Biochemistry USMLE Step 1 Exam Questions With Answers

Biochemistry USMLE Step 1 Exam Questions With Answers histones are rich in which 2 AA's -ANSWER- lysine & argenine Barr bodies (what, type of chromatin) -ANSWER- inactive X chromosomes heterochromatin (highly condensed, sterically inaccessible) less condensed, transcriptionally active, sterically accessible chromatin -ANSWER- euchromatin DNA methylation at CpG islands -ANSWER- represses transcription "CpG Methylation makes DNA Mute" DNA methylation (what is methylated and why?) -ANSWER- template strand cytosine and adenine are methyated during replication --> mismatch repair enzymes can distinguish between old and new strands Histone methylation -ANSWER- usually reversibly represses DNA transcription (can activate depending on location) "histone Methylation Mostly makes DNA Mute" Histone acetylation -ANSWER- relaxes DNA coiling --> permits transcription "histone Acetylation makes DNA Active" function of methylation and acetylation -ANSWER- change DNA transcription activity methylation mutes acetylation activates purines (which, ring number) -ANSWER- A, G 2 rings PUR As Gold pyrimidines (which, ring number) -ANSWER- C, T, U 1 ring CUT the PY (pie is a single circle) thymine vs uracil -ANSWER- thymine has a methyl, found in DNA deamination of cytosine --> uracil, found in RNA base pair with 3 H-H bonds (and consequences) -ANSWER- 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 -ANSWER- GAG + THF glycine aspartate glutamine nucleoside vs. nucleotide vs. nucleotide di/triphosphate -ANSWER- nucleoside = base + sugar nucleotide = base + sugar + phosphate nucleotide diphosphate = base + sugar + 2P's nucleotide triphosphate = base + sugar + 3P's de novo purine synthesis -ANSWER- R5P --> PRPP by PRPP synthetase (adds 2P's) add base (hypoxanthine) --> IMP by HGPRT IMP converted to AMP and GMP ribonucleotides --> deoxyribonucleotides -ANSWER- ribonucleotide reductase carbamoyl phosphate involved in which 2 metabolic pathways? -ANSWER- de novo pyrimidine synthesis (CP synthetase II) urea cycle de novo pyrimidine synthesis -ANSWER- *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 -ANSWER- 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)