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First Aid USMLE Step 1: Biochemistry Questions with
Detailed Verified Answers (100% Correct Answers)
/Already Graded A+
What is a nucleoside?
What is a nucleotide?
Which are the purines?
Which are the pyrimidines?
Ans: Side = base + (deoxy)ribose [sugar]
Tide = base + (deoxy)ribose [sugar]
*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.
Leflunomide
Methotrexatem Trimethoprim and Pyrimethamine
5-Fluorouracil
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6-Mercaptopurine
Mycophenolate and Ribavirin
Hydroxyurea
Ans: *Disrupt pyrimidine synthesis*
*Leflunomide: *inhibits dihydroorate dehydrogenase
*Methotrexatem Trimethoprim and Pyrimethamine
5-Fluorouracil*: inhibit dihydrofolate reductase, resulting in decreased
dTMP in humans, bacteria and protozoa respectively.
*Disrupt purine synthesis*
*6-Mercaptopurine* (6-MP): and its prodrug azathioprine inhibit de novo
purine synthesis
*Mycophenolate and Ribavirin*: inhibit inosine monophosphate
dehydrogenase
*Disrupt purine and pyrimidine synthesis*
*Hydroxyurea*: inhibits nucleotide reductase
Adenosine deaminase deficiency
*metabolic deficiency:*
*symptoms:*
Ans: *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*:
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*metabolic deficiency:*
*symptoms:*
Ans: *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
*symptoms:*
remember *HGPRT*
– Hyperuricemia
– Gout
– Pissed off (aggression)
– Retarded (intellectual disability)
– DsyTonia
Describe the processing of hmRNA to form an mRNA. Where does this
process occur?
Ans:
What is the function of the following polymerases:
RNA Polymerase I
RNA Polymerase II
RNA Polymerase III
Ans:
What is the splicing process of pre-mRNA?
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What is the name for antibodies against spliceosomal snRNPs and what
disease are they highly specific for?
What disease are anti-UI RNP antibodies associated with?
Ans: *What is the splicing process of pre-mRNA?*
*What is the name for antibodies against spliceosomal 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?*
– mixed connective tissue disease
For the following types of collagen, name the tissues in which they are most
commonly found/important and any relevant diseases in which they are
altered.
Type I
Type II
Type III
Type IV
Ans:
Osteogenesis imperfecta
*pattern of inheritance*:
*mutation:*
*symptoms:*
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Ans: *pattern of inheritance*:
– genetic bone disorder (brittle bone disease)
*mutation:*
– caused by a variety of gene defects but most commonly in COL1A1 and
COL1A2
– most common form is autosomal dominant with decreased production of
otherwise normal type 1 collagen
*symptoms:*
– multiple fractures with minimal trauma (can be concused with child abuse)
– blue sclerae due to translucent connective tissue over choroidal veins
– hearing loss (abnormal ossicles)
– sometimes tooth abnormalities (dentinogenesis imperfecta)
Draw the pathway for the synthesis of collagen and note the location and
steps.
Ans:
Ehlers-Danlos
*pattern of inheritance*:
*mutation:*
*symptoms:*
