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First Aid USMLE Step 1: Biochemistry Exam Practice Questions and Answers (100% Pass)
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First Aid USMLE Step 1: Biochemistry Exam Practice Questions and Answers (100% Pass) What is a nucleoside? What is a nucleotide? Which are the purines? Which are the pyrimidines? - Answer️️ -Side = base + (deoxy)ribose [sugar] Tide = base + (deoxy)ribose [sugar] + phosphaTe *Which are t...

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  • August 28, 2024
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  • 2024/2025
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©SOPHIABENNET@2024/2025 Tuesday, August 20, 202410:21 AM


First Aid USMLE Step 1: Biochemistry Exam
Practice Questions and Answers (100% Pass)

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.


1

, ©SOPHIABENNET@2024/2025 Tuesday, August 20, 202410:21 AM




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 - Answer✔️✔️-*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



2

, ©SOPHIABENNET@2024/2025 Tuesday, August 20, 202410:21 AM




*Disrupt purine and pyrimidine synthesis*

*Hydroxyurea*: inhibits nucleotide reductase

Adenosine deaminase deficiency

*metabolic deficiency:*

*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



3

, ©SOPHIABENNET@2024/2025 Tuesday, August 20, 202410:21 AM


- 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 - 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)




4

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