Flashcards in Chapter 5 (DNA Repair) Deck (25):
What would happen to DNA if it is left unrepaired?
it would under go spontaneous mutations
What are the two major pathways in which DNA is repaired?
Base excision repair
nucleotide excision repair
What is base excision repair?
DNA glycolases recognizing a damaged base and excising that base out. this is done by a process called flipping out. after removal of the base another is put in place by DNA polymerase
What does DNA glycolases do?
they recognize a altered damaged base and catalyze its hydrolytic removal
How many DNA glycolases are there and what are the type of damaged bases?
6 DNA glycolases that recognize:
deminated Cs and As
alkylated and oxidized bases
bases with opened rings
C=C accidentally converted to C-C
What is flipping out?
DNA glycolases travel along the DNA using base flipping to evaluate the bases
What is apurinic and apyrimidine endonuclease?
a nuclease that that recognizes the gap left by the DNA glycosylase and cleaves the phosphodiester backbone and the damage is repaired
What is depurination?
when a purine is cleaved of the phasphate and sugar Most common damage to DNA
What is deamination?
When a cytosine is turned into a uracil via amine group is removed and replaced by a carbonyl
What is nucleotide excision base repair?
A multienzyme scans for bulky lesions (pyrimidine dimers) in the double helix rather for specific bases .
once found it cleaves he phosphodiester bockbone on both sides ofthe lesion and DNA helicase peels it away. then it is repaired by DNA poly merase and ligase
What is a alternative to base and nucleotide excision repair?
total chemical reversal of DNA damage
What is DNA repair coupled with?
transcription, RNA polymerase stalls at the lesion, detects the damage and directs the machinery to the damaged area
How doe RNA polymerase repair DNA in bacteria?
RNA dissociates from DNA, DNA is repaired and transcription starts all over agin
How does RNA repair DNA in eukaryotes?
RNA polymerase is backed up, DNA is repaired then RNA polymerase continues transcription from the same spot
What does transcription coupled repair work with?
all types of DNA repair
repairs DNA most important DNA sequences
Why was uracil replaced by thymine in DNA?
because the repair mechanism of DNA would find it difficult to distinguish between deaminated cytosine(uracil) and cytosine
What is the deamination of methylated cytosine form?
Thymine, a special repair system is in place to recognize T-G mismatch but is very ineffective
What is used on heavily damaged DNA?
10 different DNA polymerase
they are not very accurate and do not have any exonucleolytic sites. they are used only in emergencies and only have one or a few chances to repair a base
What is a dangerous type of DNA damage?
Double strand breaks, leaves no DNA strand intact to make complementary copy from
What are the repair mechanism for Double strand breaks?
nonhomologous end joining
What is homologous end joining?
Joining together the two broken ends of DNA with the loss of one or two nucleotides.
Why is nonhomologous end joining accepted as a repair mechanism?
So little of the DNA actually codes for proteins that the damage doesnt do any harm.
by the age of 70 a human contains 2000 or so of these scars
When is homologous recombination used in humans?
shortly after replication(S and G2) when sister chromatids can serve as a template
What is the extra mechanism that is in place in eukaryotes?
They delay cell cycle. If DNA damaged is detected at certain checkpoints then the cell cycle is stopped