base pairs are known gene mutations, while those affecting a large segment of the DNA molecules including multiple genes, almost at the chromosome level, are known as chromosome mutations. This post in more about the different types of gene mutations.
(Just for info: Read about a mutation that makes people need less sleep has been found)
The mutations can be categorised into different types based on various factors like effect on phenotype, effect on protein sequence and so on. We have listed a few in this and next coming post. So read on:
1. Based on Changes in Sequence:
The different type of mutation based on the type of changes that occur in the sequence of DNA are:
a. Insertion:
Insertion is a type of mutation, in which a base gets added into the DNA molecule. As a base pair is added, the total number of base pairs increases.
For eg: As shown in fig. 1, if the original sequence is ACCGATTCCGGAT (total bases: 13) and a C gets inserted into the sequence between the two T’s, it becomes ACCGATCTCCGGAT (total bases: 14).
The insertion of additional base pairs may lead to frameshifts (see fig 2), depending on whether multiples of three base pairs are inserted.
This leads to change in the sequence of amino acids, and may in turn result into a non- functional or abnormal protein.
b. Deletion
A deletion is opposite of the insertion. It involves removal of a DNA base from the DNA molecule. Due to the loss of a base, the total number of DNA bases change (fig 3). Small deletions may remove one or a few base pairs, while larger deletions can remove an entire gene or several neighbouring genes. Like insertion, deletion, too, may cause a frameshift and alter the function of the resulting protein.
For eg: if the original sequence is ACCGATTCCGGAT (13) and a T located between T and C gets deleted, the sequence changes and becomes ACCGATCGGAT (12).
This leads to changing of the sequence of codons coding for the amino acid , and may result into a non- functional or abnormal protein, similar to insertion.
c. Duplication:
A duplication is the type of mutation wherein a region of DNA is abnormally copied one or more time. Gene duplication arises as the result of errors in DNA replication and repair. The duplicated region can be located adjacent to the original location or any other location in the genome.
This may result in the frameshift of the codons (depending on whether multiples of three are added) coding for the amino acids, which in turn causes alteration in the function of the resultant protein.
d. Substitution:
A substitution mutation is a type of mutation in which a single base is replaced by an incorrect one. This may occur due to error during DNA replication or repair or carcinogens or mutagens.
For eg: As seen in the fig. 5, if the original sequence is ACCGATTCCGGAT (13) and a T located between T and C is replaced by C, it becomes ACCGATCCCGGAT (13). This leads to change in a single codon. The resultant protein may be non-functional or abnormal. The number of bases remain the same, unlike as observed in the insertion and deletion mutations. Also there is no frameshift. Usually the substitution involving a single nucleotide is very common, which is a type of point mutation.
DNA substitution mutations are, further, of two types: Transition and Transversion.
Transitions involves the interchange among purines (A-G) or pyrimdines (C-T), which involve bases of similar shape. Transversions are the mutations which involve interchange between purine and pyrmidine bases.
Substitution can be caused by different mechanisms:
I. Depurination:
Depurination is the one process wherein the β-N-glycosidic bond between an adenine or guanine and the deoxyribose is hydrolyzed, releasing the purine bases. Depyrimidination of cytosine and thymine can also takes place at a comparatively slower rate than depurination. Depurination is repaired by the base excision repair (BER) machinery. However, few errors may be left unrepaired, resulting into substitution mutation.
II. Deamination:
Deamination is the loss of exocyclic amino group present in the structure of the bases. The four bases cytosine, adenine, guanine (see fig 4) and 5- methylcytosine are converted to uracil, hypoxanthine, xanthine and thymine, respectively on deamination.
As a result during replication, wrong base pairs are incorporated in the daughter strand leading to substitution mutation. When the DNA replicates, the new nucleotide becomes permanently integrated.
III. Carcinogens and mutagens:
These are chemicals that cause lots of mutations. Various physical factors like UV light and other ionising radiations can also cause substitution mutation (see induced mutation later in this post).
2. Based on their ability to express.
Depending on their ability to express in an individual, the mutation is either Dominant or Recessive.
