Notes on Cell Structure and Genetic Organization

Symbiosis and Organelle Evolution

  • Paramecium bursaria hosts Zoochlorella photosynthetic algae; conditions favoring genetic integration/dependence would accelerate Zoochlorella becoming permanent organelles.

  • Scientific names use binomial nomenclature (Genus capitalized, species italicized).

Cell Structures: Membranes and Walls

  • Plasma Membrane: Universal; some bacteria have two.

  • Cell Walls: Present in Prokaryotes, Fungi, Plants (e.g., bacterial peptidoglycan, plant cellulose, fungal chitin). Absent in animal cells.

    • Provide definite cell shape and protection against mechanical stress.

DNA Organization in Prokaryotes and Eukaryotes

  • Prokaryotic DNA:

    • Genomic DNA: Usually a circular molecule.

    • Plasmids: Extrachromosomal, independently replicating DNA found in bacteria, archaea, yeast, and plant mitochondria.

  • Eukaryotic DNA:

    • Nucleus: Contains genomic DNA.

    • Nucleoli: Located within the nucleus, sites of rRNA synthesis and ribosome subunit assembly.

    • Chromosomes: Genomic DNA forms linear chromosomes visible during cell division.

    • Nuclear envelope: Double, continuous membrane.

    • Additional DNA: Chloroplast DNA, Mitochondrial DNA (in plant cells).

Genetic Variation

  • Some cells are multinucleated (e.g., certain fungal hyphae).

  • Many cultivated plants are polyploid (more than 22 sets of chromosomes), such as Wheat (Hexaploid 6x6x), Potato (Tetraploid 4x4x), and Strawberry (Octoploid 8x8x).