Cell Cycle and Division Study Notes

The Cell Cycle and Cell Division Notes

Concept 7.1 Reproduction May Be Asexual or Sexual

  • Cell Division Types:

  • Cell Division: One cell gives rise to two cells.

  • Asymmetrical Division: Mother cell produces a daughter cell; e.g., yeasts.

  • Symmetrical Division: Mother cell divides to produce two identical daughter cells; typical in bacteria and protists.

  • Multicellular Organisms: Usually exhibit both division types.

  • Asexual Reproduction:

  • Offspring are clones (genetically identical to the parent).

  • Genetic variations arise solely from mutations in DNA sequences.

  • Prokaryotes: Typically reproduce via binary fission.

  • Eukaryotes: Single-celled variants reproduce through mitosis. Many multicellular eukaryotes also reproduce asexually.

  • Sexual Reproduction:

  • Occurs exclusively in eukaryotes.

  • Involves the formation of gametes (reproductive cells) that fuse to create genetically diverse offspring.

  • Gametes arise from meiosis (halves the chromosome number).

  • Meiosis enhances genetic diversity, a foundation for natural selection and evolution.

  • Chromosomes in Eukaryotes: Organized into pairs (homologous chromosomes).

  • Somatic Cells: Diploid (two sets of chromosomes)

  • Origin: One homolog from each biological parent.

  • Genetic consistency ensured during division.

Concept 7.2 Asexual Reproduction Results in Genetically Identical Daughter Cells

  • Steps in Cell Division:
  1. Initiation: Signaling triggers the start of cell division.
  2. DNA Replication: DNA is duplicated to provide complete genetic information to two new cells.
  3. Segregation: Distribution of replicated DNA into new cells.
  4. Cytokinesis: Cytoplasm is divided, leading to two separate cells.

  • Binary Fission (Prokaryotes):

  • Divides the entire organism, influenced by environmental factors (e.g., nutrient availability).

  • Starts replication at the origin of the circular chromosome, moving toward the terminus.

  • As DNA replication progresses, origins migrate to opposite sides of the cell.

  • Mitosis (Eukaryotes):

  • More complex than binary fission; must follow specific stages of the cell cycle:

    • G1 Phase: Normal cellular functions; enter G0 if not dividing.
    • S Phase: DNA replication.
    • G2 Phase: Preparation for mitosis.
    • M Phase: Mitosis and cytokinesis.

  • Cytokinesis in Animals vs. Plants:

  • In animals: Contractile ring pinches the cell membrane.

  • In plants: Vesicles from the Golgi fuse to create a new cell wall (cell plate).

Concept 7.3 Sexual Reproduction by Meiosis

  • Meiosis Overview:

  • Comprises two nuclear divisions but involves only one round of DNA replication.

  • Outcome: Four genetically different haploid cells.

  • Meiosis I:

  • Homologous chromosomes align during prophase I.

  • Anaphase I: Homologous pairs separate without centromere division.

  • Results in two nuclei with half the original chromosomes.

  • Meiosis II:

  • Similar to mitosis (no DNA replication precedes).

  • Sister chromatids separate, resulting in four haploid cells.

  • Crossing over during prophase I increases genetic variability.

Concept 7.4 Errors During Cell Division Can Result in Changes in Chromosome Number

  • Nondisjunction: Failure of homologous chromosomes to separate during anaphase, leading to extra or missing chromosomes.
  • Aneuploidy: An abnormal number of chromosomes; significant impact on viability (e.g., Down syndrome).
  • Polyploidy: Organisms with multiple chromosome sets due to errors in cell division, common in plants.

Concept 7.5 The Cell Cycle and Cell Death

  • Regulation of Cell Division:

  • In prokaryotes, the cell cycle is linked to nutrient availability; a DNA-binding protein initiates replication.

  • In eukaryotes, signals for cell division are internal and require sufficient resources, regulated via cyclin-dependent kinases (CDKs).

  • Checkpoints: G1, S, G2, and M checkpoints monitor DNA damage and replication completion.

  • Apoptosis vs. Necrosis:

  • Apoptosis: Programmed cell death that maintains homeostasis; essential for removing unwanted cells.

  • Necrosis: Results from damage or lack of nutrients; can lead to inflammation.

  • Cancer Cells:

  • Divide uncontrollably and ignore regulatory signals; often result from mutations affecting cell cycle regulation.