Approximately 60,000 DNA damages occur in a mammalian cell per day. If the cell doesn’t repair the DNA damage, it develops into DNA mutations. DNA mutations are hereditary changes in the genetic material and can have serious consequences for the affected cells or for the organism.
mutation definition
One mutation is a permanent and heritable change in the genetic information of an organism.
If the mutation occurs in a body cell, it can be passed on to the daughter cells. If the germ line cells (ovum and sperm in humans) are affected, the mutation can be passed on to the offspring.
A mutation can occur in two ways:
- spontaneous mutation:A mutation that occurs under natural conditions.
- Induced mutation:A mutation caused by mutagenic substances or radiation.
types of mutation
There are three types of mutations:
- genome mutations
- chromosomal mutations
- gene mutations
The distinction is based on the number and structure of the chromosomes or a single gene.
Genome mutations and chromosomal mutations are classified under the term chromosomal aberrations summarized.
genome mutations
Genome mutations will also numerical chromosome aberrations called because the number of chromosomes changes.
If the number of chromosomes is changed by a mutation, it is called one genome mutation.
Genome mutations are in polypoids and aneuploidies distinguished. Furthermore, a distinction is made as to whether the sex chromosomes (gonosomes) or other chromosomes (autosomes) are affected by the mutation.
polyploidy
In the polyploidy the entire set of chromosomes is multiplied. This means that the chromosome set is present at least three times. The main cause of polyploidy is a missing spindle apparatus in mitosis or meiosis. A polyploidy is always for the human embryo deadly. In some crops, a polyploidy can be produced by crossing or corresponding mutagens.
Polyploidy in agriculture
Polyloidy in a crop can bring benefits to agriculture. A multiple set of chromosomes can lead to an increase in the yield of certain plants or to a fruit containing no pips or seeds.
aneuploidy
In the aneuploidy increases or decreases the number of chromosomes. The main causes of aneuploidy are defective separations of chromosomes during the meiotic division of germ cells (Meiosis).
trisomy
A typical example of an aneuploid are trisomies in humans. A trisomy involves one of the 23 chromosomes three times instead of twice before. Depends on the affected chromosome is i.eThe trisomy is not necessarily fatal for the embryo. Chromosomes 8, 12, 18 and 21 can be present in triplicate without miscarriage occurring.
Trisomy 21 is the most common. She is also as Down syndrom known. A trisomy is accompanied by developmental disorders and physical malformations of varying severity and severity.
Figure 1: Aneuploidy & Polyploidy
Gonosomal genome mutation
Under gonosomal genome mutations one understands genome mutations of the sex chromosomes (gonosomes). The number of sex chromosomes is altered.
Typically, one sex chromosome too much or too little is inherited by the offspring. This results in gonosomal aneuploidies. Typical examples are:
- XXY trisomy (Klinefelter Syndrome)
- XYY trismoia (Jacobs Syndrome)
- Triple X Syndrome (Super Female Syndrome)
- X chromosome monosomy (Ulrich Turner Syndrome)
Because the sex chromosomes carry a small number of vital genes, the effects of a gonosomal genome mutation are less fatal. Some of those affected can lead a normal life.
You can find more detailed information on gonosomal genome mutations and details on the specific syndromes in a separate article on gonosomal genome mutations.
chromosomal mutations
If the structure of a chromosome is affected by a mutation, it is called a chromosome mutation.
Chromosome mutations will also structural chromosomal aberrations called.
Chromosome mutations are changes in the shape or structure of chromosomes. Corresponding changes affect larger sections of the genome and are typically visible under the light microscope.
There are five types of chromosomal mutation:
- deletion: A section of the chromosome is lost
- insertion: Base pairs are inserted into the chromosome
- Duplication: A section of the chromosome is duplicated
- translocation: A piece of a chromosome breaks off and joins another, non-homologous chromosome
- Inversion: A broken piece of chromosome reconnects to the same chromosome, but in reverse
In a diploid chromosome set, the two same chromosomes in different versions (one inherited from the father and one from the mother) form a homologous pair.
gene mutations
When there is a change in genetic information within a gene, it is called a gene mutation.
By a gene mutation the base sequence of a single gene is changed.
When a single base pair in the genetic information changes, it is called point mutation designated.
