Cell Cycle and Cell Division Notes

Cell Division Overview

  • Cell division enables unicellular organisms to reproduce.

  • In multicellular eukaryotes, it allows for development, renewal, repair, and replacement of cells.

  • Cell division is integral to the cell cycle: the life of a cell from formation to division.

Genetic Material Organization

  • Genome: the total DNA in a cell; can be a single DNA molecule (prokaryotes) or multiple (eukaryotes).

  • DNA is packaged into chromosomes.

  • Eukaryotic chromosomes are made of chromatin (DNA and protein).

  • Somatic cells have two sets of chromosomes; gametes have one set.

Chromosome Distribution During Cell Division

  • DNA replicates before cell division, condensing chromosomes.

  • Duplicated chromosomes have two sister chromatids, joined at the centromere.

  • During cell division, sister chromatids separate into two nuclei, becoming chromosomes.

Eukaryotic Cell Division

  • Mitosis: division of the genetic material in the nucleus.

  • Cytokinesis: division of the cytoplasm.

  • Meiosis: produces gametes with half the number of chromosomes as the parent cell.

Cell Cycle Phases

  • Mitotic (M) phase: mitosis and cytokinesis.

  • Interphase: cell growth and chromosome copying (90% of the cycle).

    • Subphases: G1 phase ("first gap"), S phase ("synthesis"), G2 phase ("second gap").

    • Chromosomes are only duplicated during the S phase.

  • Mitosis stages: prophase, prometaphase, metaphase, anaphase, telophase; cytokinesis overlaps.

Mitotic Spindle

  • Made of microtubules and associated proteins, controlling chromosome movement.

  • In animal cells, spindle assembly starts at the centrosome.

  • Centrosomes duplicate during interphase, moving to opposite cell ends during prophase and prometaphase.

  • Aster: radial array of short microtubules extending from each centrosome.

  • Spindle includes centrosomes, spindle microtubules, and asters.

  • During prometaphase, spindle microtubules attach to kinetochores on chromosomes.

  • Metaphase: chromosomes align at the metaphase plate.

Anaphase

  • Sister chromatids separate and move along kinetochore microtubules to opposite cell ends.

  • Microtubules shorten by depolymerizing at kinetochore ends.

  • Nonkinetochore microtubules overlap and push against each other, elongating the cell.

Cytokinesis

  • Animal cells: cleavage furrow forms.

  • Plant cells: cell plate forms.

Binary Fission

  • Prokaryotes reproduce through binary fission.

  • The single chromosome replicates at the origin of replication.

  • Daughter chromosomes move apart as the cell elongates and the plasma membrane pinches inward to divide the cell.

Evolution of Mitosis

  • Mitosis likely evolved from binary fission.

  • Some protists show intermediate cell division types.

Cell Cycle Regulation

  • Cell division frequency varies by cell type due to molecular-level regulation.

  • Cancer cells bypass normal cell cycle controls.

  • Cytoplasmic signals drive the cell cycle; checkpoints regulate its progression.

Cell Cycle Checkpoints

  • G1 checkpoint: determines if the cell will divide, delay division, or enter G0 phase (nondividing state).

  • M phase checkpoint: anaphase is delayed until all kinetochores are attached to spindle microtubules.

  • Regulatory proteins like kinases and cyclins control the cell cycle.

  • Growth factors stimulate cell division; density-dependent inhibition and anchorage dependence also regulate cell division.

Cancer Cells

  • Do not respond to normal cell cycle signals.

  • May produce their own growth factors or have abnormal cell cycle control systems.

  • Transformation converts a normal cell to cancerous.

  • Tumors form from cancer cells; benign tumors remain at the original site, while malignant tumors invade other tissues (metastasis).

  • Cancer treatments are becoming more personalized based on individual patient tumors.