Cell Division

Recitation 4: Cell Division

Eukaryotic Chromosomes

  • Definition and Composition of Chromosomes:

    • Chromosomes are made of chromatin, which consists of roughly equal amounts of DNA and protein molecules.
    • Protein molecules assist in organizing chromatin and controlling gene activity.
    • Chromosomes typically exist as thin fibers longer than the nucleus storing them.

  • Structural Organization:

    • Eukaryotic DNA is packed into a complex, multilevel coiling and folding system.
    • Histones: Proteins used for packaging DNA in eukaryotes.
    • Nucleosomes: Structures formed by DNA wound around several histone molecules.

  • Tight Packing of DNA:

    • Prior to cell division, DNA condenses completely (coils up neatly) after replication.

  • Sister Chromatids:

    • Each chromosome has two identical sister chromatids, representing the replicated DNA copy.

Duplicating Chromosomes

  • DNA Replication Process:

    • Before cell division begins, the DNA in each chromosome is copied through DNA replication.
    • New histone proteins attach as necessary during this process.

  • Characteristics of Sister Chromatids:

    • Each chromatid contains identical genes.
    • Sister chromatids are tightly joined at a narrow region known as the centromere.

  • Separation of Sister Chromatids:

    • During cell division, sister chromatids separate.
    • After separation, each chromatid is considered a full-fledged chromosome and remains identical to the original chromosome.

Homologous Chromosomes

  • Identity Across Individuals:

    • Different individuals of a single species possess the same number and types of chromosomes.
    • Example: A human somatic cell typically contains 46 chromosomes.

  • Karyotype Production:

    • A technician produces a karyotype by:
    • Breaking open a human cell at its metaphase of mitosis.
    • Staining the chromosomes with dyes.
    • Capturing a microscopic image.
    • Arranging chromosomes into matching pairs by size.

The Cell Cycle

  • Definition:

    • The cell cycle is a series of repeated events from the beginning of one cell division to the next. It includes the time cells are dividing (mitosis) and not dividing (interphase).

  • Cell Cycle Structure:

    • Represents the "lifetime" of the cell from formation to division with most time spent in interphase (at least 90% of the cycle).

  • Interphase Activities:

    • During interphase, the cell performs normal functions, doubles cytoplasmic contents, and grows.

  • Phases of Interphase:

    • Interphase comprises three subphases:
    • G1 Phase: (First gap)
      • Cell grows, performs normal functions, and synthesizes proteins.
      • If preparing for division, it transitions to the S phase; otherwise, it stays in G1.
    • S Phase: (Synthesis)
      • Entire genome is replicated, including all chromosomes.
      • Once this phase starts, the cell is committed to division.
    • G2 Phase: (Second gap)
      • Specialized proteins necessary for mitosis are produced.

Mitosis

  • Overview:

    • Eukaryotic cells undergo division via mitosis, generating daughter cells with identical DNA to the parent cell.

  • Mitotic Phase:

    • The mitotic (M) phase consists of two overlapping processes:
    • Mitosis: Nucleus and contents divide into two daughter nuclei.
    • Cytokinesis: Cytoplasm and organelles divided into two cells.
    • Combination results in two genetically identical daughter cells.

Phases of Mitosis

  • Prophase:
    • Chromosomes condense for organization and replication.
    • Formation of the spindle (composed of microtubules) to aid in chromosome separation.
  • Metaphase:
    • Chromosomes line up in the equatorial plane, ensuring equal distribution of chromatids between cells.
  • Anaphase:
    • Sister chromatids separate and move to opposite poles, initiated by spindle action.
  • Telophase:
    • Two sets of DNA reach opposite ends; new nuclei develop around them, preparing for cytokinesis.
  • Cytokinesis:
    • Physical separation of cytoplasm and organelles into daughter cells, concluding the cell division.

Cytokinesis Variations

  • Animal Cells:
    • Cleavage furrow formation leads to cellular division. The indentation appears at the midpoint as proteins contract under the membrane, separating daughter cells.
  • Plant Cells:
    • Formation of a cell plate from vesicles that fuse at the center, eventually becoming part of the new cell walls.

Meiosis

  • Definition and Purpose:

    • Meiosis is a type of cell division necessary for generating haploid gametes (sperm and egg), promoting genetic diversity in sexual reproduction.

  • Comparison with Mitosis:

    • Each process involves similar phases, but meiosis consists of two cell divisions, leading to four genetically unique cells.

Meiosis Stages

  • Interphase:

    • Cell prepares for division, similar to mitosis.

  • Meiosis I:

    • Homologous chromosomes separate to halve chromosome number.

  • Meiosis II:

    • Similar to mitosis, but does not involve DNA replication. Chromatids separate.

  • Crossing Over:

    • Occurs during prophase I, exchanging genetic material between homologous chromosomes.

Summary

  • Meiosis vs Mitosis:
    • Mitosis produces genetically identical cells; Meiosis produces haploid cells with unique genetic combinations.
  • Key difference: Meiosis includes a reduction in chromosome number and the shuffling of genetic information, critical for sexual reproduction. All events unique to meiosis occur during meiosis I, with meiosis II resembling mitosis but yielding haploid daughter cells.