There are three types the point mutation:
- Substitution: A base is exchanged
- deletion: A base is lost
- insertion: A base is added
Depending on whether the reading frame of the gene is affected, a distinction is made between mutations that do not affect the reading frame and grid mutations distinguished.
The genetic information for proteins lies in what are known as base triplets before. In this case, three base pairs on a gene always correspond to an amino acid of the resulting protein. The reading frame of a gene therefore consists of consecutive base triplets.
Would you like to learn more about the translation of genes into proteins and what the genetic code means? Then check out the article on protein biosynthesis!
Point mutations that do not affect the reading frame
If there is a substitution, the reading frame of the gene is not changed. By swapping a base pair, a single base triplet is changed, but the subsequent base pairs remain.
There are also deletions and insertions that do not affect the reading frame of a gene. However, these are mutations that affect several base pairs (no point mutation). Once a deletion or insertion affects 3 base pairs, or a number of base pairs divisible by 3, the reading frame of a gene is not altered.
Mutations that do not affect the reading frame can be different effects to have:
- Silent mutation: If the third position of a base triplet is exchanged, the newly created triplet may code for the same amino acid (degenerate code). In this case, the mutation has no effect and remains silent.
- missense mutation: A point mutation can result in another amino acid being built into the protein produced during protein biosynthesis. The function of the protein can be influenced or restricted as a result. It is also possible that proteins function largely unrestricted.
- Nonsense mutation: If the point mutation creates a stop codon, protein biosynthesis is terminated prematurely. A shortened amino acid sequence results, the resulting proteins are usually not functional.
- readthrough mutation: A point mutation turns a stop codon into an amino acid. The DNA continues to be read up to the next stop codon. A nonfunctional protein is formed.
grid mutations
If the reading frame is changed by a mutation, one speaks of a frame mutation, and this is also the case reading frame mutation called.
In a frame mutation, the reading frame shifts through the Insertion or removal of one or more bases in the gene. This can change the complete sequence of amino acids in the protein.
Grid mutations through point mutations arise through deletions and insertions of single base pairs. These changes have serious consequences, since not only a single base triplet, but also subsequent base triplets are changed by the mutation. Thus, as a rule, a large part of the amino acid sequence changes and the resulting protein is not functional.
A frame mutation can be a point mutation. However, it is also possible for a reading frame shift to occur as a result of the addition (insertion) or removal (deletion) of several bases. This is the case when changes in the number of base pairs are not divisible by three.
mutation causes
As already mentioned, mutations can be derived under natural conditions (spontaneous mutations) or by certain substances (induced mutations).
spontaneous mutations
Spontaneous mutations occur under natural conditions. They are typically due to errors caused by specific proteins or enzymes. These errors are normal; there are repair mechanisms in our cells that are responsible for eliminating corresponding errors.
But even these repair mechanisms are not error-free. If the repair mechanisms fail, mutations can occur that may be passed on to daughter cells or the offspring.
An example of a spontaneous mutation is defective replication during cell division. The enzymes in our cells are not 100% perfect, and the wrong base pairs are inserted into the DNA sequence during replication. There are enzymes whose job it is to fix such errors. However, these repair mechanisms do not work properly either, and so spontaneous mutations can occur during replication.
There are specific processes in our body that produce targeted mutations. These processes include:
- DNA replication during mitosis/meiosis (cell division)
- Division of the chromosomes during the 1st and 2nd meiotic division (meiosis)
- Crossing over during meiosis
Mutagenic substances and radiation (induced mutations)
mutagens are substances that can cause DNA damage and subsequent mutations.
Induced mutations can be caused by various chemical Substances (e.g. tars in tobacco smoke, nitrous acid, bromouracil) or high-energy rays (e.g. UV or X-rays) can be triggered.
Some of the processes in which mutagens interact with DNA molecules are extremely specific. As a rule, the links between the bases are broken or individual bases are removed. Other mutagens can break DNA strands.
An example of mutations caused by radiation is the influence of UV radiation. UV radiation is absorbed by DNA molecules and can lead to dimerization of thymine. This means that two adjacent thymine bases will pair up with each other instead of the opposite adenine bases.
DNA repair processes
There are different DNA repair mechanisms. Without them, all DNA damage would result in mutations. We distinguish…