Somatic mutation


A somatic mutation is a change in the DNA sequence of a somatic cell of an organism with dedicated reproductive cells.

It is any mutation that occurs in a cell other than a gamete, germ cell, or gametocyte. 

Unlike germline mutations, which can be passed on to the descendants of an organism, somatic mutations are not usually transmitted to descendants. 

Somatic mutations will be present in all descendants of a cell.

Many cancers are the result of accumulated somatic mutations.

Somatic mutations that occur earlier in development are generally present in a larger fraction of body cells.

Somatic cells make up all the internal organs, skin, bones, blood and connective tissue. 

There are approximately 220 types of somatic cells in the human body.

The separation of germ cells from somatic cells occurs during early stages of development. 

Somatic mutations are passed down to all the progeny of a mutated cell within the same organism. 

Both the nuclear DNA and mitochondrial DNA of a cell can accumulate mutations; somatic mitochondrial mutations have been implicated in development of some neurodegenerative diseases.

Mutations in somatic cells may arise due to endogenous factors, including errors during DNA replication and repair, and exposure to reactive oxygen species produced by normal cellular processes. 

Most mutagens act by causing DNA damage of alterations in DNA structure such as pyrimidine dimers, or breakage of one or both DNA strands. 

DNA repair processes can remove DNA damages that would, otherwise, cause mutation. 

Mistakes in the mechanism of DNA repair causes a change in nucleotide sequence, or if replication occurs before repair is complete, mutations occur.

Mutagens can be physical, such as radiation from UV rays and X-rays, or chemical with molecules that interact directly with DNA, such as metabolites of benzopyrene, a potent carcinogen found in tobacco smoke.

UV light can damage DNA by causing pyrimidine dimers,  adjacent bases bond with each other, resulting In a distorted DNA molecule that does not function properly, a mutation.

The frequency of mutations is generally higher in somatic cells than in cells of the germline.

There are differences in the types of mutation seen in the germ cell and in the soma cell.

Mutation frequency varies between different somatic tissues.

Somatic mutation rates are more than ten times that of the germline.

Mutation load in fibroblasts is over twenty times greater than germline mutations per base pair.

The disparity in mutation rate between the germline and somatic tissues indicates the importance of genetic integrity in the germline than in the soma.

The variation in mutation frequency may be due to differences in rates of DNA damage or to differences in the DNA repair process.

Somatic hypermutation: Somatic hypermutation, part of the adaptive immune response, allows antibody-producing B cells to experience a mutation rate many times higher than the normal rate of mutation. 

The mutation rate in antigen-binding of the immunoglobulin genes is up to 1,000,000 times higher than in cell lines outside the lymphoid system. 

Somatic hypermutation helps B cells produce antibodies with greater antigen affinity.

Somatic mutations accumulate within cells as it ages and with each round of cell division.

The role of somatic mutations in the development of cancer has been established, and is implicated in the biology of aging.

 Mutations in neuronal stem cells,  especially occurring during neurogenesis and in post-mitotic neurons lead to genomic heterogeneity of neurons resulting in somatic brain mosaicism.

It is proposed that the accumulation of age-related mutations in neurons may be linked to neurodegenerative diseases, including Alzheimer’s disease.

The majority of central-nervous system cells in the adult are post-mitotic, and adult mutations might affect only a single neuron. 

Detrimental somatic mutations might contribute to neurodegenerative disease by cell death.

Mutations occuring in a somatic cell will be present in all descendants of this cell.

Mutations over generations of somatic cells is part of the process of malignant transformation, from normal cell to cancer cell.

The heterozygous loss-of-function mutations in cells where one good copy of a gene and one mutated copy may function normally with the unmutated copy until the good copy has been spontaneously somatically mutated. 

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