Question

A 58-year-old man comes to the office due to difficulty swallowing for the past several months. He has the most trouble with sold foods and says, "They seem to get stuck in my throat if I don't chew a lot" The patient has no chest pain or heartburn and has lost 4.5 kg (10 lb) in the last 3 months. He has been an avid hunter for many years and frequently cures the meat he eats with sodium nitrite Physical examination is unremarkable. Endoscopy shows an ulcerated mass in the distal third of the esophagus, and biopsy samples ere obtained from the mass and adjacent normal mucosa. Analysis of the samples shows accelerated cytosine deamination of chromosomal DNA in both normal and malignant epithelial cells. This damage is most likely to be repaired through which of the following enzymatic sequences?

A. Endonuclease. polymerase. glycosylase, lyase, ligaee
B. Endonuclease, polymerase, lyase, glycosylase, Iigase
C. Glycosylase, endonuclease, lyase, polymerase, ligase
D. Glycosylase, ligase, lease, endonuclease, polymerase
E. Lyase, endonuclease, glycosylase, polymerase, ligase

Answer 1

For the DNA damage involving accelerated cytosine deamination, the repair process is known as base excision repair. The correct sequence of enzymes involved is DNA glycosylase, AP endonuclease, lyase, DNA polymerase, and DNA ligase.

Step-by-step explanation:

The patient's difficulty with swallowing and the discovery of an ulcerated mass in the distal third of the esophagus, when coupled with the biopsy showing accelerated cytosine deamination of chromosomal DNA, suggest a need for DNA repair in both normal and malignant epithelial cells. The most likely sequence of enzymatic actions to repair this type of damage, known as base excision repair, involves the removal and replacement of damaged bases and repair of the DNA backbone. The correct sequence is the removal of the damaged base by DNA glycosylase, cleavage of the DNA strand at the site of the damaged base by an AP endonuclease, removal of the remaining sugar phosphate by a lyase, filling in of the resulting gap by DNA polymerase, and sealing of the final 'nick' in the DNA strand by DNA ligase.

The sequence best matching this process is C. Glycosylase, endonuclease, lyase, polymerase, ligase.