BIOCHEMISTRY USMLE STEP 1
CERTIFICATION TEST 2026 QUESTIONS
WITH SOLUTIONS GRADED A+
⩥ Euchromatin. Answer: Less condensed DNA
Transcriptionally active
⩥ DNA Methylation. Answer: Occurs at *CpG Islands* to repress
transcription
'CpG Methylation Makes DNA MUTE'
Template strand *cytosine* and *adenine* are methylated, which allow
mismatch repair enzymes to distinguish between old and new strands.
⩥ Histone Methylation. Answer: Represses DNA transcription
⩥ Histone Acetylation. Answer: Relaxes DNA coiling and allows for
transcription
'Histone acetylation makes DNA ACTIVE'
⩥ Histones. Answer: Nucleosome (8):
-H2A x2
,-H2B x2
-H3 x2
-H4 x2
Linker Histone
-H1
Histones rich in *arginine* and *lysine*
⩥ Purines vs Pyrimidines. Answer: Purines: A, G (2 rings)
Pyrimidines: T, C (1 ring)
G-C bond has 3 H bonds (higher melting temperature)
A-T bond has 2 H bonds
⩥ Pyrimidine Synthesis Rate Limiting Step. Answer: Carbamoyl
Phosphate Synthetase II
Combines Glutamine, CO2, ATP to make Carbamoyl Phosphate
⩥ Purine Synthesis Rate Limiting Step. Answer: Glutamine PRPP
Aminotransferase
,Converts PRPP to IMP
⩥ Lesch-Nyhan Syndrome. Answer: Defective purine salvage due to
absent *HGPRT*
Unable to recycle Guanine and Hypoxanthine to GMP and IMP
respectively.
Excess Uric acid and de novo purine synthesis
HGPRT: hyperuricemia. gout, pissed off (aggression, self mutilation),
retardation, dysTonia
⩥ Helicase. Answer: Unwinds DNA template at replication fork
⩥ Single Stranded binding proteins. Answer: prevent strands from
reannealing
⩥ DNA Topoisomerases. Answer: Creates single or double stranded
break in helix to remove supercoils
T1: No ATP required
, T2: ATP required
⩥ Primase. Answer: Makes RNA primer for DNA polymerase III to
initiate transcription
⩥ DNA Polymerase III. Answer: Prokaryotes ONLY
Lays down new strand in 5' to 3' direction (synthesis)
and has 3' to 5' exonuclease activity (proof reading)
⩥ DNA Polymerase I. Answer: Degrades RNA primer and replaces it
with DNA
⩥ DNA Ligase. Answer: Joins Okazaki fragments
⩥ Telomerase. Answer: Eukaryotes ONLY
Adds DNA to *3' end* of chromosomes to avoid loss of genetic material
⩥ Silent Mutation. Answer: Nucleotide substitution codes for *same
amino acid*
(Synonymous)
CERTIFICATION TEST 2026 QUESTIONS
WITH SOLUTIONS GRADED A+
⩥ Euchromatin. Answer: Less condensed DNA
Transcriptionally active
⩥ DNA Methylation. Answer: Occurs at *CpG Islands* to repress
transcription
'CpG Methylation Makes DNA MUTE'
Template strand *cytosine* and *adenine* are methylated, which allow
mismatch repair enzymes to distinguish between old and new strands.
⩥ Histone Methylation. Answer: Represses DNA transcription
⩥ Histone Acetylation. Answer: Relaxes DNA coiling and allows for
transcription
'Histone acetylation makes DNA ACTIVE'
⩥ Histones. Answer: Nucleosome (8):
-H2A x2
,-H2B x2
-H3 x2
-H4 x2
Linker Histone
-H1
Histones rich in *arginine* and *lysine*
⩥ Purines vs Pyrimidines. Answer: Purines: A, G (2 rings)
Pyrimidines: T, C (1 ring)
G-C bond has 3 H bonds (higher melting temperature)
A-T bond has 2 H bonds
⩥ Pyrimidine Synthesis Rate Limiting Step. Answer: Carbamoyl
Phosphate Synthetase II
Combines Glutamine, CO2, ATP to make Carbamoyl Phosphate
⩥ Purine Synthesis Rate Limiting Step. Answer: Glutamine PRPP
Aminotransferase
,Converts PRPP to IMP
⩥ Lesch-Nyhan Syndrome. Answer: Defective purine salvage due to
absent *HGPRT*
Unable to recycle Guanine and Hypoxanthine to GMP and IMP
respectively.
Excess Uric acid and de novo purine synthesis
HGPRT: hyperuricemia. gout, pissed off (aggression, self mutilation),
retardation, dysTonia
⩥ Helicase. Answer: Unwinds DNA template at replication fork
⩥ Single Stranded binding proteins. Answer: prevent strands from
reannealing
⩥ DNA Topoisomerases. Answer: Creates single or double stranded
break in helix to remove supercoils
T1: No ATP required
, T2: ATP required
⩥ Primase. Answer: Makes RNA primer for DNA polymerase III to
initiate transcription
⩥ DNA Polymerase III. Answer: Prokaryotes ONLY
Lays down new strand in 5' to 3' direction (synthesis)
and has 3' to 5' exonuclease activity (proof reading)
⩥ DNA Polymerase I. Answer: Degrades RNA primer and replaces it
with DNA
⩥ DNA Ligase. Answer: Joins Okazaki fragments
⩥ Telomerase. Answer: Eukaryotes ONLY
Adds DNA to *3' end* of chromosomes to avoid loss of genetic material
⩥ Silent Mutation. Answer: Nucleotide substitution codes for *same
amino acid*
(Synonymous)
