BIOCHEMISTRY -USMLE STEP 1 FINAL EXAM QUESTIONS
& ANSWERS A+ GRADE STUDY GUIDE
which histone ties DNA-histone nucleosome beads into more condensed structures? -
CORRECT ANSWER >>>H1
is DNA acidic or basic? - CORRECT ANSWER >>>acidic
what type of aa are histones made of? are they acidic or basic? - CORRECT ANSWER >>>lysine
and arginine; basic
is heterochromatic more accessible or less accessible to TF? euchromatin? - CORRECT ANSWER
>>>less; more
if a cell has a hyperchromatic or condensed nucleus is it undergoing transcription etc? -
CORRECT ANSWER >>>no
what are chromosomes? during what part of the cell cycle are they found? in what pathologic
state would you see a lot of chromosomes? - CORRECT ANSWER >>>highly condensed forms of
DNA; mitosis; neoplasia- highly dividing and thus hyperchromatic
what experiment is chromosomes used for? - CORRECT ANSWER >>>karyotyping
are histones in heterochromatin or euchromatin more methylated? which had more acetylated
histones? - CORRECT ANSWER >>>heterochromatin; euchromatin
what does guanine have on it? - CORRECT ANSWER >>>a ketone
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,what does thymine have on it? - CORRECT ANSWER >>>a methyl
deamination of what pyrimidine makes what? - CORRECT ANSWER >>>cytosine to uracil
in what genetic code is uracil found in? in replace of what? - CORRECT ANSWER >>>uracil;
thymine
how many bonds does a G-C connection have? what type of bonds? how about a A-T? -
CORRECT ANSWER >>>3 hydrogen bonds; 2 hydrogen bonds
what amino acids are required in purine synthesis? which of these does pyrimidine synthesis
require also? - CORRECT ANSWER >>>glutamine, aspartate, glycine; aspartate
what is the difference between a nucleotide and a nucleoside? - CORRECT ANSWER
>>>nucleoside 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? - CORRECT ANSWER >>>later
what is PRPP? how is it made? - CORRECT ANSWER >>>its the ribose; its made from PRPP
synthetase using ribose 5 P from the HMP shunt
what is the precursor for both purines? - CORRECT ANSWER >>>IMP
what is the precursor for pyrimidines? - CORRECT ANSWER >>>orotate
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,which intermediate is involved in both pyrimidine synthesis and the urea cycle? - CORRECT
ANSWER >>>carbamoyl phosphate
why does a deficiency in ornithine transcarbamoylase (urea cycle enzyme) result in an increased
production of orotic acid, the pyrimidine precursor? - CORRECT ANSWER >>>because
carbamaoyl phosphate is involved in both urea cycle and pyrimidine synthesis
is CPSII or CPSI involved in pyrimidine synthesis? - CORRECT ANSWER >>>CPSII
what three enzymes lead to orotic acid accumulation? what reactions are they involved in? -
CORRECT ANSWER >>>ortinithine 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? - CORRECT ANSWER >>>carbamoyl
phosphate to orotic acid
what are the clinical symptoms of orotic aciduria? - CORRECT ANSWER >>>increased orotic acid
in urine, megaloblastic anemia, +/- hyperammonemia
how do you differentiate the causes of orotic aciduria? - CORRECT ANSWER >>>whether or not
the patient is also hyperammonemia (if yes, then OTC def)
how do you differentiate causes of megaloblastic anemia? - CORRECT ANSWER >>>orotic
aciduria megaloblastic anemia can not be corrected with folic acid or vit B12
how do you treat orotic aciduria? how does this work? - CORRECT ANSWER >>>oral uridine
administration; provides nucleosides and provides feedback inhibition
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, what converts dUMP to dTMP? what drug blocks this? - CORRECT ANSWER >>>thymidylate
synthase; 5-Flourouracil
what drug blocks ribonucleotide reductase? what does this enzyme do? - CORRECT ANSWER
>>>hydroxurea; deoxygenates ribose
what does ornithine transcarbamyolase do to carbamoyl phosphate? - CORRECT ANSWER
>>>converts it to citrulline
what does 6 mercaptopurine do? - CORRECT ANSWER >>>6MP blocks de novo purine synthesis
other than thymidylate synthase, what else is required to convert dUMP to dTMP? - CORRECT
ANSWER >>>N5N10 methylene THF
After N5N10 methylene THF donates a methyl for conversion of dUMP to dTMP what is it
converted to? - CORRECT ANSWER >>>DHF
what enzyme converts DHF to THF? what vitamin cofactor is used to transfer a methyl from an
amino acid to DHF? what amino acid is that? - CORRECT ANSWER >>>dihydrofolate reductase;
vitamin B12; homocysteine
what happens to homocysteine after converting a methyl? - CORRECT ANSWER >>>becomes
methionine
how is THF converted to N5N10 Methylene? - CORRECT ANSWER >>>using serine (which
turns into glycine after reaction)
what does methotrexate do? what step is this enzyme involved in? - CORRECT ANSWER >>>it
