Biochemistry USMLE step 1 Question with complete solution 2023

Biochemistry USMLE step 1 Question with complete solution 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