The Cell Cycle: Mechanisms and Regulation

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 ensures the continuity of life, allowing organisms to grow, repair tissues, and reproduce.

• Most cell division results in genetically identical daughter cells.

Functions of Cell Division

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

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

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

Molecular Organization of Genetic Material

Chromosomes and Chromatin

Genetic material in cells is organized into structures called chromosomes, which are composed of DNA and proteins (chromatin).

• 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 protein that makes up eukaryotic chromosomes.

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

• Gametes: Reproductive cells (sperm and egg) with one set of chromosomes (haploid).

The Cell Cycle: Phases and Events

Main Phases of the Cell Cycle

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

• Interphase: Period of cell growth and DNA replication, subdivided into three phases:

  • 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.

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

Mitosis: Steps and Key Events

Mitosis is conventionally divided into five stages, each characterized by specific changes in chromosome structure and cell organization.

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

2. Prometaphase: Spindle microtubules attach to kinetochores on chromosomes; nuclear membrane fully disintegrates.

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 envelopes reform; cytokinesis begins or completes.

Cytokinesis is the division of the cytoplasm, resulting in two separate daughter cells. In animal cells, this occurs via cleavage furrow formation; in plant cells, a cell plate forms.

Chromosome Duplication and Distribution

During cell division, chromosomes are duplicated and distributed equally to daughter cells.

• Each duplicated chromosome consists of two sister chromatids joined at the centromere.

• During mitosis, sister chromatids are separated to ensure each daughter cell receives an identical set of chromosomes.

Cell Division in Prokaryotes: Binary Fission

Binary Fission

Prokaryotes (bacteria and archaea) divide by a process called binary fission.

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

• The two chromosomes move apart as the cell elongates.

• The plasma membrane pinches inward, dividing the cell into two genetically identical daughter cells.

Regulation of the Eukaryotic Cell Cycle

Cell Cycle Control System

The eukaryotic cell cycle is regulated by a complex control system involving signaling molecules and checkpoints.

• Checkpoints are control points where the cell cycle can be stopped until certain conditions are met.

• Major checkpoints occur in G1, G2, and M phases.

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

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.

$\text{MPF} = \text{Cyclin} + \text{Cdk}$

External and Internal Signals

• Growth factors: Chemical signals released by certain cells to 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 substrate to divide.

Loss of Cell Cycle Control: Cancer

Cancer Cells and Tumors

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

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

• They may have abnormal cell cycle control systems.

• Cells that divide indefinitely are said to have undergone transformation.

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

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

Summary Table: Key Differences in Cell Division

Feature	Eukaryotic Mitosis	Prokaryotic Binary Fission
Genetic Material	Multiple linear chromosomes	Single circular chromosome
Process	Mitosis (nuclear division) + cytokinesis	Binary fission (no nucleus)
Regulation	Complex control system with checkpoints	Simple, less regulated
Result	Two genetically identical daughter cells	Two genetically identical daughter cells

Key Terms and Definitions

• Cell cycle: Ordered sequence of events in the life of a cell.

• Mitosis: Division of the nucleus.

• Cytokinesis: Division of the cytoplasm.

• Chromosome: DNA molecule with associated proteins.

• Chromatin: DNA-protein complex making up chromosomes.

• Centromere: Region joining sister chromatids.

• Checkpoint: Control point in the cell cycle.

• Cyclin: Regulatory protein controlling cell cycle progression.

• Cdk: Cyclin-dependent kinase, enzyme regulating cell cycle.

• MPF: Cyclin-Cdk complex promoting mitosis.

• Binary fission: Prokaryotic cell division.

• Benign tumor: Non-invasive abnormal cell mass.

• Malignant tumor: Invasive, metastatic abnormal cell mass.