Genetic expression has traditionally been understood as one-gene-one-protein, however, much genetic expression is regulated through reversible and transmissable epigenetic mechanisms, which act without an alteration of archival DNA.
Epigenetics includes the phenomenon within molecular genetics whereby a single sequence of DNA can give rise, through alternative splicing, to multiple versions of mRNA, and hence to multiple proteins, thus increasing complexity and fine-tuning genetic expression. Epigenetics also refers to DNA related mechanisms of inheritance, such as methylation and chromatin assembly.
Epigenetic regulation of gene expression is mediated through alterations in DNA methylation, covalent modifications of core nucleosomal histones, rearrangement of histones, transposon function, chromosome imprinting, type switching, telomeric silencing, and by RNA interference. Genomic imprinting is a form of mammalian epigenetic regulation which results in the silencing of one copy (allele) of specific genes, according to parental origin. Recently, protein complexes have been discovered to manipulate nucleosomes, organize larger chromatin domains, and set boundaries of chromatin structure. Thus, key histone modifications, cis-acting elements, and regulatory proteins set, maintain, and reprogram epigenetic memory.
Epigenetic mechanisms also operate as conditional, non-programmed interactions that determine individual development [s]:
1. Interactions of cell metabolism with the external and internal physicochemical environment of an organism.
2. Interactions of tissue masses with the physical environment on the basis of physical laws inherent to condensed materials.
3. Interactions among tissues themselves.
Deregulation of epigenetic mechanisms cooperates with genetic alterations in the development and progression of malignancies. Loss of epigenetic regulation is also implicated in systemic disease. Epigenetic deregulation affects several aspects of the biology of tumor cells, including cell cycle control, differentiation, cell growth, DNA repair, and cell death.
In plants, epigenetic alterations that occur during somatic growth can be transmitted to the progeny because germ cells differentiate from somatic tissues only after many cycles of mitotic divisions.
See alternative splicing
Defining epigenetic states through chromatin and RNA - Nature Genetics: "The term 'epigenetics' is used to describe heritable changes in genome function that occur without a change in DNA sequence. As such, epigenetics lies at the heart of the cellular memory crucial for development and provides an important avenue for sustained response to environmental stimuli."
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External : Transposons part 1, transposons part 2 : Barbara McClintock and mobile genetic elements :
Showing posts with label transposition. Show all posts
Showing posts with label transposition. Show all posts
12/20/2007
12/07/2007
retrotransposons
Retrotransposons are a subgroup of transposable elements that occur only in eukaryotic genomes, and which employ reverse transcriptase for their operation. Retrotransposons are particularly common in plants.
▼ Alu elements : biological evolution : cDNA : deletions : eukaryotic vs prokaryotic : integrase : inversions : inverted repeats : LINEs : LTRs : long terminal repeats : mutations : non-replicative transposition : recombination : replicative transposition : retroposons : retrotransposons : retrovirus : reverse transcriptase : RT : SINEs : stable mutations : translocations : transposable elements : transposition types I & II : transposase : transposons : viral retroposons : ▼
Retroposons are repetitive DNA fragments that were inserted into chromosomes after they had been reverse transcribed from any RNA molecule. In contrast to retrotransposons, they never encode reverse transcriptase (RT). Therefore, these elements are not capable of autonomous transposition activity, unlike transposons.
Transposable elements are relatively long DNA sequences that can act as mobile genetic elements in prokaryotic and eukaryotic genomes. Transposable elements represent a large part of the genomes in many species, and transpose by a mechanism that involves DNA synthesis followed by random integration at a new target location in the genome.
Most transposable elements contain inverted repeats at their termini. All transposable elements encode for transposase, a special enzyme activity that assists in the insertion of transposons at a new site. Prokaryotic transposable elements differ from their eukaryotic counterparts in the mechanism of transposition. Only eukaryotic genomes contain retroposons, the special type of transposable elements that employ reverse transcriptase to transpose an RNA intermediate. Alu elements are about 300 nucleotides in length and include a distinctive sequence that ends in a poly-A tail. The human gene's protein-generating capacity is considerably increased by the presence of Alu elements. Alu sequences have the potential to continue to greatly enrich the stock of meaningful genetic information available for producing new human proteins.
Transposition may result in splicing of DNA fragments into or out of the genome. During replicative transposition, the transposon is first replicated giving a new copy that is transferred to a new site, while the old copy is retained at the original site (type I transposition). Nonreplicative transposition however describes the movement of a transposon that is excised from a donor site and is integrated in a new site, usually generating a double (type II transposition). Transposons normally influence the expression of the genes in proximity of their insertion sites.
Transposons and retroposons seem to play a mechanism for biological evolution by promoting rearrangement and restructuring of genomes. Retrotransposon-induced mutations are relatively stable, because the sequence at the insertion site is retained as they transpose via the replication mechanism. Transposition may directly cause both deletion and inversion mutagenesis. Furthermore, transposable elements mediate the movement of host DNA sequences to new locations, enriching the genome with identical sequences positioned at different locations, and promoting homologous recombination. Such recombination may eventually result in deletions, inversions, and translocations.
