Which of the following statements about excision repair is correct?
a. base excision repair is initiated by dna glycosylases that recognize abnormal deoxyriboses in dna.
b. nucleotide excision repair removes large regions of dna via an excinuclease which cuts on either side of the damaged bases.
c. e. coli exonuclease activity is carried out by uvra, uvrb and uvrc, and the resulting gap is filled in by dna pol iii
d. dna glycosylases cleave the altered nucleoside (base and sugar) from the dna backbone creating an apurinic or apyrimidinic site
e. in humans, a mechanism similar to e. coli is carried out where the protein xpa acts as the exonuclease.

a. True, b. False, c.True, d. True

Explanation:

a. Base excision repair is started by a DNA glycosylase  that recognizes the changes and removes the altered base by cleavage of the glycosidic bond binding the base and the deoxyribose sugar together.

b. Nucleotide excision repair works by a cut-and patch mechanism that removes their heavy lesions, including pyrimidine dimers and nucleotides . Endonucleases are responsible for the lesion of the damaged strand.

c. Nucleotide excision repair  is initiated by the proteins namely UvrA, UvrC, and UvrB in Escherichia coli.

-UvrD (helicase II) later removes the damaged strand

-DNA polymerase I (PolI) fills in the resulting gap.

d. DNA glycolases removes the damaged nitrogenous base.

-It leaves the sugar-phosphate backbone intact and thus creating an  apurinic/apyrimidinic site, which is commonly referred to as an AP site.

e. Xeroderma pigmentosum complementation group A(XPA)

-This is an essential protein in the nucleotide excision repair pathway.

- It helps to make a pre-incision complex along with other proteins.                

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