Chapter 16: DNA, Chromosomes & the Cell Nucleus

While early scientists like Gregor Mendel identified the patterns of inheritance, it was not until the mid-20th century that experiments by Avery, MacLeod, and McCarty, as well as Hershey and Chase, confirmed that DNA—and not protein—is the primary carrier of genetic instructions. The structural architecture of DNA is characterized by the Watson-Crick double helix, which features two antiparallel strands held together by hydrogen bonds between complementary base pairs—adenine with thymine and guanine with cytosine. This molecular design is subject to topological changes such as supercoiling, which is managed by topoisomerase enzymes to facilitate processes like replication and transcription. The stability of the double helix is influenced by GC content and base stacking, which can be measured through thermal denaturation (melting) and the subsequent ability of DNA to renature or hybridize. The challenge of DNA packaging is solved differently across life forms; bacteria utilize supercoiled loops within a nucleoid, while eukaryotes employ a sophisticated hierarchy of organization involving histones. The fundamental unit of eukaryotic chromatin is the nucleosome, consisting of DNA wrapped around an eight-protein histone octamer. These units further condense into 30-nm fibers and large loops anchored to a chromosomal scaffold. Within the nucleus, DNA exists as either loosely packed euchromatin, which is transcriptionally active, or highly condensed heterochromatin, which is often inactive or performs structural roles at centromeres and telomeres. Eukaryotic genomes are notably complex, containing significant amounts of repeated DNA, including tandem repeats used for identification and interspersed transposable elements like LINEs and SINEs (Alu sequences). Furthermore, semi-autonomous organelles—mitochondria and chloroplasts—contain their own circular DNA, reflecting their endosymbiotic origins. The nucleus serves as the command center for these processes, bounded by a double-membrane nuclear envelope that is continuous with the endoplasmic reticulum. Movement between the nucleus and cytosol is strictly regulated by nuclear pore complexes (NPCs). Small molecules pass via simple diffusion, but larger proteins and RNA require active transport mediated by importins, exportins, and the Ran-GTPase cycle. Structural integrity is provided by the nuclear lamina, a meshwork of intermediate filaments; mutations here can lead to severe laminopathies like Hutchinson–Gilford progeria syndrome. Finally, the nucleolus acts as a specialized "ribosome factory" within the nucleus, where ribosomal RNA is synthesized and subunits are assembled for export to the cytoplasm.