Repair Flashcards
Dna damage
Occurs frequently in a cell
By heat, uv radiation, chemicals, free radicals
Numerous repair enzymes / mechanim exists
Without repair, genetic material quickly lost
Endogenous sources of dna damage
Replication errors
Chemical instability
- depurination
- free radical damage
- spontaneous deamination
Exogenous sources of dna errors
Toxic substances
Radiation
Depurination
Occurs spontaneously thousands a time per day
Results in loss of purine bases g a from dna
Doesn’t break phosphodiester bond
Gives rise to lesions that resemble missing teeth
Deamination
Occurs spontaneously
Results in loss of amine group - cytosine
Spontaneous
Major type of deam-converts cytosine to altered dna base uracil
Neither break ph backbone
Deamination of cytosine if uncorrected
Results in substitution of one base for another when dna is replicated
Deam of cytosine produces uracil
Which differs in its base pairing properties - prefers adenine
Dna replication machinery insets adenine when it encounters uracil on template strand
Depurination if unncorrected can lead to
Loss of nucleotide pair
When replication machinery encounter encounters a missing purine on the strand it can skip to the next nucleotide = producing daughter dna molecule missing one nucleotide pair
What is the effect of uv on nucleotide bases?
Uv in sunlight is damaging to dna
It promotes covalent linkage between two adjacent pyrimidine bases (thymine)= forming thymine dimmer
Results in disease xeroderma pigmentosum
What are the repair mechjannisms ?
Single strand - base excision, nucleotide excision, mismatch repair
Double strand - homologous end joining - non homologous end joining
What is singe;l strand- base excision repairs?
Pathway for damaged dna repair
Recognises specific base errors- delaminated bases, oxidised bases, open rings
And removes certain types of damaged bases- detected and removed by glycosylases to specific damaged bases
Functions throughout cell cycle - all phases
Deamination converts a cytosine into a uracil
Uracil is detected and removed leaving a base- less nucleotide
Baseless nucleotide, is removed, leaving a small hole in the dna backbone
Hole is filled with the right base by a dna polymerase (3,0h terminus extension )and gap is sealed by a ligase
Nucleotide excision repair
Cut and patch mechanism that removes a variety of bulky leisions
- multiple bases - often pyrimidine dimers commonly caused by uv radiation
Detects and corrects types of damage that distort the dna double helix
G1 phase prior to dna synthesis
Endonucleases removed multiple nucleotides
Dna polymerase and ligase fill gaps
Double strand breakage repair
Caused by some types of environmental factors- high energy radiation
Causes double stranded breaks in dna- splitting a chromosome in two
Dangerous because large segments of chromosomes + hundreds of genes that they contain may be lost if breakage is not repaired .
There are 2 pathways involved in repair of double stranded dna breaks
Non/ homologous pathways
What is Non- homologous ends joining
The two broken ends of the chromosome are glued back together
Is a messy mechanism, of repair - involves the loss / addition of a few nucleotides
Mechanism- tends to produce a mutation
Homologous recombination
Info from the homologous ch that matches the damaged one ( or from sister chromatid if dna has been copies) is used to repair the break
Two homolous chromosomes come together - the undamaged religion of chromatid is used as template to replace the damages religion of broken chromosome
Is cleaner - doesn’t usually cause mutations
Process occurs shortly after cells dna replication before cell division - when the duplicates helices are close to ecahother physically.
Initioation of homologous recombination
To initiate repair, a nucleate chews the 5’ ends of the two broken strands at the break
With the help of specialised enzymes one of the broken 3’ ends invades the unbroken homologous dna duplex and searches for complementary sequence through based pairing
One accurate match is found- the invading strand is elongated by repair dna polymerase using complementary strand as template
After repair polymerase has passed the point where the break has occurred , the newly repaired strand rejoins its original partner- forming base pairs that hold the two strands of broken double helix together
Repair is then completed by additional dna synthesis at 3’ end of both strands followed by dna ligation
Replication errors mechanisms
Are the mechanisms used by cells to correct replication errors and fix dna damage including
Proofreading - corrects errors during dna rep
Mismatch repair- fixes mispaired bases right after dna replication
Explain proofreading
Dna polymerases are the enzymes that build dna in cells
During dna rep most dna polymerases can check thier work with each base they add- proofreading
If polymerase detects an incorrectly paired nucleotides it will remove/replace the nucleotide right away before continuing with dna synthesis
What is mismatch repair?
Happens right after new dna has been made- job is to remove/replace mispaired bases
Can also detect /correct small insertions/deletions that happen when the polymerase slips loosing its footing on the template
Occurs in s/g2 phase after dna synthesis
Mismatch repair and cancer
Mechanism plays important role in preventing cancer- inherited predeposition to certain cancers is caused by mutations in genes that encode mismatch repair proteins
Humans inherit 2 copies of these genes - one who inherits 1 damaged m gene are unaffected until the undamaged copy of the same gene is randomly mutated in a somatic cell- mutated cell are deficient ions mismatch repair - therefore accumulate mutations more rappidly than do normal cells- because cancers arise form cells that have accumulated multiple mutations, a cell deficient in MR - has a greatly enhanced chance of becoming cancerous.
Summary
