تويت
the mechanisms of DNA repair:
There are a number of enzyme systems in the cell that perform damage to DNA. Some of these are specific for a particular type of damage (e.g. a particular base modification) while other can handle a range of mutation types. These systems also differ in the degree to which they are able to restore the wild-type sequence. The following lists some of the best describe categories of repair systems:
- Photoreactivation
is utilizing near UV light as well as an appropriate enzyme system to repair and is specific for pyrimidine dimmers( T dimmers T=T )formed by UV And we previously explained how this mechanism works
Nucleotide Excision Repair
Nucleotide Excision Repair
it Is called also UV Damage Repair or Dark Repair and it is the second major type of repair is also ubiquitous being found in all organisms.
It involves an endonucleolytic cleavage near the dimer, followed by a polymerase which cuts out the thymine dimer with a 5'-3' exonucleolytic activity. This polymerase (the polA gene product) simultaneously synthesizes an appropriate matching strand. This system and photoreactivation share the property that they both function effectively when they are able to repair the dimer before a replication fork comes to that dimer. If a growing fork or replication fork comes upon a thymine dimer before repair, one gets a large stretch of single-stranded DNA on the strand opposite the dimer.
Base Excision Repair
it allows the identification and excision of aberrant bases, typically due to deamination of normal bases, but also from chemical modification.
it allows the identification and excision of aberrant bases, typically due to deamination of normal bases, but also from chemical modification.
Mismatch repair
Mismatch repair is a multi-enzyme system that recognizes inappropriately matched bases in DNA and replaces one of the two bases with one that "matches" the other.
Genetic inactivation of the mismatch repair system causes increase the risk of getting spontaneous mutations and lead to highly elevated rates of base substitution and frameshift mutations
Inactivation of the human mismatch repair system is the cause of hereditary nonpolyposis colon cancer (HNPCC)and has been implicated in the development of subset of sporadic tumors that occur in a variety of tissues..
[
- (ii) recruitment of repair enzymes-
Inactivation of the human mismatch repair system is the cause of hereditary nonpolyposis colon cancer (HNPCC)and has been implicated in the development of subset of sporadic tumors that occur in a variety of tissues..
[
FONT="Arial"]Mismatch repair is an excision/resynthesis system that can be divided into 4 phases:[/FONT]
- (i) recognition of a mismatch by MutS proteins- (ii) recruitment of repair enzymes-
(iii) excision of the incorrect sequence
,- (iv) resynthesis by DNA polymerase using the parental strand as a template.
Recombination repair (or post-replication repair):
repairs damage by a strand exchange from the other daughter chromosome. Since it involves homologous recombination, it is largely error- free.
SOS repair
SOS repair or inducible error-prone repair is process that works on the gross DNA damage by base excision and replacement, even if there is no template to guide base selection. This process is a last resort for repair, and is often the cause of mutations.
Other repair systems are less well understood but some seem to be specific for certain mismatches.
I hope that I did not make you feel sleepy while you reading this topic
I made my best to put this topic in easy way as possible I could
good luck every one and see you in other topics
I made my best to put this topic in easy way as possible I could
good luck every one and see you in other topics
تعليق