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.

• Most cell division results in genetically identical daughter cells.

• Functions of cell division include reproduction (in single-celled organisms), growth and development (in multicellular organisms), and tissue renewal and repair.

Cellular Organization of Genetic Material

Genetic information in cells is organized into structures called chromosomes, which are composed of DNA and associated proteins.

• The genome is the complete set of genetic material in a cell.

• In prokaryotes, the genome is typically a single DNA molecule; in eukaryotes, it consists of multiple DNA molecules.

• Chromatin is the complex of DNA and proteins that makes up eukaryotic chromosomes.

• Somatic cells have two sets of chromosomes (diploid), while gametes have one set (haploid).

The Cell Cycle Phases

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

• Interphase is 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.

• M phase includes mitosis (nuclear division) and cytokinesis (cytoplasmic division).

Mitosis: Steps and Mechanisms

Mitosis is conventionally divided into five stages, each with distinct events that ensure accurate chromosome segregation.

• Prophase: Chromatin condenses into visible chromosomes; mitotic spindle begins to form; nuclear envelope breaks down.

• Prometaphase: Spindle microtubules attach to kinetochores on chromosomes.

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

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

• 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 a cleavage furrow; 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.

Binary Fission in Bacteria

Prokaryotic cells, such as bacteria, divide by a process called binary fission.

• Chromosome replication begins 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 genetically identical cells.

Regulation of the Eukaryotic Cell Cycle

The eukaryotic cell cycle is tightly regulated by molecular control systems to ensure proper division and prevent errors.

• Regulation occurs at specific checkpoints (G1, G2, and M phases).

• Cells may enter a non-dividing state called G0 phase if they do not receive the proper signals.

• Regulatory proteins include cyclins and cyclin-dependent kinases (Cdks).

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

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

External and Internal Signals Influencing Cell Division

Cell division is influenced by both internal surveillance mechanisms and external signals.

• Growth factors are external 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 substrate to divide.

Loss of Cell Cycle Controls in Cancer Cells

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 remain at the original site; malignant tumors invade surrounding tissues and can metastasize to other parts of the body.

Summary Table: Key Differences in Cell Division

Feature	Eukaryotic Mitosis	Prokaryotic Binary Fission
Type of Cell	Somatic (body) cells	Bacterial cells
Genetic Material	Multiple linear chromosomes	Single circular chromosome
Process	Mitosis (prophase, metaphase, anaphase, telophase) + cytokinesis	Binary fission (chromosome replication, segregation, division)
Regulation	Complex checkpoints, cyclins, Cdks	Less complex, regulated by cell size and DNA replication
Result	Two genetically identical daughter cells	Two genetically identical daughter cells