Dr. Israa ayoub alwan Lec -9- AL-Ma’moon University College Medical Laberatory techniques Department Molecular biology/ Second stage Dr. Israa ayoub alwan Lec -9-

Mutations

The Nature of Mutations A mutation, used as a noun, is a change in a DNA sequence that is present in less than 1 percent of individuals in a population. is the process of altering a DNA sequence. Mutations range in size from alteration of a single DNA base; to deletion or duplication of tens, hundreds, thousands, or even millions of bases; to missing or extra entire chromosomes. Mutation can affect any part of the genome, including sequences that encode proteins or control transcription; in introns; repeats; and sites critical to intron removal and exon splicing.

The effects of mutation vary The effects of mutation vary. Mutations may impair a function, have no effect, or even be beneficial. A deleterious (harmful) mutation can stop or slow production of a protein, overproduce it, or impair the protein’s function—such as altering its secretion, location, or interaction with another protein.

A single nucleotide polymorphism (SNP)

a single nucleotide polymorphism, or SNP is a single base change. So are many mutations. Geneticists define a mutation as being present in less than 1 percent of a population and a polymorphism as being present in more than 1 percent of a population. The consequences of this distinction provide the logic underlying the more popular view that a mutation is “bad” but a polymorphism is a “harmless variant.” That is, if a genetic change greatly impairs health, individuals with it are unlikely to reproduce, and the mutant allele remains uncommon.

  The term mutation refers to genotype—that is, a change at the DNA or chromosome level. The familiar term mutant refers to phenotype. The nature of a mutant phenotype depends upon how the mutation affects the gene’s product or activity, and usually connotes an abnormal or unusual characteristic.

However, a mutant phenotype may also be an uncommon variant that is nevertheless “normal,” such as red hair. In an evolutionary sense, mutation has been essential to life, because it produces individuals with variant phenotypes who are better able to survive specific environmental challenges, including illnesses.

*** A mutation may be present in all the cells of an individual or just in some cells. * In a germline mutation, the change occurs during the DNA replication that precedes meiosis. The resulting gamete and all the cells that descend from it after fertilization have the mutation—that is, every cell in the body. * In contrast, a somatic mutation happens during DNA replication before a mitotic cell division. All the cells that descend from the original changed cell are altered, but they might only comprise a small part of the body. Somatic mutations are more likely to occur in cells that divide often, such as hair root cells, because there are more opportunities for replication errors.

*** Molecular Consequences of Mutation Gain of Function: over-expression of the protein products or inappropriate expression (Dominant Diseases). Loss of Function: The mutation results in the loss of 50% of the protein product; for instance a metabolic enzyme (Recessive Diseases).

Source of DNA Damage

DNA damage can be subdivided into two main types: Source of DNA Damage DNA damage can be subdivided into two main types: Endogenous damage such as attack by reactive oxygen species produced from normal metabolic byproducts (Cause:- spontaneous mutation), especially the process of oxidation. Exogenous damage caused by external agents(Cause:- Induced Mutation ) such as: Ultraviolet radiation from the sun Other radiation frequencies, including x-rays and gamma rays Hydrolysis or thermal disruption Certain plant toxins Human-made mutagenic chemicals, especially aromatic compounds that act as DNA intercalating agents Cancer chemotherapy and radiotherapy Viruses

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