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

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 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. ©SOPHIABENNET@2024/2025 Tuesday, August 20, 202410:21 AM 2 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 - Answer️️ -*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 ©SOPHIABENNET@2024/2025 Tuesday, August 20, 202410:21 AM 3 *Disrupt purine and pyrimidine synthesis* *Hydroxyurea*: inhibits nucleotide reductase Adenosine deaminase deficiency *metabolic deficiency:* *symptoms:* - Answer️️ -*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:* - Answer️️ -*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 ©SOPHIABENNET@2024/2025 Tuesday, August 20, 202410:21 AM 4 - 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 - Answer️️ -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) ©SOPHIABENNET@2024/2025 Tuesday, August 20, 202410:21 AM 5 Regulation of gene expression - Answer️️ --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: RNA Polymerase I RNA Polymerase II RNA Polymerase III Describe the processing of hmRNA to form an mRNA and where does this process occur - Answer️️ - Lac operon - Answer️️ -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 - Answer️️ -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 ©SOPHIABENNET@2024/2025 Tuesday, August 20, 202410:21 AM 6 Base excision repair - Answer️️ -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 - Answer️️ -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?*