Lecture 5 (AT) Flashcards
(18 cards)
What are the main sources of replication errors?
- Keto/enol equilibrium for G and T.
- Amino/imino equilibrium for A and C.
T(enol) will form…
3 H bonds with G (instead of 2 with A).
G(imino) will form…
2 H bonds with A (instead of 3 with C).
This is mispairing, which is classified as a transition. Define this.
Purines/pyrimidines are switched.
Base slipping can occur in two different ways. Explain these mechanisms.
- Occurs in the template strand –> results in a DNA strand that is one base short.
- Occurs in the new strand –> results in DNA that is one base longer.
There are 3 types of spontaneous lesions. Name these.
- Spontaneous deamination.
- Loss of bases at apurinic/apyrimidinic sites = AP sites.
- Base pairing between 8-oxoguanine and adenine.
Loss of bases at AP sites.
This is when there is a backbone but no bases are attached. This is usually repaired. However, if not, a random base is usually incorporated (ofetn A) and this causes a mutation.
Spontaneous deamination.
- The amino group of the cytosine can be hydrolysed into uracil.
- 5-methylcytosine can be deaminated into thymine.
Base pairing between 8-oxoguanine and adenine.
Forms 2 H bonds with A. This is a transversion - purine is replaced with a pyrimidine.
Induced lesions.
Alteration of bases, such as alkylating agents and bulky adducts.
Base pair damage by bulky adducts.
They bind to DNA, e.g. aflatoxin and prevent progression of the replication fork. Replication fork arrest.
How does UV light damage DNA?
Causes cross-linking between adjacent bases, which can lead to the formation of a cyclobutane pyrimidine dimer.
What does intercalculate mean?
To insert themselves between planar bases of DNA.
Causes errors in DNA replication through strand slippage. So either deletions or insertions.
Give an example of an intercalculating agent.
Acridine orange.
What does the cell do when it encounters one of these lesions?
These lesions will cause replication fork arrest. The high fidelity Pol III will be replaced with Pol V, which is less accurate. The cell favours continuing replication, even if inaccurate, over no replication.
What does the Ames test take into account?
The fact that some compounds must be metabolised before they become mutagenic, e,g. aflatoxin.
Process of the Ames test?
- Add aflatoxin to a liver extract so that it is metabolised.
- Add to a plate of salmonella bacteria that have a mutant gene that is involved in histidine synthesis.
- If you get colony growth this indicates that you have revertants = mutagenic compound has fixed the original mutation.
A mutation in which strand will cause replication fork arrest?
LEADING. In the lagging strand it is not a barrier because of the repeating annealing of the DNA primer.
