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What is NER?

Removing a group of damaged bases called an oligonucleotide fragment


What causes damage that requires NER?

UV light


What are the products of UV damage?

UV photoproducts e.g. cyclobutane pyrimidine dimers


What is UVRA?

A protein that has DNA binding ability


What is the function of UVRB?

It is a helicase that unwinds DNA and a translocase that moves along the DNA


What is the function of UVRC?

It is a nuclease that cuts DNA at the damaged site


What is the function of UVRD?

Helicase 2


How are UVRA and UVRB involved in NER in prokaryotes? (STEP 1 IN NER)

1. UVRA2B2 heterotetramer forms
2. This complex binds to DNA using UVRA zinc fingers
3. This complex moves along DNA using UVRB translocase (also uses ATP hydrolysis)
4. The complex then tightly binds to damaged site = kink forms
5. UVRA released = UVRB still tightly bound to the damaged site


What is the role of UVRC in NER in prokaryotes? (STEP 2 IN NER)

1. UVRC uses nuclease activity to cut DNA at the damaged site


What are the 2 cuts that UVRC makes at the site of damage? (STEP 3 IN NER)

1. 3-5 bases from the 3' end of the thymine dimer
2. 7 bases from the 5' end of the thymine dimer


Describe the role of UVRD in NER in prokaryotes? (STEP 4 IN NER)

1. UVRD helicase unwinds the clipped piece of DNA
= damaged part is taken out of helix so left with oligonucleotide fragment being released


How big is the oligonucleotide fragment in prokaryotes?

Usually about 12 bases


What happens after UVRD in NER? (STEP 5 IN NER)

UVRB and UVRC proteins leave


What is the last step of NER in prokarytoes? (STEP 6 IN NER)

Left with gap in helix that can be filled with complementary base pairings using the template strand. DNA is linearised.


What proteins are used to finish up NER? (STEP 6 IN NER)

Pol 1, DNTP and DNA ligase


How many comlemntation groups are in Xerodoma Pigmentosum?



What is XP?

Autosomal recessive skin disease


What are the most common and the least common complentation groups of XP?

Common: A, C, D
Least common: B, E, F, G, U


What characterises XP?

XP patients don't have NER or UDS (unshceduled DNA synthesis)


What would cells without UV exposure show in the lab?

They would show S phase nuclei = lots of DNA synthesis


What do XP cells show in the lab and how is this different to normal UV irradiated cells?

Normal = shows G1 and G2 cells, this shows that cells are trying to recover from UV damage, this is UDS.

XP = G1 and G2 phase cells are not present, that means that the cells cannot recover from damage (don't have UDS)


Describe the diagnostic test pathway for XP

1. Take cells and UV irradiate them
2. Incubate with flurescent precurser of DNA
3. Measure how much of the precurser is taken up by the cells
= XP cells take up less precurser, therefore showing low levels of UDS


Describe the symptoms of XP

- Freckling
- Increase/decrease of skin pigmentation
- Skin cancers leading to death
- Neurological abnormalities
- Sunburn sensitivity in 50% of cases


Why are Cockane Syndrome cells negatively affected by UV, despite having normal UDS?

Because cells need the TC-NER subpathway to help the cell to survive after UV damage. But, this pathway is defective in CS, therefore they are UV sensitive and UDS is not enough for survival.


Do CS cells have normal GG-NER?



Which complementation groups of CS are common vs rare?

Common: CSB
Rare: CSA


What are the symptoms of CS?

- Beaked nose
- Loss of adipose tissue
- Premature ageing
- Dwarfism
- Sunken eyes
- Severe mental retardation
- Sun sensitivity


How is CS different to XP with regards to symptoms?

In XP, sun sensitivity = cancer
In CS, sun sensitivity = no cancer


Describe the diagnostic test in CS?

1. Incubate UV irradiated cells for 24 hours
2. Measure levels of RNA synthesis
= Normal cells = little effect on RNA synthesis
= CS cells = big effect on RNA synthesis