inhibits conversion of DHF to THF by blocking dihyrdofolate reductase
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& ANSWERS A+ GRADE STUDY GUIDE
which histone ties DNA-histone nucleosome beads into more condensed structures? -
CORRECT ANSWER >>>H1
is DNA acidic or basic? - CORRECT ANSWER >>>acidic
what type of aa are histones made of? are they acidic or basic? - CORRECT ANSWER >>>lysine
and arginine; basic
is heterochromatic more accessible or less accessible to TF? euchromatin? - CORRECT ANSWER
>>>less; more
if a cell has a hyperchromatic or condensed nucleus is it undergoing transcription etc? -
CORRECT ANSWER >>>no
what are chromosomes? during what part of the cell cycle are they found? in what pathologic
state would you see a lot of chromosomes? - CORRECT ANSWER >>>highly condensed forms of
DNA; mitosis; neoplasia- highly dividing and thus hyperchromatic
what experiment is chromosomes used for? - CORRECT ANSWER >>>karyotyping
are histones in heterochromatin or euchromatin more methylated? which had more acetylated
histones? - CORRECT ANSWER >>>heterochromatin; euchromatin
what does guanine have on it? - CORRECT ANSWER >>>a ketone
1|Page
,what does thymine have on it? - CORRECT ANSWER >>>a methyl
deamination of what pyrimidine makes what? - CORRECT ANSWER >>>cytosine to uracil
in what genetic code is uracil found in? in replace of what? - CORRECT ANSWER >>>uracil;
thymine
how many bonds does a G-C connection have? what type of bonds? how about a A-T? -
CORRECT ANSWER >>>3 hydrogen bonds; 2 hydrogen bonds
what amino acids are required in purine synthesis? which of these does pyrimidine synthesis
require also? - CORRECT ANSWER >>>glutamine, aspartate, glycine; aspartate
what is the difference between a nucleotide and a nucleoside? - CORRECT ANSWER
>>>nucleoside 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? - CORRECT ANSWER >>>later
what is PRPP? how is it made? - CORRECT ANSWER >>>its the ribose; its made from PRPP
synthetase using ribose 5 P from the HMP shunt
what is the precursor for both purines? - CORRECT ANSWER >>>IMP
what is the precursor for pyrimidines? - CORRECT ANSWER >>>orotate
2|Page
,which intermediate is involved in both pyrimidine synthesis and the urea cycle? - CORRECT
ANSWER >>>carbamoyl phosphate
why does a deficiency in ornithine transcarbamoylase (urea cycle enzyme) result in an increased
production of orotic acid, the pyrimidine precursor? - CORRECT ANSWER >>>because
carbamaoyl phosphate is involved in both urea cycle and pyrimidine synthesis
is CPSII or CPSI involved in pyrimidine synthesis? - CORRECT ANSWER >>>CPSII
what three enzymes lead to orotic acid accumulation? what reactions are they involved in? -
CORRECT ANSWER >>>ortinithine 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? - CORRECT ANSWER >>>carbamoyl
phosphate to orotic acid
what are the clinical symptoms of orotic aciduria? - CORRECT ANSWER >>>increased orotic acid
in urine, megaloblastic anemia, +/- hyperammonemia
how do you differentiate the causes of orotic aciduria? - CORRECT ANSWER >>>whether or not
the patient is also hyperammonemia (if yes, then OTC def)
how do you differentiate causes of megaloblastic anemia? - CORRECT ANSWER >>>orotic
aciduria megaloblastic anemia can not be corrected with folic acid or vit B12
how do you treat orotic aciduria? how does this work? - CORRECT ANSWER >>>oral uridine
administration; provides nucleosides and provides feedback inhibition
3|Page
, what converts dUMP to dTMP? what drug blocks this? - CORRECT ANSWER >>>thymidylate
synthase; 5-Flourouracil
what drug blocks ribonucleotide reductase? what does this enzyme do? - CORRECT ANSWER
>>>hydroxurea; deoxygenates ribose
what does ornithine transcarbamyolase do to carbamoyl phosphate? - CORRECT ANSWER
>>>converts it to citrulline
what does 6 mercaptopurine do? - CORRECT ANSWER >>>6MP blocks de novo purine synthesis
other than thymidylate synthase, what else is required to convert dUMP to dTMP? - CORRECT
ANSWER >>>N5N10 methylene THF
After N5N10 methylene THF donates a methyl for conversion of dUMP to dTMP what is it
converted to? - CORRECT ANSWER >>>DHF
what enzyme converts DHF to THF? what vitamin cofactor is used to transfer a methyl from an
amino acid to DHF? what amino acid is that? - CORRECT ANSWER >>>dihydrofolate reductase;
vitamin B12; homocysteine
what happens to homocysteine after converting a methyl? - CORRECT ANSWER >>>becomes
methionine
how is THF converted to N5N10 Methylene? - CORRECT ANSWER >>>using serine (which
turns into glycine after reaction)
what does methotrexate do? what step is this enzyme involved in? - CORRECT ANSWER >>>it
inhibits conversion of DHF to THF by blocking dihyrdofolate reductase
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