Final: DNA Repair Systems Flashcards
3 basic mechanisms of lesion repair
direct base repair
nucleotide excision repair
base excision repair
how to the 3 basic mechanisms of lesion repair work in general?
use proteins to perform corrective chemistry on defective DNA (heal)
what are the 1st and 2nd lines of defense against aging and cancer?
1st: antioxidants
2nd: DNA repair
4 types of direct repair and # of steps
repair lesions in a single step
alkyl transferase
photoreactivation
nick repair
gap repair
alkyl transferases
enzymes pick off alkyl (methyl/ethyl) groups from DNA bases
MGMT methylguanine methyl transferase (1 molecule per methyl..can’t reuse)
photoreactivation
in bacteria, thymine dimer repaired by breaking it into two thymines again
uses a photon of blue light
nick repair
ligase (same enzyme used in last step of DNA replication)
gap repair
DNA polymerase fills the gap, then ligase
what is excision repair?
cut out a piece of damaged ssDNA
leaves a gap, which is fixed by gap repair
Xeroderma pigmentosum
genetic disease caused by defective dimer excision repair system
thymine dimers caused by short-wave UV light (sun)
leads to skin cancer
AP site repair system
turn AP site into a gap (nick above missing base)
then gap repair fixes
glycosylase repair system
recognize bad bases and cut it out by cutting bond between base and sugar (glycosyl bond)
this cutting leaves an AP site, AP site repair fixes
transcription-coupled repair (TCR)
brings an excision repair system to a damaged site during transcription
Cockayne Syndrome
genetic disease caused by defective TCR
cells get stalled when transcribing genes (leads to apoptosis)
causes stunted growth
symptoms of Cockayne Syndrome
shortness, skeletal deformation, premature aging, MR
post-replication repair
fixes some lesions after DNA has been replicated
mismatch repair (MMR)
type of post-replication repair
replication error puts wrong base on new daughter strand
identifies parent strand by finding which has methylated adenines
changes incorrect bases on daughter strand to match parent strand
2 types of double-strand break repair (DSBR) & why needed?
homologus repair (HR)
non-homologus end joining (NHEJ)
when a double strand break occurs, a small bit of chromosome is lost
homologus repair (HR)
repair break in chromosome by lining up broken piece with matching chromatid of DNA (or chromosome)
restores missing sequence and prevents translocation/inversion
non-homologus end joining
used when HR fails
stick two broken ends back together
could be the wrong ends, causing translocation/inversion
doesn’t replace missing info accurately leadings to deletions/insertions
loss of heterozygosity (HR)
if HR uses the homologus chromosome, not chromatid then LOH could occur
other chromosome may have different alleles, making repaired chromosome have recessive alleles
