BIOCHEMISTRY USMLE COMPREHENSIVE
STUDY GUIDE 2026 QUESTIONS WITH
SOLUTIONS GRADED A+
⩥ What histone protein is responsible for binding to the nucleosome and
to the "linker" DNA thereby stabilizing the chromatin fiber? Answer: H1
⩥ When does DNA and histone synthesis occur in the cell cycle?
Answer: S-phase
⩥ Will the following be increased or decreased or neutral in
heterochromatin?
methylation
acetylation Answer: methylation increased
acetylation decreased
⩥ Which two bases are methylated in DNA replication on the template
strand in prokaryotes? Answer: Cytosine
Adenine
(Protip: this is done so the mismatch repair enzymes can distinguish
between old and new strands)
,⩥ Where does DNA methylation occur in eukaryotes? Answer: CpG
islands
(Protip: this represses transcription)
⩥ Deamination of cytosine will produce.... Answer: Uracil
⩥ Deamination of adenine will produce.... Answer: Guanine
⩥ Methylation of uracil will produce.... Answer: Thymine
⩥ What amino acids are necessary for purine synthesis? Answer:
Glycine
Aspartate
Glutamine
⩥ What enzyme does Leflunomide inhibit?
What will be the general effect of Leflunomide? Answer: inhibits
dihydroorotate dehydrogenase
,(results in increased carbamoyl1-P concentration and a blockage of the
early steps of pyrimidine base production)
⩥ What enzyme does Methotrexate inhibit?
What will be the general effect of Methotrexate? Answer: Dihydrofolate
reductase in humans
Causes a decrease in deoxythmidine monophosphate (dTMP)
⩥ What enzyme does trimethorpim inhibit? Answer: Dihydrofolate
reductase in bacteria
⩥ What enzyme does pyrimethamine inhibit? Answer: Dihydrofolate
reductase in protozoa
⩥ What enzyme does 5-Fluorouracil inhibit?
What will be the general effect of 5-FU? Answer: Thymidylate synthase
Decreased conversion of dUMP to dTMP
, ⩥ What will be the general effect of 5-mercaptopurine? Answer: Inhibit
de novo purine synthesis
⩥ What will be the general effect of azathioprine? Answer: Inhibit de
novo purine synthesis
(Protip: This is the prodrug for 5-mercaptopurine.)
⩥ What enzyme will be inhibited by Mycophenolate?
What will be the general effect of Mycophenolate? Answer: Inosine
monophosphate dehydrogenase
Disruption of purine synthesis
⩥ What enzyme will be inhibited by Ribavirin?
What will be the general effect of Ribavirin? Answer: Inosine
monophosphate dehydrogenase
Disruption of purine synthesis
⩥ What enzyme will be inhibited by Hydroxyurea?
STUDY GUIDE 2026 QUESTIONS WITH
SOLUTIONS GRADED A+
⩥ What histone protein is responsible for binding to the nucleosome and
to the "linker" DNA thereby stabilizing the chromatin fiber? Answer: H1
⩥ When does DNA and histone synthesis occur in the cell cycle?
Answer: S-phase
⩥ Will the following be increased or decreased or neutral in
heterochromatin?
methylation
acetylation Answer: methylation increased
acetylation decreased
⩥ Which two bases are methylated in DNA replication on the template
strand in prokaryotes? Answer: Cytosine
Adenine
(Protip: this is done so the mismatch repair enzymes can distinguish
between old and new strands)
,⩥ Where does DNA methylation occur in eukaryotes? Answer: CpG
islands
(Protip: this represses transcription)
⩥ Deamination of cytosine will produce.... Answer: Uracil
⩥ Deamination of adenine will produce.... Answer: Guanine
⩥ Methylation of uracil will produce.... Answer: Thymine
⩥ What amino acids are necessary for purine synthesis? Answer:
Glycine
Aspartate
Glutamine
⩥ What enzyme does Leflunomide inhibit?
What will be the general effect of Leflunomide? Answer: inhibits
dihydroorotate dehydrogenase
,(results in increased carbamoyl1-P concentration and a blockage of the
early steps of pyrimidine base production)
⩥ What enzyme does Methotrexate inhibit?
What will be the general effect of Methotrexate? Answer: Dihydrofolate
reductase in humans
Causes a decrease in deoxythmidine monophosphate (dTMP)
⩥ What enzyme does trimethorpim inhibit? Answer: Dihydrofolate
reductase in bacteria
⩥ What enzyme does pyrimethamine inhibit? Answer: Dihydrofolate
reductase in protozoa
⩥ What enzyme does 5-Fluorouracil inhibit?
What will be the general effect of 5-FU? Answer: Thymidylate synthase
Decreased conversion of dUMP to dTMP
, ⩥ What will be the general effect of 5-mercaptopurine? Answer: Inhibit
de novo purine synthesis
⩥ What will be the general effect of azathioprine? Answer: Inhibit de
novo purine synthesis
(Protip: This is the prodrug for 5-mercaptopurine.)
⩥ What enzyme will be inhibited by Mycophenolate?
What will be the general effect of Mycophenolate? Answer: Inosine
monophosphate dehydrogenase
Disruption of purine synthesis
⩥ What enzyme will be inhibited by Ribavirin?
What will be the general effect of Ribavirin? Answer: Inosine
monophosphate dehydrogenase
Disruption of purine synthesis
⩥ What enzyme will be inhibited by Hydroxyurea?
