The Cell Cycle: Structure, Function, 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 gives rise to two genetically identical daughter cells.

• It is a defining characteristic that distinguishes living things from nonliving matter.

Functions of Cell Division

Cell division serves several critical functions in 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 old cells in multicellular organisms.

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

Molecular 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 in its nucleus.

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

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

Overview of the Cell Cycle

Main Phases of the Cell Cycle

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

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

• 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 in Mitosis

Mitosis is conventionally divided into five stages, each with distinct events:

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

• Prometaphase: Nuclear membrane dissolves; 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 envelope reforms; cytokinesis begins or completes.

Cytokinesis

Cytokinesis is the division of the cytoplasm, resulting in two separate daughter cells.

• In animal cells, cytokinesis occurs via cleavage, forming a cleavage furrow.

• In plant cells, a cell plate forms, eventually developing into a new cell wall.

Chromosome Duplication and Distribution

Chromosome Structure and Replication

Before cell division, chromosomes are duplicated, resulting in two sister chromatids joined at the centromere.

• Sister chromatids: Identical copies of a chromosome, attached by cohesins at the centromere.

• During mitosis, sister chromatids are separated and distributed to daughter cells.

Cell Division in Prokaryotes: Binary Fission

Binary Fission

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

• Chromosome replication begins at the origin of replication.

• 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

Cell Cycle Control System

The eukaryotic cell cycle is regulated by a molecular control system that ensures proper timing and fidelity of cell division.

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

• Checkpoints are controlled by internal and external signals.

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

Cyclins and Cyclin-Dependent Kinases (Cdks)

Two key 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.

External Signals Influencing Cell Division

• Growth factors: Chemical signals released by certain cells to stimulate division in others (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 and Cell Cycle Regulation

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 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 mitosis)
Regulation	Complex control system (checkpoints, cyclins, Cdks)	Less complex, regulated by cell size and DNA replication
Result	Two genetically identical daughter cells	Two genetically identical daughter cells

Key Equations and Terms

• Chromosome number in somatic cells: $2n$

• Chromosome number in gametes: $n$

• MPF activity: $\text{MPF} = \text{Cyclin} \times \text{Cdk}$

Example: Cell Cycle Checkpoint

If a cell receives a go-ahead signal at the G1 checkpoint, it will proceed through S, G2, and M phases and divide. If not, it enters G0 phase and does not divide.

Additional info: Some details, such as the molecular mechanisms of checkpoint regulation and the role of specific growth factors, were expanded for academic completeness.