Lecture 26 - Replication fidelity, DNA repair and recombination (pt 2) Flashcards
What nucleotide excision repair mechanism repairs
Chemical changes in DNA that affect normal helical structure
What induces chemical changes in DNA that affect normal helical structure
UV light
How chemical changes in DNA that affect normal helical structure are detected
Sensor proteins slide along for DNA looking for kinks in the helical structure
Example of chemical changes in DNA due to UV light that affect helical structure
Adjacent thymine bases covalent bonding
Protein that recognizes thymine dimers
XP-C/23B complex (dimer)
XP-C = big protein that surrounds it
23 B = protein that binds XP-C
First protein that XP-C/23B complex recruits and what it does
Transcription factor TFIIH (is a DNA helicase). Unwinds the DNA
2 proteins that further unwind the DNA after TFIIH and how what they exactly do
XP-G and RPA (replication fork protein A). Further unwind until a 25 nt bubble is formed
How TFIIH, XP-G and RPA are positionned together
TFIIH thymine dimer side (figure : top)
RPA on A A strand side (figure : bottom)
XP-G on right
2 proteins required/recruited after 25 nt bubble is formed and what they do
XP-G (already there) and XP-F = 2 ENDOnucleases
XP-G cuts at 3’ end of bubble (on T dimer strand)
XP-F cuts at 5’ end of bubble (on T dimer strand)
After endonucleases (XP-G and XP-F) action, what happens to 25nt strand that was cut away
is degraded
How 25nt strand that was cut away is replaced
DNAP and DNA ligase fill the gap
Name of cancer syndrome in families that lack genes for proteins in nucleotide excision repair
xeroderma pigmenosum (XP)
What induces double-strand DNA breaks
X-rays and gamma-rays
2 systems that repair double-strand DNA breaks
1) Homologous recombination
2) Nonhomologous end joining
Principle of homologous recombination and how perfect repair can be
Uses unbroken homologous chromosome as a repair template
Can make a perfect repair
Nonhomologous end joining difference with homologous recombination and how perfect repair can be
does not use unbroken homologous chromosome as repair template
error-prone repair (always errors)
Homologous recombination step 1
Generate 3’ ssDNA tails at the break site (on broken chromosome) by using 5’-exonucleases
Homologous recombination step 2
Use Rad-51 to invade (hybridize with) the template chromosome with one of the two tails (part of a strand on the template chromosome makes a bubble cause not coupled anymore)
What hybridization of 3’ ssDNA tail of break site to template chromosome allows
perfect alignement of the two chromosomes
Homologous recombination step 3
3’ end of invading tail is extended until DNA in ‘‘bubble’’ base-pairs with other 3’ ssDNA tail. (bubble is now bigger + one of two 3’ ssDNA tails is now repaired)
Homologous recombination step 4
Extension of the second (non-invading 3’ ssDNA tail) using the strand of the template chromosome that is not hybridized (that forms a bubble)
Homologous recombination step 5
DNA ligase ligates the repaired strands
Structure left after Homologous recombination + name
4-stranded DNA structure, all bases paired, but with a bubble of 2 strands mixing with other chromosome ->HOLLIDAY STRUCTURE = EACH CROSSING AT BOTH ENDS OF BUBBLE (so there are 2)
When nonhomologous end joining is used
when homologous chromosome that can act as a template is not nearby
