Step 1 First Aid - Biochemistry 2023 LATEST EDITION

Chromatin structure Answer- 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 Answer- Condensed, transcriptionally inactive ("H eteroC hromatin = H ighly C ondensed.") Euchromatin Answer- Less condensed, transcriptionally active (Eu = true, "truly transcribed") Purines Answer- A, G 2 Rings ("PUR e A s G old = PUR ines") <img src="67b - Purine strxr.JPG" /> Pyrimidines Answer- C, T, U 1 ring ("CUT the PY (pie): PY rimidines") <img src="67c - Pyrimidine strxr.JPG" /> Functional groups of the nucleosides Answer- Guanine has a ketone. Thy mine has a methy l. Deamination of cytosine makes uracil.