USMLE Biochemistry Exam Questions with Correct Answers

USMLE Biochemistry Exam Questions with Correct Answers Heterochromatin - ANSWER Highly Condensed DNA Transcriptionally inactive Ex. Barr bodies (inactivated X chromosomes) Euchromatin - ANSWER Less condensed DNA Transcriptionally active DNA Methylation - ANSWER Occurs at *CpG Islands* to repress transcription 'CpG Methylation Makes DNA MUTE' Template strand *cytosine* and *adenine* are methylated, which allow mismatch repair enzymes to distinguish between old and new strands. Histone Methylation - ANSWER Represses DNA transcription Histone Acetylation - ANSWER Relaxes DNA coiling and allows for transcription 'Histone acetylation makes DNA ACTIVE' Histones - ANSWER Nucleosome (8): -H2A x2 -H2B x2 -H3 x2 -H4 x2 Linker Histone -H1 Histones rich in *arginine* and *lysine* Purines vs Pyrimidines - ANSWER Purines: A, G (2 rings) Pyrimidines: T, C (1 ring) G-C bond has 3 H bonds (higher melting temperature) A-T bond has 2 H bonds Pyrimidine Synthesis Rate Limiting Step - ANSWER Carbamoyl Phosphate Synthetase II