USMLE Step 1 First Aid - Biochemistry 2022 (100% Verified)

Chromatin structure - Negatively charged DNA loops twice around histone octamer (2 each of the positively charged H2A, H2B, H3, and H4) to form nucleosome bead. H1 ties nucleosomes together in a string. (Think of "beads on a string"; H1 is the only histone that is not in the nucleosome core.) In mitosis, DNA condenses to form mitotic chromosomes. <img src="66a - Euchramatin strxr.JPG" /> Heterochromatin - Condensed, transcriptionally inactive ("H eteroC hromatin = H ighly C ondensed.") Euchromatin - Less condensed, transcriptionally active (Eu = true, "truly transcribed") Purines - A, G 2 Rings ("PUR e A s G old = PUR ines") <img src="67b - Purine strxr.JPG" /> Pyrimidines - C, T, U 1 ring ("CUT the PY (pie): PY rimidines") <img src="67c - Pyrimidine strxr.JPG" /> Functional groups of the nucleosides - Guanine has a ketone. Thy mine has a methy l. Deamination of cytosine makes uracil. Base differences btw RNA and DNA - Uracil is found in RNA; Thymine in DNA Base pair bonds - G-C bond (3 H-bonds) is stronger than A-T bond (2 H-bonds). Incr G-C content --< higher melting temperature. AA's necessary for purine synthesis - G lycine A spartate G lutamine Nucleoside - Base + riboseNucleotide - Base + ribose + phosphate; linked by 3'-5' phosphodiester bond. Purines are made from...? - IMP precursor (see bottom/right) <img src="68a - Nucleotide synthesis.JPG" /> Pyrimidines are made from...? - Orotate precursor, with PRPP added later. <img src="68a - Nucleotide synthesis.JPG" /> Deoxyribonucleotide synthesis - Ribonucleotides are synthesized first and are converted to deoxyribonucleotides by ribonucleotide reductase. <img src="68a - Nucleotide synthesis.JPG" /> ABX and anti-neoplastic drugs that function by interfering w/ nucleotide synthesis (list) - Hydroxyurea 6-mercaptopurine 5-fluorouracil Methotrexate Trimethoprim Hydroxyurea - Inhibits ribonucleotide reductase. <img src="68a - Nucleotide synthesis.JPG" /> 6-mercaptopurine (6-MP) - Blocks de novo purine synthesis. <img src="68a - Nucleotide synthesis.JPG" /> 5-Fluorouracil (5-FU) - Inhibits thymidilate synthase (decr dTMP). <img src="68a - Nucleotide synthesis.JPG" /> Methotrexate - Inhibits dihydrofolate reductase (decr dTMP) <img src="68a - Nucleotide synthesis.JPG" /> Trimethoprim - Inhibits bacterial dihydrofolate reductase (decr dTMP) <img src="68a - Nucleotide synthesis.JPG" />Transition vs. transversion - Transition: Substituting purine for purine or pyrimidine for pyrimidine ("TransI tion = I dentical type") Transversion: Substituting purine for pyrimidine or vice versa ("TransV ersion = conV ersion btw types")