Types of mutations
There are many different ways that DNA can be changed, resulting in different types of mutation. Here is a quick summary of a few of these:
Substitution
A substitution is a mutation that exchanges one base for another (i.e., a change in a single "chemical letter" such as switching an A to a G). Such a substitution could:change a codon to one that encodes a different amino acid and cause a small change in the protein produced. For example,sickle cell anemia is caused by a substitution in the beta-hemoglobin gene, which alters a single amino acid in the protein produced.change a codon to one that encodes the same amino acid and causes no change in the protein produced. These are called silent mutations.change an amino-acid-coding codon to a single "stop" codon and cause an incomplete protein. This can have serious effects since the incomplete protein probably won't function.
Insertion
Insertions are mutations in which extra base pairs are inserted into a new place in the DNA.
Deletion
Deletions are mutations in which a section of DNA is lost, or deleted.
Frameshift
Since protein-coding DNA is divided into codons three bases long, insertions and deletions can alter a gene so that its message is no longer correctly parsed. These changes are called frameshifts.
For example, consider the sentence, "The fat cat sat." Each word represents a codon. If we delete the first letter and parse the sentence in the same way, it doesn't make sense.
In frameshifts, a similar error occurs at the DNA level, causing the codons to be parsed incorrectly. This usually generates truncated proteins that are as useless as "hef atc ats at" is uninformative.
There are other types of mutations as well, but this short list should give you an idea of the possibilities.
The causes of mutations
Mutations happen for several reasons.
1. DNA fails to copy accurately
Most of the mutations that we think matter to evolution are "naturally-occurring." For example, when a cell divides, it makes a copy of its DNA — and sometimes the copy is not quite perfect. That small difference from the original DNA sequence is a mutation.
2. External influences can create mutations
Mutations can also be caused by exposure to specific chemicals or radiation. These agents cause the DNA to break down. This is not necessarily unnatural — even in the most isolated and pristine environments, DNA breaks down. Nevertheless, when the cell repairs the DNA, it might not do a perfect job of the repair. So the cell would end up with DNA slightly different than the original DNA and hence, a mutation.
Chromosomal mutation
At the molecular level, single nucleotide to several hundred nucleotides are altered but at the chromosomal level large segment of the chromosome with one gene to many, are deleted or added. Depending upon the type of action, chromosomal mutations are categorized into addition, deletion, duplication, translocation, inversion or ring chromosome.
Addition: when some of the extra segment is added to a chromosome, is called as an addition.
Deletion: when some segment of an entire chromosome is deleted and loses several genes, it is called as a deletion.
The image is a representation of chromosomal mutation
Duplication: one segment of a chromosome is duplicated in a manner which alters the structural hierarchy of the entire chromosome and creates chromosomal abnormality is called as a chromosomal duplication.
chromosomal duplication
Translocation:
Balanced translocation: when two segments (nearly same) are exchanged, resulting in balanced translocation. Here the two segments are approximately the same in size hence it is not possible to detect such type of translocation by cytogenetic analysis.
The image represents a general type of chromosomal translocation.
Generally, balanced translocation results in recurrent abortion in females and it results in abnormalities in the fetus. Balance translocation can be screened by FISH.
Reciprocal translocation: translocation occurred between two non-homologous chromosomes, is called as a reciprocal translocation. This type of translocation is most common in nature.
Robertsonian translocation: Robertsonian translocation occurs between two acrocentric chromosomes. Acrocentric chromosomes are small short chromosome with one long q-arm and a short or very smaller p-arm.
In humans 13, 14, 15, 21, 22 and Y chromosomes are acrocentric. When translocation between two acrocentric chromosomes occurs, one (nearly) metacentric and one “only centromeric” chromosomes are formed.
Here long arms of both chromosome are fused together. Therefore, one chromosome become larger metacentric and another chromosome remain without any arms or only with centromere.
Different type of chromosomal translocation.
Though the “only centromeric” chromosome does not have any chromatid, still it contains many genes of both chromosomes. Balanced Robertsonian translocation does not cause any physical abnormality but recurrent abortion in females is the common abnormality.
Imbalanced Robertsonian translocation results in mental retardation, trisomy and other structural abnormalities.
Post a Comment