USMLE: BIOCHEMISTRY FINAL 2025/2026 QUESTIONS AND VERIFIED CORRECT SOLUTIONS GRADED A+ (LATEST!)

USMLE: BIOCHEMISTRY FINAL
2025/2026 QUESTIONS AND VERIFIED
CORRECT SOLUTIONS GRADED A+
(LATEST!)

Heterochromatin
Highly Condensed DNA
Transcriptionally inactive
Ex. Barr bodies (inactivated X chromosomes)




Euchromatin
Less condensed DNA
Transcriptionally active




DNA Methylation
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
Represses DNA transcription

,Histone Acetylation
Relaxes DNA coiling and allows for transcription
'Histone acetylation makes DNA ACTIVE'




Histones
Nucleosome (8):
-H2A x2
-H2B x2
-H3 x2
-H4 x2


Linker Histone
-H1


Histones rich in arginine and lysine




Purines vs Pyrimidines
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
Carbamoyl Phosphate Synthetase II


Combines Glutamine, CO2, ATP to make Carbamoyl Phosphate




Purine Synthesis Rate Limiting Step
Glutamine PRPP Aminotransferase


Converts PRPP to IMP




Lesch-Nyhan Syndrome
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
Unwinds DNA template at replication fork




Single Stranded binding proteins
prevent strands from reannealing