First Aid USMLE Step 1: Biochemistry | Exam Questions and Answers | Latest Update 2026/2027 | Graded A+ | Guaranteed Pass.

First Aid USMLE Step 1: Biochemistry | Exam
Questions and Answers | Latest Update 2026/2027 |
Graded A+ | Guaranteed Pass.


What is a nucleoside?

What is a nucleotide?

Which are the purines?

Which are the pyrimidines? - 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.

Leflunomide
Methotrexatem Trimethoprim and Pyrimethamine
5-Fluorouracil
6-Mercaptopurine
Mycophenolate and Ribavirin
Hydroxyurea - *Disrupt pyrimidine synthesis*
*Leflunomide: *inhibits dihydroorate dehydrogenase
*Methotrexatem Trimethoprim and Pyrimethamine: inhibit dihydrofolate reductase,
resulting in decreased dTMP in humans, bacteria and protozoa respectively.
5-Fluorouracil*: inhibits thymidylate syhtnase
*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:* - *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:* - *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 organization of eukaryotic DNA/gene - 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 - -Promoter - CAAT and TATA box region upstream.

,RNA poly II and other transcription factors bind here.
-Enhancer -
- Silencer


*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?* - *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?*
- mixed connective tissue disease

How do cyclins and tumor supressors regulate cell cycle - CDK - Constitutive and
inactive
Cyclin - activate CDKs
Cyclin-CDK complexes - phosphorylate other proteins to coordinate cell cycle
progression; must be activated and inactivated at appropriate times for cell cycle to
progress

Tumor suppressor - p53 induces p21, which inhibits CDKs -> hypophos (activation) of
Rb -> hypophos Rb binds and inactivates E2F -> inhibit G1-S progression. ------
Mutation in these genes recsult un unrestrained cell division (Li-frau)

Permanent cells - -Neurons, skeletal and cardiac muscle, RBC
-Remain in Go phase

Stabile (quiescent) cells - -Hepatocytes, lymphocytes
-Enter G1 from Go when stimulated

Labile cells - -Bone marrow, gut epithelium, skin, hair follicles, germ cells
-Never go to Go but rapidly divide

Cells rich in RER
Cells rich in SER - RER - mucus secreting goblet cells of small intesting and antibody
secreting plasma cells

, SER - liver hepatocytes and steroid hormone-producing cells of the adrenal cortex and
gonads

I cell disease - -Inherited lysosomal storage disorder.
-Defect in phosphotransferase -> golgi cant phosphorylate manonose on glycoproteins
(low mannose 6 phosphate) -> proteins/hydrolytic enzymes cannot be transported to
lysosomes and is transported extracellularly -> generalized inflam

-Symptoms - coarse facial features, clouded corneas, restricted joint movement, high
plasma levels of lysosome enzymes

Kartagener syndrome - -aka primary ciliary diskynesia -immotile
cilia due to dynein arm defect.
-Symptoms - male and female infertility, bronchiectasis, recurrent sinnusitis, situs
inversus - know difference between this and CF

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 -

Osteogenesis imperfecta *pattern
of inheritance*:
*mutation:*
*symptoms:* - *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. and