Biochemistry USMLE step 1 questions correctly 2023

Biochemistry USMLE step 1 questions correctly 2023histones are rich in which 2 AA's lysine & argenine Barr bodies (what, type of chromatin) inactive X chromosomes heterochromatin (highly condensed, sterically inaccessible) less condensed, transcriptionally active, sterically accessible chromatin euchromatin DNA methylation at CpG islands represses transcription "CpG Methylation makes DNA Mute" DNA methylation (what is methylated and why?) template strand cytosine and adenine are methyated during replication --> mismatch repair enzymes can distinguish between old and new strands Histone methylation usually reversibly represses DNA transcription (can activate depending on location) "histone Methylation Mostly makes DNA Mute" Histone acetylation relaxes DNA coiling --> permits transcription "histone Acetylation makes DNA Active" function of methylation and acetylation change DNA transcription activity methylation mutes acetylation activates purines (which, ring number) A, G 2 rings PUR As Gold pyrimidines (which, ring number) C, T, U 1 ring CUT the PY (pie is a single circle) thymine vs uracil thymine has a methyl, found in DNA deamination of cytosine --> uracil, found in RNA base pair with 3 H-H bonds (and consequences) G-C G-C is stronger than A-T (2 H-H) inc G-C content --> inc melting temperature of DNA amino acids necessary for purine synthesis GAG + THF glycine aspartate glutamine nucleoside vs. nucleotide vs. nucleotide di/triphosphate nucleoside = base + sugar nucleotide = base + sugar + phosphate nucleotide diphosphate = base + sugar + 2P's nucleotide triphosphate = base + sugar + 3P's de novo purine synthesis R5P --> PRPP by PRPP synthetase (adds 2P's) add base (hypoxanthine) --> IMP by HGPRT IMP converted to AMP and GMP ribonucleotides --> deoxyribonucleotides ribonucleotide reductase carbamoyl phosphate involved in which 2 metabolic pathways? de novo pyrimidine synthesis (CP synthetase II) urea cycle de novo pyrimidine synthesis Make temporary base: 1. glutamine + CO2 + ATP --> carbamoyl phosphate by CP synthetase II 2. carbamoyl phosphate + aspartate --> orotic acid via dihydroorotate dehydrogenase Add sugar + P (PRPP): 3. orotic acid + PRPP --> UMP --> UDP Modify base 4a. UDP --> CTP 4b. UDP --> dUDP by ribonucleotide reductase 5. dUDP --> dUMP 6. dUMP + THF --> dTMP by thymidylate synthase = methylates uracil into thymadine *THF (which becomes DHF) regenerated by dihydrofolate reductase differences in de novo purine and pyrimidine synthesis purines: start with sugar + phos (PRPP), add base pyrimidines: make temporary base (orotic acid), add sugar + phos (PRPP), modify base (requires THF for thymidylate synthase) inhibits dihydroorotate dehydrogenase leflunomide (tx: RA)