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Category Archives: Genetics
DNA Repair
DNA Repair
DNA repair occurs through many mechanisms.
DNA Proofreading
Proofreading 3′ to 5′ Exonuclease, seen in DNA polymerases I, II and III
Removal of Lesions
This is the first choice of DNA repair as it causes the less mistakes and DNA damage. This occurs through photolyase and alkyltransferase.
Excision Repair
Excision repair through UvrABC endonucleases.
Recombination Repair
Recombination repair through RecA and endonucleases.
SOS Repair
SOS repair through RecA and UvrABC endonuclease. SOS repair is for excessive damaged DNA. It is a last resort of the cell for DNA repair.
Photoligases
Photoligases are enzymes that use light to repair DNA. Photoligases use light to cleave cyclobutyl ring, thymine dimers. Light is required for the repair.
Excision DNA Repair
Enzymes begin by scanning DNA and detecting DNA distortions secondary to changes, such as with dimerization. UvrA and UvrB scan the DNA for mutations. UvrB and UvrC endonuclease activity incize the damaged DNA strand and the damaged DNA strand diffuses away. UvrD is a helicase that excises the DNA fragment. Then DNA polymerase I fills the gap and DNA ligase seals the remaining DNA nick.
Recombination Repair
Recombination repair occurs when DNA polymerases skip the damaged DNA region and recA protein binds to the DNA at the single-stranded segment. Endonucleases are then able to locate the region, and cut out the homologous region of the undamaged strand and there is exchange of the undamaged segment and ligation. Ligases seal this region and DNA polymerase begins the reparative process.
Cells with Damaged DNA Die
Cells that contain irrepairable DNA die. This is because the base structure is altered beyond repair, there may be no remaining template to allow for repair, as both strands may have become damaged, DNA damage may have hit an essential gene and repair systems may have become themselves damaged.
DNA Mutations
DNA Mutations
Mutation of DNA Molecules
Mutations of DNA molecules can occur through many ways. The most common are listed below:
Base substitutions occuring during DNA replication
Base changes due to chemical instability of the base and the N-glycosidic bond
Alterations on the DNA from external factors, such as chemicals and environmental agents that induce DNA breakdown
DNA Molecule Alterations
Incorrect DNA bases in one strand that are unable to form hydrogen bonds with the opposite corresponding base in the other strand. This can occur there two ways, through replication error such as with tautomeric shifts and through deamination of bases, such as cytosine to uracil.
Missing bases through depurination through alkylating agents
Altered bases such as through thymine dimers
Single-strand breaks termed Nicks. There can occur spontaneously, through Dnases or through ionizing radiation. DNA ligases can often repair these.
Double-strand breaks in which two single strand breaks occur in a small region of the DNA
Cross-linking of DNA strands. This occurs across strands and is caused by agents such as mitomycin C and nitrite ions. The DNA duplex cannot unwind subsequently to be cross-linked at such sites.
Posted in Genetics
Tagged , dna, dna alterations, DNA Mutations, dna strands, mutations
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Transposon
Transposon
Transposable Elements
Transposition
Transposition is the process by which a DNA fragment or gene is able to move or ‘jump’ to another place of the genome and incorporate there.
Transposable Elements
Transposable elements are genetic elements that are able to transpose, move, from one site of the genome to another. These transposable elements include insertion sequences and transposons.
Excision of Transposons
How do transposons go in and come out of the genome?
Transposition and Evolution
Transposons have continued to be around as they have evolved to persist. Bacteria are know to be able to control the level of transposition. This is seen as when bacteria incorporate a transposon, the transposon spreads in a population rapidly, and a reaches a certain number of copies per cell, and then the levels slow down and the amount per cell remains fairly constant. Transpositions benefit bacteria by increasing their mutation rates. It is well-known that bacteria growing in chemostat cultures that have transposons compete out the bacteria that lack transposons. Even if there is only one cell against 5000, if that cell has a transposon, it will outcompete the other cells. This is because that cell while have a higher error rate with the formation of some beneficial mutations secondary to the errors and will have increased fitness and survival advantages.
Posted in Genetics
Tagged , elements, Evolution, Transposable Elements, Transposition, Transposon
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