First Aid USMLE Step 1: Biochemistry
Exam Questions With Correct Answers
What is a nucleoside?
What is a nucleotide?
Which are the purines?
Which are the pyrimidines? - answer✔✔Side = base + (deoxy)ribose [sugar]
Tide = base + (deoxy)ribose [sugar] + phosphaTe
*Which are the purines?*
"pure as gold" = Adenosine, Guanosine
*Which are the pyrimidines?*
"CUT the pyramid" = Cytosine, Uracil, Thymine
In the nucleotide synthesis pathway, describe how purine synthesis differs from pyrimidine
synthesis.
Fr the following drugs, describe how they interfere with purine or pyrimidine synthesis.
*symptoms:* - answer✔✔*metabolic deficiency:*
- absence of adenosine deaminase resulting in increased dATP which is toxic to lymphocytes
- one of the major causes of autosomal recessive SCID
*symptoms:*
Lesch-Nyhan Syndrome
*pattern of inheritance*:
*metabolic deficiency:*
*symptoms:* - answer✔✔*pattern of inheritance*:
- x-linked recessive
*metabolic deficiency:*
- defective purine salvage due to absence of HGPRT which converts hypoxanthine to IMP and
guanine to GMP
- this results in excess uric acid production and de novo purine synthesis
Describe organization of eukaryotic DNA/gene - answer✔✔Coding strand : 5'
..Enhancer.............CAATbox(-75)....TATAA box(-25)....transcription start
site(+1)......exon....intron...exon.....AAATAAA..3'
-on exam always assume it's the 5'-3' coding strand that you see
-5'-3' coding strand (same as RNA)
-3'-5' template strand (compl. To RNA)
-5'-3' mRNA
-(CAAT & TATAA are the promoter region)
Regulation of gene expression - answer✔✔-Promoter - CAAT and TATA box region upstream.
RNA poly II and other transcription factors bind here.
-Enhancer -
- Silencer
What is the function of the following polymerases:
Lac operon - answer✔✔Low glucose: hi AC activity -> hi cAMP -> activation of CAP induces
transcription only if repressor is not bound
High lactose: unbinds repressor from the opersator and increases transcription
Nucleotide excision repair - answer✔✔excision endonucleases release oligonucleotide
containing damaged bases. DNA polymerase fills in gap and ligase seals it. Repairs bulky kelix-
distortine lesions. Occurs in G1 phase. Defect in xoderma pigmentosum, which prevents repair
of pyrimidine dimers because of UV exposure
Base excision repair - answer✔✔Base specific glycosylases remove altered base and creates AP
site. One or more nucelotides are removed by AP endonucleases that clease the 5' end, Lyase
cleases the 3' end, and DNA poly 3 fillgs gap and ligase seals it. ........... Important fo cytosine
deamination and nitrates from meat?
Mismatch repair - answer✔✔Newly synthesized strand is recognized, mismatched nucleotides
are removed, and the gap is filled and resealed. Occurs in G2 phase
-MSH2 finds mismatch
-MCH1 takes it out
-Defective in Lynch syndrome (HNPCC)
*What is the splicing process of pre-mRNA?*
*What is the name for antibodies againsr splicesosome snRNPs and what disease are they highly
specific for?*
What disease are anti-UI RNP antibodies associated with?* - answer✔✔*What is the splicing
process of pre-mRNA?*
*What is the name for antibodies againsr splicesosome snRNPs and what disease are they highly
specific for?
- antibodies to spliceosomal snRNPS (aka *anti-SMITH antibodies) are highly specific for SLE
*What disease are anti-UI RNP antibodies associated with?*
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