In eukaryotes, retroposons are more common than transposons. They are either retroviral or nonviral. Viral retroposons encode for the enzymes reverse transcriptase and integrase and, in the same manner as retroviruses, are flanked by long terminal repeats (LTRs). Short interspersed elements (SINEs) and long interspersed elements (LINEs) are the most abundant nonviral retroposons. These are usually repeated many times in the mammalian genome.
Both SINEs and LINEs lack LTRs and are thought to transpose through a special retrotransposition mechanism that involves transcription of one strand of the retroposon into RNA. The RNA then undergoes conformation change (looping) and provides a primer for the synthesis of single stranded cDNA (cellular DNA). The cDNA later serve as template for the synthesis of a double stranded DNA that is inserted in the genome by as yet unknown mechanisms.
▲ Alu elements ₪ Alu elements : biological evolution »» Biological Evolution : cDNA : deletions »» Deletion : eukaryotic vs prokaryotic : integrase : inversions »» Inversion : inverted repeats : LINEs : LTRs : long terminal repeats : mutations »» Mutation : non-replicative transposition : recombination »» Recombination : replicative transposition : retroposons : retrotransposons : retrovirus ۰۰ retroviruses : reverse transcriptase : RT : SINEs : stable mutations : translocations »» Translocation : transposable elements ~ transposable elements : transposition types I & II : transposase : transposons : viral retroposons : ▲
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External : Transposons part 1, transposons part 2 : Barbara McClintock and mobile genetic elements :
▼ Alu elements : biological evolution : cDNA : deletions : eukaryotic vs prokaryotic : integrase : inversions : inverted repeats : LINEs : LTRs : long terminal repeats : mutations : non-replicative transposition : recombination : replicative transposition : retroposons : retrotransposons : retrovirus : reverse transcriptase : RT : SINEs : stable mutations : translocations : transposable elements : transposition types I & II : transposase : transposons : viral retroposons : ▼
Retroposons are repetitive DNA fragments that were inserted into chromosomes after they had been reverse transcribed from any RNA molecule. In contrast to retrotransposons, they never encode reverse transcriptase (RT). Therefore, these elements are not capable of autonomous transposition activity, unlike transposons.
Transposable elements are relatively long DNA sequences that can act as mobile genetic elements in prokaryotic and eukaryotic genomes. Transposable elements represent a large part of the genomes in many species, and transpose by a mechanism that involves DNA synthesis followed by random integration at a new target location in the genome.
Most transposable elements contain inverted repeats at their termini. All transposable elements encode for transposase, a special enzyme activity that assists in the insertion of transposons at a new site. Prokaryotic transposable elements differ from their eukaryotic counterparts in the mechanism of transposition. Only eukaryotic genomes contain retroposons, the special type of transposable elements that employ reverse transcriptase to transpose an RNA intermediate. Alu elements are about 300 nucleotides in length and include a distinctive sequence that ends in a poly-A tail. The human gene's protein-generating capacity is considerably increased by the presence of Alu elements. Alu sequences have the potential to continue to greatly enrich the stock of meaningful genetic information available for producing new human proteins.
Transposition may result in splicing of DNA fragments into or out of the genome. During replicative transposition, the transposon is first replicated giving a new copy that is transferred to a new site, while the old copy is retained at the original site (type I transposition). Nonreplicative transposition however describes the movement of a transposon that is excised from a donor site and is integrated in a new site, usually generating a double (type II transposition). Transposons normally influence the expression of the genes in proximity of their insertion sites.
Transposons and retroposons seem to play a mechanism for biological evolution by promoting rearrangement and restructuring of genomes. Retrotransposon-induced mutations are relatively stable, because the sequence at the insertion site is retained as they transpose via the replication mechanism. Transposition may directly cause both deletion and inversion mutagenesis. Furthermore, transposable elements mediate the movement of host DNA sequences to new locations, enriching the genome with identical sequences positioned at different locations, and promoting homologous recombination. Such recombination may eventually result in deletions, inversions, and translocations.
In eukaryotes, retroposons are more common than transposons. They are either retroviral or nonviral. Viral retroposons encode for the enzymes reverse transcriptase and integrase and, in the same manner as retroviruses, are flanked by long terminal repeats (LTRs). Short interspersed elements (SINEs) and long interspersed elements (LINEs) are the most abundant nonviral retroposons. These are usually repeated many times in the mammalian genome.
Both SINEs and LINEs lack LTRs and are thought to transpose through a special retrotransposition mechanism that involves transcription of one strand of the retroposon into RNA. The RNA then undergoes conformation change (looping) and provides a primer for the synthesis of single stranded cDNA (cellular DNA). The cDNA later serve as template for the synthesis of a double stranded DNA that is inserted in the genome by as yet unknown mechanisms.
