Study Notes on Mitosis and the Cell Cycle

Mitosis and the Cell Cycle

Overview of Mitosis and the Cell Cycle

  • Mitosis is a critical process in the cell cycle that ensures division of replicated DNA into daughter cells.

  • The cell cycle consists of phases that prepare the cell for division and execute the division process.

Key Phases of Cell Cycle

  • Interphase: Longest phase of the cell cycle where preparations for division take place.

    • S Phase: DNA replication occurs, doubling the amount of DNA; additional cytosolic components and organelles are synthesized.

    • G1 Phase (Gap 1): Cell grows and prepares for DNA synthesis.

    • G2 Phase (Gap 2): Further growth and final preparations for mitosis.

  • Mitotic Phase: DNA and cytoplasm are divided, resulting in two genetically identical daughter cells.

Interphase Sub-phases

  • G1 Phase (Gap 1):

    • Cell growth occurs upon receiving a division signal.

  • S Phase (Synthesis):

    • DNA replication and centrosome duplication happen, ensuring the cell has two centrosomes.

  • G2 Phase (Gap 2):

    • Cell continues to grow and prepares for mitosis.

Vocabulary

  • Chromatin: Uncondensed form of DNA.

  • Chromosome: Condensed DNA, visible during cell division with sister chromatids joined at the centromere.

  • Sister Chromatids: Two identical copies of a chromosome joined together.

  • Homologous Chromosomes: A pair of chromosomes (one from each parent).

Mitotic Spindle

  • Formed from microtubules and associated proteins.

  • Responsible for moving chromosomes during mitosis.

  • Centrosomes: Organizing centers for microtubules; duplicated during interphase.

  • Kinetochore: Protein structures on chromatids that attach to microtubules during mitosis.

Phases of Mitosis

  1. Prophase:

    • Chromatin condenses into visible chromosomes.

    • Nucleolus disappears and the mitotic spindle forms from centrosomes moving towards opposite poles.

  2. Prometaphase:

    • Nuclear envelope breaks down.

    • Chromosomes condense further, kinetochores form, and microtubules attach to kinetochores.

    • Kinetochore microtubules pull chromatid pairs towards the cell equator.

  3. Metaphase:

    • Centrosomes reach opposite poles; chromosomes align at the metaphase plate (equatorial line).

  4. Anaphase:

    • Cohesin proteins cleave, separating sister chromatids into daughter chromosomes.

    • Microtubules shorten, pulling daughter chromosomes to opposite poles, elongating the cell.

  5. Telophase/Cytokinesis:

    • Nuclear envelope reforms around each set of chromosomes as nucleolus reappears.

    • Chromosomes decondense, and cytokinesis divides the cytoplasm through formation of a cleavage furrow.

Cell Cycle Control

  • Different cells divide at varying rates; skin cells often divide more frequently than nerve cells.

  • Molecules regulate cell division through checkpoints.

  • Internal Checkpoints: G1, G2, and M phase checkpoints control progression through the cell cycle.

    • Example of G1 Checkpoint: Checks for growth factors, nutrients, cell size, and DNA damage. If the signal is not received, the cell enters a non-dividing state (G0 phase).

Regulatory Molecules in Cell Cycle

  • Cyclins: Proteins whose concentration varies during the cell cycle.

  • Cyclin-Dependent Kinases (Cdks): Enzymes that catalyze phosphorylation, activating other proteins when bound to cyclins.

  • Mitosis Promoting Factor (MPF): Active when cyclin levels are high; allows transition from G2 to mitosis.

Mechanical and External Controls

  • Density-Dependent Inhibition: Cells cease division when crowded, ensuring proper tissue structure.

  • Anchorage-Dependence: Cells require attachment to a substrate for division; e.g., red blood cells divide in bone marrow.

Cancer and Cell Cycle Dysfunction

  • Cancer cells bypass key regulatory signals of the cell cycle leading to uncontrolled growth and division.

  • Failure at checkpoints can result in tumor formation and aberrantly dividing cells can spread through metastasis to other tissues.

Key Questions to Know

  • List the four phases of mitosis.

  • Describe the characteristics of each phase of mitosis including Prophase, Metaphase, Anaphase, and Telophase/Cytokinesis.

  • Define the difference between diploid and haploid cells.

  • Given a diploid number of 30 (2n=30), how many chromosomes are in somatic cells, and how many are in gametes?

Mitosis Practice Questions

  1. Identify the phases of mitosis based on provided images.

  2. Count sister chromatids during different phases.

  3. Discuss the number of chromosomes present in cells during different stages of the cell cycle, particularly in G0 and Prophase.