Roxanita
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Q.- The enzyme DNA glycosylase is necessary in order to:
a) Convert cytosine to uracil.
b) Add the Deoxyribose to form deoxynucleotides
c) Form Thymine dimers
d) Reseal nicks in a DNA molecule
e) Remove a uracil base from DNA
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| mjl1717
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answer-b ( it cleaves abnormal bases from the deoxyribose phosphate backbone)
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| Roxanita
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let's try again :wink:
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| mjl1717
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answer-e, although I dont remember seeing choice e previously!
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| mash
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E removes uracil frm DNA creating abasic sites
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| rida
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Agree with E
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| Roxanita
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yup the answer is e)
sorry mjl1717, I forgot to put the last choice before :oops:
quick read  ...
A number of diseases exist which are due to DEFECTS IN DNA REPAIR MECHANISMS
XERODERMA PIGMENTOSUM is the most widespread of these, but is still rather rare. Skin cells from these patients cannot carry out the excision repair that is necessary to repair DNA damage caused by UV light. The first step in the repair process: the removal of the pyrimidine dimer by endonuclease activity, occurs with reduced efficiency due to a defective enzyme. The patients with this disease, as well as the carriers of this disease, have a higher incidence of skin cancer.
NUCLEOTIDE EXCISION REPAIR is a mechanism of DNA repair in which oligonucleotides containing damaged bases are removed from a DNA molecule. The damaged DNA (a thymine dimer, for example) is recognized and cleaved on both sides of the damaged area by a 3’ and a 5’ nuclease. A helicase unwinds the DNA, resulting in excision of the oligonucleotide. DNA polymerase then fills in the missing nucleotides and the nick is sealed by a DNA ligase.
DNA GLYCOSILASE is an enzyme that cleaves the bond linking a base, such as URACIL, to the Deoxyribose of the DNA backbone. This leaves a sugar that is not attached to a base and is called an AP (for apurinic or apyrimidinic) site. The enzyme AP ENDONUCLEASE cleaves adjacent to the AP site, the Deoxyribose moiety is removed, and DNA polymerase adds the correct nucleotide. The "nick" is then ligated by DNA ligase.
The DEAMINATION OF CYTOSINEresults in the formation of Uracil.
Bloom’s syndrome is an inherited human disease in which individuals are partially defective in DNA ligation. These patients are consequently defective in DNA repair and have a dramatically increased incidence of cancer.
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