USMLE Step 1- Biochemistry (2022) Questions and Answers

Histones are rich in which 2 amino acids? - Lysine Arginine What histone protein is responsible for binding to the nucleosome and to the "linker" DNA thereby stabilizing the chromatin fiber? - H1 When does DNA and histone synthesis occur in the cell cycle? - S-phase Will the following be increased or decreased or neutral in heterochromatin? methylation acetylation - methylation increased acetylation decreased Which two bases are methylated in DNA replication on the template strand in prokaryotes? - 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? - CpG islands (Protip: this represses transcription) Deamination of cytosine will produce.... - UracilDeamination of adenine will produce.... - Guanine Methylation of uracil will produce.... - Thymine What amino acids are necessary for purine synthesis? - Glycine Aspartate Glutamine What enzyme does Leflunomide inhibit? What will be the general effect of Leflunomide? - 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? - Dihydrofolate reductase in humans Causes a decrease in deoxythmidine monophosphate (dTMP) What enzyme does trimethorpim inhibit? - Dihydrofolate reductase in bacteria What enzyme does pyrimethamine inhibit? - Dihydrofolate reductase in protozoa What enzyme does 5-Fluorouracil inhibit?What will be the general effect of 5-FU? - Thymidylate synthase Decreased conversion of dUMP to dTMP What will be the general effect of 5-mercaptopurine? - Inhibit de novo purine synthesis What will be the general effect of azathioprine? - 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? - Inosine monophosphate dehydrogenase Disruption of purine synthesis What enzyme will be inhibited by Ribavirin? What will be the general effect of Ribavirin? - Inosine monophosphate dehydrogenase Disruption of purine synthesis What enzyme will be inhibited by Hydroxyurea? What will be the general effect of Hydroxyurea? - Ribonucleotide reductase(Protip: can't add deoxy to sugars without this enzyme) Disruption of both purine and pyrimidine synthesis What enzyme deficiency is one of the major causes of SCID? - ADA deficiency What type of inheritance pattern is in SCID? - AR