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, transcriptionally active, sterically accessible chromatin - CORRECT
ANSWER✔✔- euchromatin
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)
"histone Methylation Mostly makes DNA Mute"
Histone acetylation - CORRECT ANSWER✔✔- relaxes DNA coiling --> permits
transcription
"histone Acetylation makes DNA Active"
function of methylation and acetylation - CORRECT ANSWER✔✔- change DNA
transcription activity
,methylation mutes
acetylation activates
purines (which, ring number) - CORRECT ANSWER✔✔- A, G
2 rings
PUR As Gold
pyrimidines (which, ring number) - CORRECT ANSWER✔✔- C, T, U
1 ring
CUT the PY (pie is a single circle)
thymine vs uracil - CORRECT ANSWER✔✔- thymine has a methyl, found in DNA
deamination of cytosine --> uracil, found in RNA
base pair with 3 H-H bonds (and consequences) - CORRECT ANSWER✔✔- G-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 - CORRECT ANSWER✔✔- GAG + THF
glycine
aspartate
glutamine
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
ribonucleotides --> deoxyribonucleotides - CORRECT ANSWER✔✔- ribonucleotide
reductase
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
*Add sugar + P (PRPP):*
3. orotic acid + PRPP --> UMP --> UDP
*Modify base*
4a. UDP --> CTP
4b. UDP --> dUDP by ribonucleotide reductase
5. dUDP --> dUMP
6. dUMP + THF --> dTMP by thymidylate synthase = methylates uracil into thymadine
*THF (which becomes DHF) regenerated by dihydrofolate reductase
, differences in de novo purine and pyrimidine synthesis - CORRECT ANSWER✔✔- purines:
start with sugar + phos (PRPP), add base
pyrimidines: make temporary base (orotic acid), add sugar + phos (PRPP), modify base
(requires THF for thymidylate synthase)
inhibits dihydroorotate dehydrogenase - CORRECT ANSWER✔✔- leflunomide (tx: RA)
prevents carbamoyl phosphate --> orotic acid in pyrimidine synthesis
inhibits IMP dehydrogenase - CORRECT ANSWER✔✔- mycophenolate (tx: organ
transplant rejection)
ribavirin (tx: viral infx)
prevents IMP --> GMP in purine synthesis
inhibits ribonucleotide reductase - CORRECT ANSWER✔✔- hydroxyurea (tx: SC dz > inc
HbF)
prevents UDP --> dUDP (RNA > DNA) in pyrimidine synthesis
prodrug of 6-mercaptopurine (6-MP) - CORRECT ANSWER✔✔- Azathioprine
MOA of 6-MP and Azathioprine (and use) - CORRECT ANSWER✔✔- inhibit de novo
purine synthesis
tx: leukemia, immune dzs (RA, IBD)
prevents PRPP --> IMP
inhibits thymidylate synthase - CORRECT ANSWER✔✔- 5-fluorouracil
prevents dUMP --> dTMP, decreases thymine
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, transcriptionally active, sterically accessible chromatin - CORRECT
ANSWER✔✔- euchromatin
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)
"histone Methylation Mostly makes DNA Mute"
Histone acetylation - CORRECT ANSWER✔✔- relaxes DNA coiling --> permits
transcription
"histone Acetylation makes DNA Active"
function of methylation and acetylation - CORRECT ANSWER✔✔- change DNA
transcription activity
,methylation mutes
acetylation activates
purines (which, ring number) - CORRECT ANSWER✔✔- A, G
2 rings
PUR As Gold
pyrimidines (which, ring number) - CORRECT ANSWER✔✔- C, T, U
1 ring
CUT the PY (pie is a single circle)
thymine vs uracil - CORRECT ANSWER✔✔- thymine has a methyl, found in DNA
deamination of cytosine --> uracil, found in RNA
base pair with 3 H-H bonds (and consequences) - CORRECT ANSWER✔✔- G-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 - CORRECT ANSWER✔✔- GAG + THF
glycine
aspartate
glutamine
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
ribonucleotides --> deoxyribonucleotides - CORRECT ANSWER✔✔- ribonucleotide
reductase
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
*Add sugar + P (PRPP):*
3. orotic acid + PRPP --> UMP --> UDP
*Modify base*
4a. UDP --> CTP
4b. UDP --> dUDP by ribonucleotide reductase
5. dUDP --> dUMP
6. dUMP + THF --> dTMP by thymidylate synthase = methylates uracil into thymadine
*THF (which becomes DHF) regenerated by dihydrofolate reductase
, differences in de novo purine and pyrimidine synthesis - CORRECT ANSWER✔✔- purines:
start with sugar + phos (PRPP), add base
pyrimidines: make temporary base (orotic acid), add sugar + phos (PRPP), modify base
(requires THF for thymidylate synthase)
inhibits dihydroorotate dehydrogenase - CORRECT ANSWER✔✔- leflunomide (tx: RA)
prevents carbamoyl phosphate --> orotic acid in pyrimidine synthesis
inhibits IMP dehydrogenase - CORRECT ANSWER✔✔- mycophenolate (tx: organ
transplant rejection)
ribavirin (tx: viral infx)
prevents IMP --> GMP in purine synthesis
inhibits ribonucleotide reductase - CORRECT ANSWER✔✔- hydroxyurea (tx: SC dz > inc
HbF)
prevents UDP --> dUDP (RNA > DNA) in pyrimidine synthesis
prodrug of 6-mercaptopurine (6-MP) - CORRECT ANSWER✔✔- Azathioprine
MOA of 6-MP and Azathioprine (and use) - CORRECT ANSWER✔✔- inhibit de novo
purine synthesis
tx: leukemia, immune dzs (RA, IBD)
prevents PRPP --> IMP
inhibits thymidylate synthase - CORRECT ANSWER✔✔- 5-fluorouracil
prevents dUMP --> dTMP, decreases thymine
