Cell Cycle and Mitosis Overview

The Key Roles of Cell Division

• Cell division is crucial in distinguishing living organisms from nonliving matter.

  • Unicellular organisms: division of one cell leads to the reproduction of the entire organism.

  • Multicellular eukaryotes: rely on cell division for:

    • Development from a fertilized cell

    • Growth

    • Repair

• Cell division is a central component of the cell cycle, which represents the life cycle of a cell from its formation through to its division.

Chromosomal Structure and Genetic Material

• The genome of a cell comprises all its DNA.

  • Can be a single DNA molecule (prokaryotic cells) or multiple DNA molecules (eukaryotic cells).

• DNA in eukaryotic cells is organized into chromosomes.

  • Chromosomes in eukaryotic cells are composed of chromatin, a combination of DNA and protein that condenses during division.

• Each eukaryotic species has a definitive number of chromosomes:

  • Humans: 46 chromosomes

  • Cabbage: 18 chromosomes

  • Elephants: 56 chromosomes

  • Hedgehogs: 90 chromosomes

  • Carp: 104 chromosomes

Types of Cells

• Somatic cells (non-reproductive) have two sets of chromosomes.

• Gametes (reproductive cells: sperm and eggs) carry half the number of chromosomes compared to somatic cells.

Overview of Eukaryotic Cell Division

• In preparation for cell division:

  • DNA is replicated, leading to visible chromatin packaged as chromosomes.

• Each duplicated chromosome consists of two sister chromatids connected by cohesins.

  • The centromere is the region where the chromatids are most tightly bound.

Stages of the Cell Cycle

Cell Cycle Phases

• The cell cycle comprises:

  • Mitotic (M) phase (includes mitosis and cytokinesis)

  • Interphase:

    • G1 phase: First gap

    • S phase: Synthesis (chromosome duplication)

    • G2 phase: Second gap

• Interphase lasts approximately 90% of the cycle, with chromosome duplication only occurring during the S phase.

Mitosis Phases

• Interphase is prior to mitosis

• Mitosis occurs in 5 stages:

  • Prophase - chromosomes start condensing , the mitotic spindle begins to form, and the nuclear envelope starts to break down.

  • Prometaphase - the chromosomes become even more condensed, and the microtubules attach to the kinetochores on the chromosomes.

  • Metaphase - chromosomes line up across the center of the cell so each sster chromatid is alligned

  • Anaphase - sister chromatids sperate and mov towards the centromere

  • Telophase - chromosomes art at each end of the cell, nuclues sorrounds them, cytoplasm starts to divide

The Mitotic Spindle

• The mitotic spindle is crucial for chromosome movement during mitosis and is formed from microtubules.

• hollow tubes made from tubilin

• aster - radial array of short microtubulues that extends from each centrosome

• kinetochores - protein structures on the chromosome that attach to the mitotic spindle,

• Centrosomes: Organizing centers of microtubules that form the spindle.

Key Points of Mitosis

• During prometaphase, spindle microtubules attach to the kinetochores of chromosomes.

• In anaphase, sister chromatids are pulled apart toward opposite cell poles due to action of microtubules and separation of cohesins.

  • enzyme called separase makes them move to opposite ends of the cell

• Telophase sees the formation of daughter nuclei.

Cytokinesis in Animal and Plant Cells

• Animal cells: Cytokinesis occurs via cleavage furrow formed by a contractile ring of actin and myosin.

• Plant cells: Vesicles form a cell plate, which develops into a new cell wall dividing the daughter cells.

Binary Fission in Prokaryotes

• Prokaryotes divide through binary fission:

  • Chromosome replicates and origins move to opposite sides; then, the cell membrane pinches inward.

Cell Cycle Regulation

• Cell division frequency varies among cell types (e.g., skin cells divide more frequently than nerve cells).

• The cell cycle control system regulates the process via chemical signals and checkpoints:

  • Major checkpoints: G1, G2, and M phases.

Cyclins and Cyclin-Dependent Kinases

• Cyclins and cyclin-dependent kinases (Cdks) are regulatory proteins that control the cell cycle progression.

  • The activity of Cdks fluctuates with cyclin concentrations.

  • MPF (maturation-promoting factor) mediates the G2 to M phase transition.

Cancer and Cell Cycle Regulation

• Cancer cells evade normal regulatory mechanisms and divide uncontrollably.

• Characteristics of cancer cells:

  • May not require growth factors.

  • Can lead to tumor formation (benign or malignant).

• Metastasis refers to the spread of cancer cells to distant parts of the body.

Treatment of Cancer

• Localized tumors can be treated with radiation, while chemotherapy targets cell cycle processes.

  • Example: Taxol stabilizes microtubules, halting mitosis, affecting both cancerous and normal rapidly dividing cells.

• Advances in molecular understanding are aiding in the development of personalized cancer treatments.