USMLE STEP 1 BIOCHEMISTRY EXAM 2025/2026
QUESTIONS WITH ANSWERS GRADED A+
✔✔What type of Antibiotics inhibit bacterial topoisomerase 4? - ✔✔Quinolones
✔✔_____chormatin is tightly coiled and transcriptionally inactive while _____chromatin
is less condensed and is transcriptionally active. - ✔✔Heterochromatin; Euchromatin
✔✔DNA methylation often ______ genes while histone acetylation ______. -
✔✔Inactivates (decreases gene transcription); Loosens DNA making it more accessible
and more easily transcribed.
✔✔Sjogren's syndrome (lymphocytic infiltration of salvivary and lacrimal glands
resulting in dry mouth/dryeyes) is often associate with antibodies against SS-A/B (single
strand binding proteins). What role do these proteins play in prokaryotic DNA
replication? - ✔✔SS-DNA binding protein bind and stabilize ssdna to prevent re-
annealing
✔✔DNA polymerase III has polymerase activity in the _______ direction and
exonuclease activity in the _______ direction. - ✔✔5 to 3; 3 to 5
✔✔The primer for DNA replication synthesized by primase is made of DNA or RNA? -
✔✔RNA
✔✔RNA primer removal in DNA synthesis is done by dna polymerase _____. - ✔✔I
✔✔What DNA replication enzyme is responsible for sealing the remaining nicks? -
✔✔DNA ligase
✔✔What are the 3 stop codons? - ✔✔UGA, UAG, UAA
✔✔Proteins are synthesized from ___ to ____ terminus. - ✔✔N; C
✔✔1 codon for ____ amino acid. - ✔✔1
✔✔DNA to RNA is known as _________. - ✔✔Transcription
✔✔RNA to protein is known as ________. - ✔✔Translation
✔✔What is the Translation start codon and what amino acid does is code for? -
✔✔AUG; Methionine
, ✔✔How does Rifampin work and what is it most commonly used for? - ✔✔Binds the
beta subunit of RNA polymerase and inhbitis the intiation of transcription; Tuberculosis
✔✔How does actinomycin D work? - ✔✔Binds DNA and prevents RNA polymerase
(both eukaryotic and prokaryotc) from moving along template.
✔✔Eukaryotic RNA polymerase 1 makes _______. - ✔✔rRNA
✔✔Eukaryotic RNA poylmerase 2 makes_______. - ✔✔mRNA
✔✔Eukaryotic RNA polymerase 3 makes_______. - ✔✔tRNA
✔✔The splicesome exsises introns or exons? - ✔✔Introns
✔✔What chemotherap agent inhibits thymidylate synthase (therefore decreasing
pyrmidine synthesis for DNA)? - ✔✔5-fluorouracil
✔✔What chemotherapuetic agent inhibits dihyrdofolate reductase (thus no THF for
pyrimadine synthesis)? - ✔✔Methotrexate
✔✔_______ are made from and IMP precursor while _________ are made from orotate
precusor, with PRPP added later. - ✔✔Purine; Pyrimidines
✔✔Ribnucleotides are converted to deoxynucleotides by what enzyme? -
✔✔Riboncleotide reductase
✔✔Methotrexate inhibits _________. - ✔✔dihydrofolate reductase, cant regenerate
N5N10THF so reduces dTMP, but will have enough dUMP for RNA synthesis-- NOT
SPECIFIC FOR BACTERIAL CELLS THUS IS TOXIC TO ALL DIVDING CELLS (e.g.
GI tract, skin, hair follicles etc)
✔✔Bactrim works by inhibiting bacterial ________. - ✔✔dihydrofolate reductase, thus
reducing available THF, so reduces dTMP, pyrimidine synthesis
✔✔What two antibiotics are combined in Bactrim? - ✔✔Trimethoprim and
Sulfamethazole
✔✔5-fluorouracil (5-FU) inhibits ________. - ✔✔thymidylate synthase, reducing dTMP,
pyrimidine synthesis
✔✔6-mercaptone (6-MP) inhibits ________. - ✔✔blocks de novo purine synthesis by
blocking rate limiting PRPP step
QUESTIONS WITH ANSWERS GRADED A+
✔✔What type of Antibiotics inhibit bacterial topoisomerase 4? - ✔✔Quinolones
✔✔_____chormatin is tightly coiled and transcriptionally inactive while _____chromatin
is less condensed and is transcriptionally active. - ✔✔Heterochromatin; Euchromatin
✔✔DNA methylation often ______ genes while histone acetylation ______. -
✔✔Inactivates (decreases gene transcription); Loosens DNA making it more accessible
and more easily transcribed.
