Biochemistry USMLE Step 1 ACTUAL Exam Questions and CORRECT Answers
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Course
Biochemistry USMLE
Institution
Biochemistry USMLE
Biochemistry USMLE Step 1 ACTUAL
Exam Questions and CORRECT Answers
histones are rich in which 2 AA's - CORRECT ANSWER- lysine & argenine
Barr bodies (what, type of chromatin) - CORRECT ANSWER- inactive X chromosomes
heterochromatin (highly condensed, sterically inaccessible)
less condensed, ...
DNA methylation at CpG islands - CORRECT ANSWER✔✔- represses transcription
"CpG Methylation makes DNA Mute"
DNA methylation (what is methylated and why?) - CORRECT ANSWER✔✔- template
strand cytosine and adenine are methyated during replication --> mismatch repair enzymes
can distinguish between old and new strands
Histone methylation - CORRECT ANSWER✔✔- usually reversibly represses DNA
transcription (can activate depending on location)
nucleoside vs. nucleotide vs. nucleotide di/triphosphate - CORRECT ANSWER✔✔-
nucleoside = base + sugar
,nucleotide = base + sugar + phosphate
nucleotide diphosphate = base + sugar + 2P's
nucleotide triphosphate = base + sugar + 3P's
de novo purine synthesis - CORRECT ANSWER✔✔- R5P --> PRPP by PRPP synthetase
(adds 2P's)
add base (hypoxanthine) --> IMP by HGPRT
IMP converted to AMP and GMP
carbamoyl phosphate involved in which 2 metabolic pathways? - CORRECT ANSWER✔✔-
de novo pyrimidine synthesis (CP synthetase II)
urea cycle
de novo pyrimidine synthesis - CORRECT ANSWER✔✔- *Make temporary base:*
1. glutamine + CO2 + ATP --> carbamoyl phosphate by CP synthetase II
2. carbamoyl phosphate + aspartate --> orotic acid via dihydroorotate dehydrogenase
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