Biochemistry Section-USMLE Step 1 2024 Questions and Answers

Biochemistry Section-USMLE Step 1 2024 Questions and Answers which histone ties DNA-histone nucleosome beads into more condensed structures? - answerH1 is DNA acidic or basic? - answeracidic what type of aa are histones made of? are they acidic or basic? - answerlysine and arginine; basic is heterochromatic more accessible or less accessible to TF? euchromatin? - answerless; more if a cell has a hyperchromatic or condensed nucleus is it undergoing transcription etc? - answerno what are chromosomes? during what part of the cell cycle are they found? in what pathologic state would you see a lot of chromosomes? - answerhighly condensed forms of DNA; mitosis; neoplasia- highly dividing and thus hyperchromatic what experiment is chromosomes used for? - answerkaryotyping are histones in heterochromatin or euchromatin more methylated? which had more acetylated histones? - answerheterochromatin; euchromatin what does guanine have on it? - answera ketone what does thymine have on it? - answera methyl deamination of what pyrimidine makes what? - answercytosine to uracil in what genetic code is uracil found in? in replace of what? - answeruracil; thymine how many bonds does a G-C connection have? what type of bonds? how about a A-T? - answer3 hydrogen bonds; 2 hydrogen bonds what amino acids are required in purine synthesis? which of these does pyrimidine synthesis require also? - answerglutamine, aspartate, glycine; aspartate what is the difference between a nucleotide and a nucleoside? - answernucleoside is just the ribose plus the base; nucleotide is the ribose plus the base but the phosphate bond Is PRPP added later or first in pyrimidine synthesis? - answerlater what is PRPP? how is it made? - answerits the ribose; its made from PRPP synthetase using ribose 5 P from the HMP shunt what is the precursor for both purines? - answerIMP what is the precursor for pyrimidines? - answerorotate which intermediate is involved in both pyrimidine synthesis and the urea cycle? - answercarbamoyl phosphate why does a deficiency in ornithine transcarbamoylase (urea cycle enzyme) result in an increased production of orotic acid, the pyrimidine precursor? - answerbecause carbamaoyl phosphate is involved in both urea cycle and pyrimidine synthesis is CPSII or CPSI involved in pyrimidine synthesis? - answerCPSII what three enzymes lead to orotic acid accumulation? what reactions are they involved in? - answerortinithine transcarbamoylase, orotic acid phosphoribosyltransferase, and orotidine 5 phosphate decarboxylase; urea cycle; last two are involved in conversion of orotic acid to UMP (adding PRPP to orotic acid) what step in pyrimidine synthesis requires aspartate? - answercarbamoyl phosphate to orotic acid what are the clinical symptoms of orotic aciduria? - answerincreased orotic acid in urine, megaloblastic anemia, +/- hyperammonem