Biochemistry Flashcards
7 cards from real NBME practice questions. Tap to flip, then mark Knew It or Still Learning — missed cards come back until you master them.
Read the first 7 Biochemistry flashcards as text
During eukaryotic DNA replication, which enzyme complex synthesizes the RNA primer at origins of replication?
Answer: DNA polymerase alpha-primase complex
DNA polymerase alpha associates with primase to synthesize short RNA primers that provide the 3'-OH needed by replicative DNA polymerases.
Huntington disease is caused by which type of mutation in the HTT gene?
Answer: Trinucleotide (CAG) repeat expansion
Expansion of CAG repeats beyond ~36 copies in the HTT gene produces a mutant huntingtin protein with an abnormally long polyglutamine tract causing neurodegeneration.
A child with severe combined immunodeficiency (SCID) is found to have adenosine deaminase (ADA) deficiency. Toxic accumulation of which compound destroys lymphocytes?
Answer: Deoxyadenosine triphosphate (dATP)
ADA deficiency causes deoxyadenosine accumulation, which is phosphorylated to dATP; excess dATP inhibits ribonucleotide reductase and induces apoptosis in lymphocytes.
Xeroderma pigmentosum presents with extreme UV sensitivity and high skin cancer risk. Which DNA repair pathway is defective?
Answer: Nucleotide excision repair (NER)
NER removes bulky helix-distorting lesions such as pyrimidine dimers caused by UV radiation; XP mutations in NER endonucleases allow these lesions to persist.
Which post-transcriptional modification is added to the 3' end of eukaryotic mRNA to protect against exonuclease degradation and aid nuclear export?
Answer: Poly-A tail of ~200 adenosine residues
A poly-A tail added by poly-A polymerase to the 3' end of mRNA protects against nucleolytic degradation and facilitates cytoplasmic export and translation initiation.
Translation initiation in eukaryotes begins when the ribosome scans the 5' mRNA cap to find the start codon. Which amino acid is encoded by AUG in eukaryotes?
Answer: Methionine
In eukaryotes, AUG encodes methionine (Met) as the initiator amino acid, whereas prokaryotes use N-formyl-methionine (fMet) for initiation.
The lac operon of E. coli is induced when lactose is present and glucose is absent. The actual inducer molecule that binds and inactivates the lac repressor is:
Answer: Allolactose (a galactose-glucose isomer of lactose)
Beta-galactosidase converts a small amount of lactose to allolactose, which binds the lac repressor and causes it to dissociate from the operator, allowing transcription.