As is known, the higher organisms are diploid and have homologous chromosomes. The different forms of gene are called as alleles. If an individual carry two identical alleles on both the chromosomes, they are called as homozygous. If an individual carry different alleles, they are said to be heterozygous for a gene. The genes can be either dominant or recessive based on the ability to express itself in presence of another allele.
Dominant gene is the one which is expressed in a heterogenous individual, while recessive is the one whose effect is suppressed or concealed. Recessive phenotype is observes in individuals homologous for the recessive allele. Similarly, the mutations are also of two types, i.e. Dominant or Recessive, based on their ability to effect the phenotype of an individual.
• Dominant mutation:
Dominant mutation is the one which exhibits its effect on the phenotype, even when a single copy of the mutant gene (p) is present alongwith the normal gene (P). That is the mutant phenotype is observed in the heterozygous individual (P/p) as shown in fig 7a.
The dominant mutation may cause either inefficient amount of the gene product, abnormally functioning or interfering product or increased protein activity.
In cases where the mutation in one gene copy, leads to insufficient production of protein needed for the normal functioning, the genes are referred to as haplo-insufficient (that is one is insufficient). Hence for the normal phenotype, both the normal copies are required.
In some cases, mutation in one allele may result in production of an abnormally functioning protein, which may in turn interfere with function the wild type protein. These mutations are termed as dominant negative mutations.
Examples of disorders due to dominant mutation includes;
– polycystic kidney disease (see the NIH page) and
– osteogenesis imperfecta (see the NIH page).
(Just for info: Read more on Dominant mutations.)
• Recessive mutation:
Recessive mutation is one in which both the copies of the gene must be mutant (p/p) in order for the mutant phenotype to be observed; that is, the individual must be homozygous for the mutant allele to show the mutant phenotype (see fig 7b).
If the individual carries one copy of mutant allele and one normal allele (heterozygous), the phenotype will be normal (fig 7b). However the individual will be the carrier of the mutant allele. Recessive mutations causes a gene to get inactivated and leads to loss of function.
Examples of autosomal recessive disorders include;
– cystic fibrosis (see the NIH page) and
– sickle cell anemia (see the NIH page).
3. Based on the cause of mutation:
Based on the cause of the mutation, they can be Spontaneous or Induced.
– Spontaneous mutations:
Few mutations arise spontaneously due to the chemical instability of purine and pyrimidine bases as well as due to the errors during DNA replication.
A common cause of spontaneous point mutations is the deamination especially of cytosine to uracil. As mentioned before, during replication Adenine is incorporated into the daughter strand instead on Guanine, leading to substitution mutation.
Another cause of spontaneous mutation is the mistakes in DNA replication. Usually an incorrect nucleotide is added by the replication machinery into the DNA daughter strands.
(Just for info: Read about the spontaneous mutations in more details.)
– Induced Mutation:
Exposure to some physical and chemical factors like UV light, ionising radiations and chemical carcinogens can cause mutations. The mutations arising due to such environmental factors are called induced mutations.
Ionising radiations include gamma or X-rays while chemical carcinogens include chemicals like alkyl or aryl epoxides, nitrosoureas, nitrosamides, polycyclic aromatic hydrocarbons, aromatic amines and aflatoxin B1. Generally, chemical mutagens induce point mutations, whereas ionizing radiation causes aberrations at chromosomal level.
(Just for info: Read our post on structure of Chromosome.)
For e.g.: Ethylmethane sulfonate (EMS), a mutagen causes alkylation of guanine in DNA to give O6-ethylguanine. O6-ethylguanine is paired with thymine (T), instead of the original cytosine (C), resulting in substitution of G·C by A·T base pair.
~ The mutation can also be categorised based on their origin (hereditary or induced) and on the basis of their effect on the structure a protein (missense mutation, nonsense mutation, etc). These types of mutation will be discussed in the next post.
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References:
Lodish et al. (2000) Mutations: Types and Causes. Molecular Cell Biology. 4th edition. New York: W. H. Freeman.
Fay and Spencer (2005) Dominant mutations. Copyright © 2005, WormBook Research Community.
Ziad & Stypczynska (2013) Clustering algorithms in radiobiology and DNA damage quantification. Data Security, Data Mining and Data Management: Technologies and Challenges, Nova Science Pub Inc.
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