USMLE STEP 1 BIOCHEMISTRY Questions and Answers with Complete Solutions 2024

USMLE STEP 1 BIOCHEMISTRY Questions and Answers with Complete Solutions 2024 histones are rich in which 2 AA's - ANSlysine & argenine Barr bodies (what, type of chromatin) - ANSinactive X chromosomes heterochromatin (highly condensed, sterically inaccessible) less condensed, transcriptionally active, sterically accessible chromatin - ANSeuchromatin DNA methylation at CpG islands - ANSrepresses transcription "CpG Methylation makes DNA Mute" DNA methylation (what is methylated and why?) - ANStemplate strand cytosine and adenine are methyated during replication --> mismatch repair enzymes can distinguish between old and new strands Histone methylation - ANSusually reversibly represses DNA transcription (can activate depending on location) "histone Methylation Mostly makes DNA Mute" Histone acetylation - ANSrelaxes DNA coiling --> permits transcription "histone Acetylation makes DNA Active" function of methylation and acetylation - ANSchange DNA transcription activity methylation mutes acetylation activates purines (which, ring number) - ANSA, G 2 rings PUR As Gold pyrimidines (which, ring number) - ANSC, T, U 1 ring CUT the PY (pie is a single circle) thymine vs uracil - ANSthymine has a methyl, found in DNA deamination of cytosine --> uracil, found in RNA base pair with 3 H-H bonds (and consequences) - ANSG-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 - ANSGAG + THF glycine aspartate glutamine nucleoside vs. nucleotide vs. nucleotide di/