▲ Alu elements ₪ Alu elements : biological evolution »» Biological Evolution : cDNA : deletions »» Deletion : eukaryotic vs prokaryotic : integrase : inversions »» Inversion : inverted repeats : LINEs : LTRs : long terminal repeats : mutations »» Mutation : non-replicative transposition : recombination »» Recombination : replicative transposition : retroposons : retrotransposons : retrovirus ۰۰ retroviruses : reverse transcriptase : RT : SINEs : stable mutations : translocations »» Translocation : transposable elements ~ transposable elements : transposition types I & II : transposase : transposons : viral retroposons : ▲
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External : Transposons part 1, transposons part 2 : Barbara McClintock and mobile genetic elements :
12/05/2007
transposable elements
Transposable elements are relatively long DNA sequences that can act as mobile genetic elements in prokaryotic and eukaryotic genomes. Transposable elements represent a large part of the genomes in many species, and transpose by a mechanism that involves DNA synthesis followed by random integration at a new target location in the genome.
Most of transposable elements contain inverted repeats at their ends. All transposable elements encode for transposase, a special enzyme activity that assists in the insertion of transposons at a new site. Prokaryotic transposable elements differ from their eukaryotic counterparts in the mechanism of transposition.
Only eukaryotic genomes contain retrotransposons, the special type of transposable elements that employ reverse transcriptase to transpose an RNA intermediate. Alu elements are about 300 nucleotides in length and include a distinctive sequence that ends in a poly-A tail. The human gene's protein-generating capacity is considerably increased by the presence of Alu elements. Alu sequences have the potential to continue to greatly enrich the stock of meaningful genetic information available for producing new human proteins.
Transposition may result in splicing of DNA fragments into or out of the genome. During replicative transposition, the transposon is first replicated giving a new copy that is transferred to a new site, while the old copy is retained at the original site (type I transposition). Nonreplicative transposition, however, involves a transposon that is excised from a donor site and is relocated to a new site (type II transposition). Transposons normally influence the expression of the genes in proximity of their insertion sites.
Transposons and retroposons seem to play a mechanism for biological evolution by promoting rearrangement and restructuring of genomes. Retrotransposon-induced mutations are relatively stable, because the sequence at the insertion site is retained as they transpose via the replication mechanism. Transposition may directly cause both deletion and inversion mutagenesis. Furthermore, transposable elements mediate the movement of host DNA sequences to new locations, enriching the genome with identical sequences positioned at different locations, and promoting homologous recombination. Such recombination may eventually result in deletions, inversions, and translocations.
retrotransposons > Alu elements ₪ Alu elements : biological evolution »» Biological Evolution : cDNA : deletions »» Deletion : eukaryotic vs prokaryotic : integrase : inversions »» Inversion : inverted repeats : LINEs : LTRs : long terminal repeats : mutations »» Mutation : non-replicative transposition : recombination »» Recombination : replicative transposition : retroposons : retrotransposons : retrovirus ۰۰ retroviruses : reverse transcriptase : RT : SINEs : stable mutations : translocations »» Translocation : transposable elements ~ transposable elements : transposition types I & II : transposase : transposons : viral retroposons < retrotransposons
▲ Top ▲
External : Transposons part 1, transposons part 2 : Barbara McClintock and mobile genetic elements :
Most of transposable elements contain inverted repeats at their ends. All transposable elements encode for transposase, a special enzyme activity that assists in the insertion of transposons at a new site. Prokaryotic transposable elements differ from their eukaryotic counterparts in the mechanism of transposition.
Only eukaryotic genomes contain retrotransposons, the special type of transposable elements that employ reverse transcriptase to transpose an RNA intermediate. Alu elements are about 300 nucleotides in length and include a distinctive sequence that ends in a poly-A tail. The human gene's protein-generating capacity is considerably increased by the presence of Alu elements. Alu sequences have the potential to continue to greatly enrich the stock of meaningful genetic information available for producing new human proteins.
Transposition may result in splicing of DNA fragments into or out of the genome. During replicative transposition, the transposon is first replicated giving a new copy that is transferred to a new site, while the old copy is retained at the original site (type I transposition). Nonreplicative transposition, however, involves a transposon that is excised from a donor site and is relocated to a new site (type II transposition). Transposons normally influence the expression of the genes in proximity of their insertion sites.
Transposons and retroposons seem to play a mechanism for biological evolution by promoting rearrangement and restructuring of genomes. Retrotransposon-induced mutations are relatively stable, because the sequence at the insertion site is retained as they transpose via the replication mechanism. Transposition may directly cause both deletion and inversion mutagenesis. Furthermore, transposable elements mediate the movement of host DNA sequences to new locations, enriching the genome with identical sequences positioned at different locations, and promoting homologous recombination. Such recombination may eventually result in deletions, inversions, and translocations.
retrotransposons > Alu elements ₪ Alu elements : biological evolution »» Biological Evolution : cDNA : deletions »» Deletion : eukaryotic vs prokaryotic : integrase : inversions »» Inversion : inverted repeats : LINEs : LTRs : long terminal repeats : mutations »» Mutation : non-replicative transposition : recombination »» Recombination : replicative transposition : retroposons : retrotransposons : retrovirus ۰۰ retroviruses : reverse transcriptase : RT : SINEs : stable mutations : translocations »» Translocation : transposable elements ~ transposable elements : transposition types I & II : transposase : transposons : viral retroposons < retrotransposons
▲ Top ▲
External : Transposons part 1, transposons part 2 : Barbara McClintock and mobile genetic elements :