Study Notes on Cell Division and Genetics

Concept 9.1: Most Cell Division Results in Genetically Identical Daughter Cells

  1. The Fundamentals of Cell Division

    • Life's Continuity: The ability of organisms to produce more of their own kind is a core characteristic of life. This process is driven by cell division, which is part of the integral cell cycle—the life of a cell from its formation from a parent cell until its own division into two daughter cells.

    • Comparison with Nonliving Matter: Unlike nonliving matter, which may break apart or erode, living cells reproduce through a highly regulated and complex mechanism to ensure survival across generations.

  2. Key Roles of Cell Division

    • Asexual Reproduction:

    • In prokaryotes (bacteria and archaea) and unicellular eukaryotes (such as Amoeba), cell division results in the creation of a whole new organism. This is often referred to as binary fission (in prokaryotes).

    • Growth and Development:

    • For multicellular organisms, cell division allows for growth from a single fertilized egg (zygote). For example, a zygote undergoes billions of mitotic divisions to form a complex human body with trillions of specialized cells.

    • Tissue Renewal and Repair:

    • Division functions to replace cells that die from normal wear and tear or accidents. For instance, cells in your bone marrow divide continuously to produce fresh blood cells, replacing those that are aged or damaged.

  3. Cellular Organization of Genetic Material

    • The Genome:

    • A cell’s total endowment of DNA is called its genome.

    • Prokaryotic Genomes: Usually consist of a single, long, circular DNA molecule.

    • Eukaryotic Genomes: Consist of multiple linear DNA molecules. The amount of DNA is immense; a typical human cell contains approximately 2 meters of DNA, which must be packed into a nucleus only a few micrometers in diameter.

    • Chromosomes and Chromatin:

    • To manage the large volume of DNA, it is packaged into chromosomes.

    • Chromatin: This is a complex of DNA and protein molecules (specifically histones). Chromatin exists as thin, loose fibers when the cell is not dividing, but it condenses into thick, visible chromosomes during division.

    • Genes: Each chromosome contains hundreds to thousands of genes, which are the units of information that specify an organism’s inherited traits.

  4. Chromosome Count and Cellular Identity

    • Somatic Cells: These are all body cells except the reproductive cells. In humans, somatic cells contain 46 chromosomes, organized into 23 pairs.

    • Gametes: These are reproductive cells (sperm and eggs). They contain half as many chromosomes as somatic cells. In humans, gametes have a total of 23 chromosomes.

    • Species Variation: The number of chromosomes is characteristic of a species but does not necessarily correlate with complexity. For example:

    • Cabbage: 18 chromosomes.

    • Chimpanzees: 48 chromosomes.

    • Hedgehogs: 90 chromosomes (varies by species).

  5. The Mechanism of Chromosome Distribution

    • DNA Replication and Condensation:

    • Before a cell can divide, it must replicate all its DNA. After replication, the chromatin condenses, making the chromosomes shorter and thicker.

    • Sister Chromatids:

    • Each duplicated chromosome consists of two sister chromatids, which are joined copies of the original chromosome containing identical DNA.

    • They are held together by protein complexes called cohesins (a process known as sister chromatid cohesion).

    • The Centromere:

    • Each sister chromatid has a centromere, a region of repetitive DNA sequences where the chromatid is attached most closely to its sister chromatid.

    • The portions of a chromatid on either side of the centromere are called the arms.

    • Separation:

    • During mitosis, the two sister chromatids of each duplicated chromosome separate and move into two new nuclei. Once separated, they are considered individual chromosomes.

  6. Mitosis vs. Meiosis

    • Mitosis: The division of the genetic material in the nucleus. It is typically followed by cytokinesis, the division of the cytoplasm. This results in two genetically identical somatic cells.

    • Meiosis: A specialized type of cell division occurring in the gonads (testes or ovaries).

    • It reduces the chromosome number from diploid (2n) to haploid (n).

    • In humans, meiosis reduces the count from 46 to 23.

    • Fertilization: When two gametes (sperm and egg) fuse, the diploid number of 46 is restored in the resulting zygote.

  7. Concept Check Analysis

    • Chromosome Logic: If a chicken has 78 chromosomes in its somatic cells (updated estimate), it inherited 39 from its mother and 39 from its father. Following DNA replication but before division, it would have 78 duplicated chromosomes consisting of 156 sister chromatids.