Cell Division and Mitosis Review

Student Learning Objectives

  • List reasons cells divide.

  • Describe the events during phases of the cell cycle: G1, S, G2 (and G0) during interphase, and during mitosis and cytokinesis in M phase.

  • Explain the structural differences between a chromosome in G1 and the same chromosome in G2.

  • Explain the relationships among chromosomes, replicated chromosomes, and sister chromatids.

  • Identify the mitosis stage based on chromosome visibility and arrangement.

  • Explain how mitotic cell division produces two genetically identical cells.

  • Explain the structural relationships among double-stranded DNA molecules, chromosomes, replicated chromosomes, and sister chromatids.

  • Predict effects on M phase if microtubules function is disrupted.

  • Predict effects on M phase if actin microfilaments function is disrupted.

Cell Division - Mitosis

Reasons for Cell Division

  • All cells divide for several key reasons:

    • Reproduction: A method of asexual reproduction where organism duplicates itself.

    • Growth: Cells multiply to help organisms grow larger.

    • Regeneration: The process of replacing worn out or damaged cells, aiding healing and repair.

Cell Division Mechanisms

Binary Fission

  • Definition: A method of cell division used by prokaryotic cells, mitochondria, and chloroplasts.

  • Steps in Binary Fission:

    1. Circular bacterial DNA molecule attaches to the inner membrane.

    2. DNA replication begins at a specific location and proceeds bidirectionally.

    3. Newly synthesized DNA molecule attaches near the original attachment site.

    4. As the cell elongates, DNA attachment sites separate.

    5. A constriction forms at the cell's midpoint.

    6. The formation of a new membrane and cell wall separates the two daughter cells.

Eukaryotic vs Prokaryotic Cells

  • Genome Characteristics:

    • Eukaryotes: Large and linear genome, multiple separate chromosomes, DNA found in the nucleus.

    • Prokaryotes: Smaller, circular genome, DNA found in the cytoplasm.

  • Division: Eukaryotic cells divide via mitosis or meiosis, unlike prokaryotic cells.

Mitotic vs Meiotic Cell Division

  • Mitotic Cell Division:

    • Results in two identical daughter cells (diploid).

  • Meiotic Cell Division:

    • Results in four genetically different daughter cells (haploid).

Chromosomal Anatomy

Key Definitions

  • Haploid (n): Cells with one complete set of chromosomes, e.g., gametes (sperm, eggs).

  • Diploid (2n): Cells with two complete sets of chromosomes, e.g., somatic (body) cells.

Chromosome Structure

  • A chromosome is a DNA molecule complexed with histone proteins.

  • A gene is a specific region of a chromosome containing genetic information encoded in nucleotide sequences.

  • A replicated chromosome comprises two DNA molecules joined at the centromere.

  • Sister chromatids are the individual DNA molecules in a replicated chromosome.

    • Example: In G1, a chromosome is a single DNA molecule; in G2, it is two DNA molecules (sister chromatids).

The Cell Cycle

  • Comprises two main phases:

    1. Interphase: Consists of G1, S, G2 phases.

    2. M Phase: Includes mitosis and cytokinesis.

Interphase Phases

  1. G1 Phase:

    • Cell growth, organelle duplication, regulatory protein expression.

    • Chromosome number: 4 (single chromatids).

  2. S Phase:

    • DNA synthesis occurs: each chromosome duplicates.

    • Chromosome number remains 4, but DNA molecules double to 8 (2 chromatids per chromosome).

  3. G2 Phase:

    • Further cell growth and preparation for division.

    • Chromosome structure: identical sister chromatids, essentially 2 chromosome structures present per original chromosome.

M Phase: Mitosis

  • Occurs after interphase, divided into five stages:

    1. Prophase:

    • Nuclear membrane disappears, chromosomes condense and become visible, spindle fibers form.

    • Centrioles duplicate and migrate to opposite poles, forming the mitotic spindle.

    1. Prometaphase:

    • Nuclear envelope breaks down, spindle microtubules attach to centromeres of chromosomes.

    1. Metaphase:

    • Chromosomes align along the equatorial plane of the cell.

    1. Anaphase:

    • Sister chromatids separate and move towards opposite poles, centromeres divide.

    1. Telophase:

    • Nuclear envelope reforms around two sets of chromosomes, which decondense, resulting in two nuclei in one cell.

  • Cytokinesis:

    • Division of the cytoplasm resulting in two distinct daughter cells. In animal cells, this involves the formation of a contractile ring made of actin filaments; in plant cells, a cell plate forms.

Impact of Cytoskeleton on Cell Division

  • Microtubules: Essential for chromosome movement during mitosis, forming the spindle apparatus.

  • Actin Microfilaments: Necessary for cytokinesis in animal cells, forming the contractile ring.

Disruption Effects

  • Microtubule Disruption:

    • Prevents chromosome movement from metaphase to anaphase, leaving chromosomes aligned in the metaphase plate.

    • Leads to problems such as failure to divide correctly resulting in aneuploidy or cell cycle stalling.

  • Actin Disruption:

    • Prevents proper cytokinesis, resulting in multinucleate cells if cytokinesis fails.

Conclusion

  • Cell division is a fundamental process necessary for reproduction, growth, and tissue repair. Understanding the cell cycle, the mechanisms of division, and the consequences of disruptions to these processes is essential in cell biology and has applications in medical science.