The Cell Cycle: Structure, Function, and Regulation

The Cell Cycle

Overview and Importance

The cell cycle is the series of events that cells undergo as they grow and divide. It is fundamental for the reproduction of unicellular organisms and for growth, development, and tissue repair in multicellular organisms.

• Asexual reproduction: In unicellular organisms, cell division produces genetically identical offspring.

• Growth and development: Multicellular organisms rely on cell division for development from a fertilized egg and subsequent growth.

• Tissue renewal: Cell division replaces damaged or dead cells.

Key Functions of Cell Division

• Reproduction: Division of one cell reproduces the entire organism in unicellular species.

• Growth: Enables multicellular organisms to increase in size.

• Repair: Essential for healing and replacing cells.

Genetic Material and Chromosome Structure

Organization of Genetic Material

All the DNA in a cell constitutes its genome. DNA molecules are packaged into chromosomes, which are visible during cell division.

• Chromatin: The complex of DNA and protein that condenses to form chromosomes.

• Somatic cells: Nonreproductive cells with two sets of chromosomes.

• Gametes: Reproductive cells (sperm and eggs) with half the chromosome number of somatic cells.

Chromosome Structure

• Sister chromatids: Duplicated chromosomes consist of two sister chromatids joined at the centromere.

• Centromere: The region where chromatids are most closely attached.

Chromosome Replication and Distribution

Chromosome Replication During the Cell Cycle

Chromosomes are replicated during the S phase of interphase, resulting in two identical sister chromatids.

• G1 phase: Chromosomes are unreplicated.

• S phase: DNA replication creates replicated chromosomes.

• G2 phase: Chromosomes are replicated, each containing two exact copies of its DNA.

Phases of the Cell Cycle

Interphase and Mitotic Phase

The cell cycle consists of interphase (G1, S, G2) and the mitotic (M) phase. Interphase is the period of cell growth and DNA replication, while the M phase includes mitosis and cytokinesis.

• G1 phase: Cell growth.

• S phase: DNA synthesis.

• G2 phase: Preparation for mitosis.

• M phase: Division of nucleus (mitosis) and cytoplasm (cytokinesis).

Mitosis and Cytokinesis

Stages of Mitosis

Mitosis is the process by which the nucleus divides, followed by cytokinesis, which divides the cytoplasm. The stages of mitosis include:

• Prophase: Chromosomes condense, spindle forms.

• Prometaphase: Nuclear envelope fragments, spindle fibers attach to kinetochores.

• Metaphase: Chromosomes align at the metaphase plate.

• Anaphase: Sister chromatids separate and move to opposite poles.

• Telophase: Nuclear envelope reforms, chromosomes decondense.

Cytokinesis

Cytokinesis differs in animal and plant cells.

• Animal cells: Cleavage furrow forms, pinching the cell in two.

• Plant cells: Cell plate forms, leading to separation by a new cell wall.

Regulation of the Cell Cycle

Cell Cycle Control System

The cell cycle is regulated by a molecular control system, which acts like a clock with checkpoints.

• Checkpoints: G1, G2, and M checkpoints ensure proper progression.

• Go-ahead signals: Required for the cell to proceed past checkpoints.

• G0 phase: Cells that do not receive signals exit the cycle and enter a nondividing state.

Cyclins and Cyclin-Dependent Kinases (Cdks)

• Cyclins: Regulatory proteins whose levels fluctuate during the cell cycle.

• Cdk: Enzymes that, when bound to cyclins, regulate cell cycle progression.

• MPF: A cyclin-Cdk complex that triggers passage past the G2 checkpoint into M phase.

External Regulation and Cancer

Growth Factors and External Controls

External factors such as growth factors influence cell division.

• Platelet-derived growth factor (PDGF): Stimulates cell division in connective tissue.

• Anchorage dependence: Cells require a surface to divide.

• Density-dependent inhibition: Cells stop dividing when they form a single layer.

Summary Table: Key Features of the Cell Cycle

Phase	Main Events	Regulation
G1	Cell growth	G1 checkpoint
S	DNA replication	Controlled by cyclins/Cdks
G2	Preparation for mitosis	G2 checkpoint
M	Mitosis and cytokinesis	M checkpoint
G0	Nondividing state	Exit from cycle if no signal

Key Equations and Concepts

• DNA Replication: $\text{DNA}_{\text{parent}} \rightarrow 2 \times \text{DNA}_{\text{daughter}}$

• Chromosome Number: $\text{Somatic cells:} \, 2n \, \text{chromosomes}$ $\text{Gametes:} \, n \, \text{chromosomes}$

Additional info:

• Cell cycle regulation is crucial for preventing uncontrolled cell division, which can lead to cancer.

• Checkpoint failures are often associated with tumorigenesis.