Chapter 12: The Cell Cycle

Introduction to the Cell Cycle

The cell cycle is a fundamental process by which cells grow, duplicate their genetic material, and divide to produce new cells. This process is essential for growth, development, and maintenance in all living organisms.

• Cell division is the process by which a parent cell divides into two or more daughter cells.

• It is crucial for reproduction, growth, repair, and renewal in organisms.

• Most cell division results in genetically identical daughter cells.

Functions of Cell Division

Cell division serves several key functions in living organisms:

• Reproduction: Single-celled organisms reproduce by cell division.

• Growth and Development: Multicellular eukaryotes undergo embryonic development through repeated cell divisions.

• Renewal and Repair: Cell division replaces damaged or dead cells in multicellular organisms.

• Genetic Consistency: Ensures distribution of identical genetic material to daughter cells.

Cellular Organization of Genetic Material

Genome and Chromosomes

The genetic material of a cell is organized into a genome, which is packaged into chromosomes.

• Genome: The complete set of DNA in a cell.

• Prokaryotic genomes typically consist of a single DNA molecule.

• Eukaryotic genomes consist of multiple DNA molecules.

• Chromatin: The complex of DNA and proteins that makes up eukaryotic chromosomes.

• Each eukaryotic species has a characteristic number of chromosomes.

• Somatic cells: Body cells with two sets of chromosomes (diploid).

• Gametes: Reproductive cells with half as many chromosomes as somatic cells (haploid).

Distribution of Chromosomes During Eukaryotic Cell Division

Before a cell divides, its DNA is replicated and chromosomes condense. Each duplicated chromosome consists of two sister chromatids joined at the centromere.

• Mitosis: Division of the nucleus, ensuring each daughter cell receives an identical set of chromosomes.

• Cytokinesis: Division of the cytoplasm, resulting in two separate cells.

The Cell Cycle Phases

Overview of the Cell Cycle

The cell cycle consists of two major phases: the mitotic (M) phase and interphase.

• Mitotic (M) phase: Includes mitosis and cytokinesis.

• Interphase: Period of cell growth and DNA replication, divided into three subphases:

  • G1 phase (first gap): Cell grows and carries out normal functions.

  • S phase (synthesis): DNA is replicated; chromosomes are duplicated.

  • G2 phase (second gap): Cell prepares for division.

Steps of Mitosis

Mitosis is conventionally divided into five stages:

1. Prophase: Chromatin condenses into visible chromosomes; mitotic spindle forms; nuclear envelope breaks down.

2. Prometaphase: Spindle microtubules attach to kinetochores on chromosomes.

3. Metaphase: Chromosomes align at the metaphase plate (center of the cell).

4. Anaphase: Sister chromatids are separated and pulled to opposite poles of the cell.

5. Telophase: Chromosomes decondense; nuclear envelope reforms; cytokinesis begins.

Cytokinesis completes cell division, resulting in two genetically identical daughter cells.

Example: Mitosis in Animal Cells

During animal cell cytokinesis, a cleavage furrow forms, pinching the cell into two. In plant cells, a cell plate forms to separate the daughter cells.

Binary Fission in Bacteria

Prokaryotic Cell Division

Prokaryotes (bacteria and archaea) divide by binary fission, a simpler process than mitosis.

• The chromosome replicates, starting at the origin of replication.

• Daughter chromosomes move apart as the cell elongates.

• The plasma membrane pinches inward, dividing the cell into two.

Regulation of the Eukaryotic Cell Cycle

Molecular Control System

The frequency of cell division varies with cell type and is regulated by molecular mechanisms.

• Cell cycle is controlled by signaling molecules in the cytoplasm.

• Checkpoints exist at key stages (G1, G2, M) to ensure proper progression.

• If a cell does not receive a go-ahead signal at the G1 checkpoint, it enters a nondividing state called G0.

Cyclins and Cyclin-Dependent Kinases (Cdks)

Two types of regulatory proteins control the cell cycle:

• Cyclins: Proteins whose concentrations fluctuate cyclically during the cell cycle.

• Cyclin-dependent kinases (Cdks): Enzymes that must bind to cyclins to be active.

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

Internal and External Signals

Cell division is influenced by both internal and external factors:

• Growth factors: Chemical signals that stimulate cell division (e.g., PDGF for fibroblasts).

• Density-dependent inhibition: Cells stop dividing when crowded.

• Anchorage dependence: Most animal cells must be attached to a surface to divide.

Loss of Cell Cycle Controls in Cancer Cells

Cancer and the Cell Cycle

Cancer cells evade normal cell cycle controls, leading to uncontrolled division.

• Cancer cells may produce their own growth factors or signal without them.

• They may have abnormal cell cycle control systems.

• Cells that divide indefinitely have undergone transformation.

• Benign tumors: Abnormal cells remain at the original site.

• Malignant tumors: Invade surrounding tissues and can metastasize to other parts of the body.

Summary Table: Comparison of Cell Division Types

Key Equations

• DNA replication during S phase:

• MPF activity:

Example Application

Understanding the cell cycle is essential for research in cancer biology, regenerative medicine, and developmental biology.

Additional info: Some explanations and definitions have been expanded for clarity and completeness.