✔✔Sjogren's syndrome (lymphocytic infiltration of salvivary and lacrimal glands
resulting in dry mouth/dryeyes) is often associate with antibodies against SS-A/B (single
strand binding proteins). What role do these proteins play in prokaryotic DNA
replication? - ✔✔SS-DNA binding protein bind and stabilize ssdna to prevent re-
annealing
✔✔DNA polymerase III has polymerase activity in the _______ direction and
exonuclease activity in the _______ direction. - ✔✔5 to 3; 3 to 5
✔✔The primer for DNA replication synthesized by primase is made of DNA or RNA? -
✔✔RNA
✔✔RNA primer removal in DNA synthesis is done by dna polymerase _____. - ✔✔I
✔✔What DNA replication enzyme is responsible for sealing the remaining nicks? -
✔✔DNA ligase
✔✔What are the 3 stop codons? - ✔✔UGA, UAG, UAA
✔✔Proteins are synthesized from ___ to ____ terminus. - ✔✔N; C
✔✔1 codon for ____ amino acid. - ✔✔1
✔✔DNA to RNA is known as _________. - ✔✔Transcription
✔✔RNA to protein is known as ________. - ✔✔Translation
✔✔What is the Translation start codon and what amino acid does is code for? -
✔✔AUG; Methionine
, ✔✔How does Rifampin work and what is it most commonly used for? - ✔✔Binds the
beta subunit of RNA polymerase and inhbitis the intiation of transcription; Tuberculosis
✔✔How does actinomycin D work? - ✔✔Binds DNA and prevents RNA polymerase
(both eukaryotic and prokaryotc) from moving along template.
✔✔Eukaryotic RNA polymerase 1 makes _______. - ✔✔rRNA
✔✔Eukaryotic RNA poylmerase 2 makes_______. - ✔✔mRNA
✔✔Eukaryotic RNA polymerase 3 makes_______. - ✔✔tRNA
✔✔The splicesome exsises introns or exons? - ✔✔Introns
✔✔What chemotherap agent inhibits thymidylate synthase (therefore decreasing
pyrmidine synthesis for DNA)? - ✔✔5-fluorouracil
✔✔What chemotherapuetic agent inhibits dihyrdofolate reductase (thus no THF for
pyrimadine synthesis)? - ✔✔Methotrexate
✔✔_______ are made from and IMP precursor while _________ are made from orotate
precusor, with PRPP added later. - ✔✔Purine; Pyrimidines
✔✔Ribnucleotides are converted to deoxynucleotides by what enzyme? -
✔✔Riboncleotide reductase
✔✔Methotrexate inhibits _________. - ✔✔dihydrofolate reductase, cant regenerate
N5N10THF so reduces dTMP, but will have enough dUMP for RNA synthesis-- NOT
SPECIFIC FOR BACTERIAL CELLS THUS IS TOXIC TO ALL DIVDING CELLS (e.g.
GI tract, skin, hair follicles etc)
✔✔Bactrim works by inhibiting bacterial ________. - ✔✔dihydrofolate reductase, thus
reducing available THF, so reduces dTMP, pyrimidine synthesis
✔✔What two antibiotics are combined in Bactrim? - ✔✔Trimethoprim and
Sulfamethazole
✔✔5-fluorouracil (5-FU) inhibits ________. - ✔✔thymidylate synthase, reducing dTMP,
pyrimidine synthesis
✔✔6-mercaptone (6-MP) inhibits ________. - ✔✔blocks de novo purine synthesis by
blocking rate limiting PRPP step
