The Cell Cycle

Cytokinesis: Mechanisms in Animal and Plant Cells

Cytokinesis is the process by which the cytoplasm of a parent cell is divided into two daughter cells, following mitosis. The mechanism differs between animal and plant cells.

• Animal Cells: Cytokinesis occurs via cleavage. The first sign is the appearance of a cleavage furrow, a shallow groove in the cell surface near the old metaphase plate. A contractile ring of microfilaments pinches the cell in two.

• Plant Cells: Cytokinesis occurs through the formation of a cell plate. Vesicles derived from the Golgi apparatus move to the center of the cell, coalesce, and form the cell plate, which eventually develops into a new cell wall separating the two daughter cells.

• Key Terms: Cleavage furrow, cell plate, contractile ring, microfilaments

Mitosis in Plant Cells

Mitosis is the process of nuclear division in eukaryotic cells, followed by cytokinesis. In plant cells, the stages of mitosis are similar to those in animal cells, but cytokinesis involves cell plate formation.

• Stages: Prophase, Prometaphase, Metaphase, Anaphase, Telophase

• Cell Plate Formation: Occurs during telophase, leading to the separation of daughter cells

Cell Division in Prokaryotes: Binary Fission

Binary Fission in Bacteria

Prokaryotes reproduce by a process called binary fission, which is simpler than mitosis.

• Process: The bacterial chromosome replicates, beginning at the origin of replication. The two daughter chromosomes move apart as the cell elongates. The plasma membrane pinches inward, dividing the cell into two.

• Key Terms: Origin of replication, plasma membrane, chromosome replication

Evolution of Mitosis

Mitosis likely evolved from binary fission, as prokaryotes predate eukaryotes. Some unicellular eukaryotes exhibit intermediate forms of cell division.

• Comparative Mechanisms: Bacteria (binary fission), dinoflagellates, diatoms, yeasts, and most eukaryotes (mitosis)

Regulation of the Eukaryotic Cell Cycle

Molecular Control System

The frequency of cell division varies with cell type and is regulated at the molecular level. Cancer cells escape these controls.

• Cell Cycle Control System: Driven by signaling molecules in the cytoplasm. Experiments show that cytoplasmic signals can induce cell cycle progression in fused cells.

• Checkpoints: The cell cycle has built-in checkpoints (G1, G2, M) where it stops until a go-ahead signal is received.

Cyclins and Cyclin-Dependent Kinases (Cdks)

Two types of regulatory proteins control the cell cycle: cyclins and cyclin-dependent kinases (Cdks).

• Cyclins: Proteins whose concentrations fluctuate cyclically in the cell.

• Cdk: Enzymes that must be attached to a cyclin to be active. Their activity rises and falls with cyclin concentration.

• MPF (Maturation-Promoting Factor): A cyclin-Cdk complex that triggers passage past the G2 checkpoint into M phase.

Cell Cycle Checkpoints: Internal and External Signals

Internal and External Regulation

Checkpoints are regulated by internal surveillance mechanisms and external signals.

• Internal Signals: Cells will not begin anaphase until all chromosomes are properly attached to the spindle at the metaphase plate, ensuring correct chromosome number in daughter cells.

• External Signals: Growth factors, such as platelet-derived growth factor (PDGF), stimulate cell division. Physical factors like density-dependent inhibition and anchorage dependence also regulate cell division.

Loss of Cell Cycle Controls in Cancer Cells

Cancer Cell Characteristics

Cancer cells do not respond to normal cell cycle controls.

• Growth Factor Independence: Cancer cells may produce their own growth factors, convey signals without growth factors, or have abnormal control systems.

• Transformation: Cells that divide indefinitely have undergone transformation. Tumors form when cancer cells are not eliminated by the immune system.

• Benign vs. Malignant Tumors: Benign tumors remain at the original site; malignant tumors invade tissues and can metastasize.

• Metastasis: The spread of cancer cells to other parts of the body, forming additional tumors.

• Treatment: Localized tumors may be treated with radiation; metastatic tumors are treated with chemotherapeutic drugs targeting the cell cycle.

Summary Table: Cell Cycle Regulation and Cancer

Key Equations and Concepts

• Cell Cycle Phases: G1 → S → G2 → M

• Checkpoint Regulation:

• Cyclin-Cdk Activity:

Additional info:

• Modern cancer treatments are increasingly personalized, targeting specific cell-signaling pathways unique to each patient's tumor.

• Research continues into the mechanisms of chromosome movement during binary fission and mitosis.