Скачать или смотреть Enzyme Deficiencies & Metabolic Pathways Quiz-1

DNA Structure and Regulation
DNA, the genetic blueprint, condenses into chromatin to fit inside the cell nucleus. It wraps twice around a histone octamer (H2A, H2B, H3, H4) to form nucleosomes like "beads on a string." H1 histone binds nucleosomes and linker DNA, stabilizing the fiber. DNA's negative charge from phosphates pairs with histones' positive charge from lysine and arginine.
During mitosis, DNA condenses into chromosomes. Synthesis of DNA and histones occurs in S phase. Mitochondrial DNA is circular and histone-free.
Chromatin types: Heterochromatin is condensed, dark on electron microscopy, transcriptionally inactive due to high methylation and low acetylation (e.g., Barr bodies in X inactivation). Euchromatin is less condensed, light, active, and accessible.
Epigenetics: DNA methylation (e.g., in CpG islands) silences genes without sequence change, linked to aging, cancer, imprinting. Histone methylation often suppresses transcription; acetylation relaxes coiling for increased expression; deacetylation tightens it, reducing expression (e.g., in Huntington disease).
Nucleotides and Synthesis
Nucleosides: base + sugar. Nucleotides: add phosphate, linked 3'-5'. Purines (A, G: two rings); pyrimidines (C, U, T: one ring). C-G bonds (3 H) are stronger than A-T (2 H), raising melting temperature.
De novo purine synthesis needs glycine, aspartate, glutamine. Pyrimidine: carbamoyl phosphate, aspartate.
Drugs target synthesis: 5-FU inhibits thymidylate synthase; methotrexate blocks dihydrofolate reductase; hydroxyurea stops ribonucleotide reductase.
Purine salvage defects: ADA deficiency causes SCID via dATP buildup; Lesch-Nyhan (HGPRT absent) leads to hyperuricemia, gout, self-mutilation. Treated with allopurinol.
Genetic code: Unambiguous (one codon per amino acid), degenerate (multiple codons per amino acid), commaless, universal (conserved, except mitochondria).