BIOCHEMISTRY SECTION – USMLE STEP 1
2026 | COMPLETE STUDY GUIDE &
PRACTICE QUESTIONS
| GRADED A+ | GUARANTEED SUCCESS
Updated 2026 Questions and Answers
100% Verified Exam Prep and Comprehensive
Rationales Included
,if a cell has a hyperchromatic or no
condensed nucleus is it undergoing
transcription etc?
what are chromosomes? during what highly condensed forms of DNA; mitosis;
part of the cell cycle are they found? neoplasia- highly dividing and thus hyperchromatic
in what pathologic state would you
see a lot of chromosomes?
what experiment is chromosomes karyotyping
used for?
are histones in heterochromatin or heterochromatin; euchromatin
euchromatin more methylated?
which had more acetylated
histones?
what does guanine have on it? a ketone
what does thymine have on it? a methyl
deamination of what pyrimidine cytosine to uracil
makes what?
in what genetic code is uracil found uracil; thymine
in? in replace of what?
,how many bonds does a G-C 3 hydrogen bonds; 2 hydrogen bonds
connection have? what type of
bonds? how about a A-T?
what amino acids are required in glutamine, aspartate, glycine; aspartate
purine synthesis? which of these
does pyrimidine synthesis require
also?
what is the difference between a nucleoside is just the ribose plus the base;
nucleotide and a nucleoside? nucleotide is the ribose plus the base but the
phosphate bond
Is PRPP added later or first in later
pyrimidine synthesis?
what is PRPP? how is it made? its the ribose; its made from PRPP synthetase using
ribose 5 P from the HMP shunt
what is the precursor for both IMP
purines?
what is the precursor for orotate
pyrimidines?
which intermediate is involved in carbamoyl phosphate
both pyrimidine synthesis and the
urea cycle?
, why does a deficiency in ornithine because carbamaoyl phosphate is involved in both
transcarbamoylase (urea cycle urea cycle and pyrimidine synthesis
enzyme) result in an increased
production of orotic acid, the
pyrimidine precursor?
is CPSII or CPSI involved in CPSII
pyrimidine synthesis?
what three enzymes lead to orotic ortinithine transcarbamoylase, orotic acid
acid accumulation? what reactions phosphoribosyltransferase, and orotidine 5
are they involved in? 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 carbamoyl phosphate to orotic acid
requires aspartate?
what are the clinical symptoms of increased orotic acid in urine, megaloblastic
orotic aciduria? anemia, +/- hyperammonemia
how do you differentiate the causes whether or not the patient is also hyperammonemia
of orotic aciduria? (if yes, then OTC def)
2026 | COMPLETE STUDY GUIDE &
PRACTICE QUESTIONS
| GRADED A+ | GUARANTEED SUCCESS
Updated 2026 Questions and Answers
100% Verified Exam Prep and Comprehensive
Rationales Included
,if a cell has a hyperchromatic or no
condensed nucleus is it undergoing
transcription etc?
what are chromosomes? during what highly condensed forms of DNA; mitosis;
part of the cell cycle are they found? neoplasia- highly dividing and thus hyperchromatic
in what pathologic state would you
see a lot of chromosomes?
what experiment is chromosomes karyotyping
used for?
are histones in heterochromatin or heterochromatin; euchromatin
euchromatin more methylated?
which had more acetylated
histones?
what does guanine have on it? a ketone
what does thymine have on it? a methyl
deamination of what pyrimidine cytosine to uracil
makes what?
in what genetic code is uracil found uracil; thymine
in? in replace of what?
,how many bonds does a G-C 3 hydrogen bonds; 2 hydrogen bonds
connection have? what type of
bonds? how about a A-T?
what amino acids are required in glutamine, aspartate, glycine; aspartate
purine synthesis? which of these
does pyrimidine synthesis require
also?
what is the difference between a nucleoside is just the ribose plus the base;
nucleotide and a nucleoside? nucleotide is the ribose plus the base but the
phosphate bond
Is PRPP added later or first in later
pyrimidine synthesis?
what is PRPP? how is it made? its the ribose; its made from PRPP synthetase using
ribose 5 P from the HMP shunt
what is the precursor for both IMP
purines?
what is the precursor for orotate
pyrimidines?
which intermediate is involved in carbamoyl phosphate
both pyrimidine synthesis and the
urea cycle?
, why does a deficiency in ornithine because carbamaoyl phosphate is involved in both
transcarbamoylase (urea cycle urea cycle and pyrimidine synthesis
enzyme) result in an increased
production of orotic acid, the
pyrimidine precursor?
is CPSII or CPSI involved in CPSII
pyrimidine synthesis?
what three enzymes lead to orotic ortinithine transcarbamoylase, orotic acid
acid accumulation? what reactions phosphoribosyltransferase, and orotidine 5
are they involved in? 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 carbamoyl phosphate to orotic acid
requires aspartate?
what are the clinical symptoms of increased orotic acid in urine, megaloblastic
orotic aciduria? anemia, +/- hyperammonemia
how do you differentiate the causes whether or not the patient is also hyperammonemia
of orotic aciduria? (if yes, then OTC def)
