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USMLE Step 1: Biochemistry & other

Biochemistry

Biochemistry

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Kartei Details

Karten 288
Sprache English
Kategorie Medizin
Stufe Universität
Erstellt / Aktualisiert 02.03.2015 / 27.03.2020
Lizenzierung Namensnennung (CC BY)    (First Aid for the USMLE Step 1 2012: Vikas Bhushan and Tao Le; © USMLEWorld, LLC)
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Chromatin structure

  • negative DNA loops twice around positively charged histone octamer, which then forms the nucleosome core (H2A, H2B, H3, H4)
  • H1 ties beads together

  1. Heterochromatin
  2. Euchromatin

  1. condensed, inactive, inaccessible
  2. less condensed, active, accessible
  1. Methylation
  2. Hypermethylation
  3. Acetylation

  1. C-G base pairs on the template strand are methylated –> allows mismatch repair enzymes to distinguish between old & new strands
  2. inactivates transcription of DNA
  3. relaxes DNA coiling –>  transcription

  1. Purines
  2. Pyrimidines
  3. Bonds

  1. 2 rings
    1. Adenosine
    2. Guanine: has a ketone
    3. Amino acides necessary for purine synthesis: glycine, aspartate, glutamine
  2. 1 rings
    1. Thymine: has a methyl; in DNA
    2. Cytosine: becomes uracil when deaminated
    3. Uracil: in RNA
  3. Bonds
    1. G-C bond stronger (3H)
    2. A-T bond (2H)

  1. Nucleoside
  2. Nucleotide

  1. Base + ribose
  2. Base + ribose + phosphate

De novo pyrimidine and purine synthesis

Melde dich an, um Bilder anzusehen.

  • Purines:
    • start with sugar + phosphate (PRPP)
    • add base
  • Pyrimidines:
    • make temporary base (orotic acid)
    • add sugar + phosphate (PRPP)
    • modify base

Antineoplastic and antibiotic drugs interfering with nucleotide synthesis (5)

  • Hydroxyurea: inhibits ribonucleotide reductase
  • 6-mercaptopurine: blocks de novo purine synthesis
  • 5-fluorouracil: inhibits thymidylate synthase (dTMP decreased)
  • Methotrexate: inhibits dihydrofolate reductase (dTMP decreased)
  • Trimethoprim: inhibits bacterial dihydrofolate reductase (dTMP decreased)

Orotic aciduria

  1. Definition
  2. Findings (4)
  3. Treatment

  1. Inability to convert orotic acid to UMP
    1. due to defect in either orotic acid phosphoribosyltransferase or orotidine 5'-phosphate decarboxylase
    2. autosomal recessive
  2. Findings
    1. increased orotic acid in urine
    2. megaloblastic anemia
    3. failure to thrive
    4. No hyperammonemia
  3. Treament: oral uridine administration