The Cell Cycle: Regulation, Control, and Implications for Cell Division

Chapter 12: The Cell Cycle

Introduction to the Cell Cycle

The cell cycle is a series of events that cells go through as they grow and divide. It is fundamental to the development, growth, and maintenance of all living organisms. The length and regulation of the cell cycle can vary greatly among different cell types and in response to environmental conditions.

• Cell cycle length varies due to differences in the duration of the G1 phase.

• Rapidly dividing cells may eliminate the G1 phase, while non-dividing cells can become permanently arrested in G1.

• The G0 state is a quiescent phase where cells are metabolically active but do not divide. Examples include skeletal muscle cells, cardiac muscle cells, and nerve cells.

• Some cells, such as intestinal cells, divide frequently (twice a day), while others, like liver cells, divide infrequently (about once a year).

• Cell cycle length can change in response to external conditions, indicating that the cycle is regulated.

Phases of the Cell Cycle

The cell cycle consists of distinct phases, each with specific roles in cell growth and division.

• G1 phase: Cell growth and preparation for DNA replication.

• S phase: DNA synthesis, where chromosomes are replicated.

• G2 phase: Further growth and preparation for mitosis; the cell checks for DNA errors and repairs them.

• M phase: Mitosis and cytokinesis, where the cell divides into two daughter cells.

Regulation of the Cell Cycle

Regulatory molecules control the progression of the cell cycle, ensuring that cells only divide when appropriate. These molecules are especially important at key checkpoints.

• Cell-cycle checkpoints are control points where the cell assesses whether to proceed with division.

• Checkpoints occur at the end of G1, G2, and during M phase (metaphase and anaphase).

• Defective regulatory molecules can lead to uncontrolled cell division and tumor formation.

Experimental Evidence for Cell-Cycle Control Molecules

Experiments have shown that entry into M phase is controlled by molecules present in the cytoplasm of M-phase cells.

• Microinjection of cytoplasm from M-phase cells into interphase cells can induce mitosis in the recipient cell.

• This demonstrates that M-phase promoting factors (MPF) are present in the cytoplasm and are responsible for initiating mitosis.

M-Phase Promoting Factor (MPF)

MPF is a key regulatory complex that triggers the onset of mitosis in eukaryotic cells.

• MPF consists of two subunits: a protein kinase (cyclin-dependent kinase, Cdk) and a cyclin.

• The kinase subunit catalyzes the transfer of phosphate groups from ATP to target proteins, a process known as phosphorylation.

• Cyclin concentrations fluctuate throughout the cell cycle, peaking during M phase.

• MPF is active only when cyclin is bound to Cdk and specific phosphorylation events have occurred.

Key equation:

• $\text{MPF Activity} \propto [\text{Cyclin}] \times [\text{Cdk}]$

Functions of MPF

MPF initiates several processes required for mitosis:

• Formation of the mitotic spindle

• Fragmentation of the nuclear envelope

• Chromosomal compaction

Regulation and Inactivation of MPF

MPF activity is tightly regulated to ensure proper cell division.

• MPF is inactivated by the destruction of cyclin via ubiquitination and proteasomal degradation.

• This feedback mechanism ensures that mitosis does not proceed unchecked.

Cell-Cycle Checkpoints

Checkpoints are critical for maintaining genomic integrity and preventing uncontrolled cell division.

• G1 checkpoint: Assesses cell size, nutrient availability, growth signals, and DNA integrity.

• G2 checkpoint: Ensures DNA replication is complete and undamaged before mitosis.

• M-phase checkpoints: Ensure proper chromosome alignment and attachment to the spindle before separation.

Role of p53 Protein

The p53 protein is a tumor suppressor that plays a key role in the G1 checkpoint.

• p53 activates proteins that pause the cell cycle to allow for DNA repair or trigger apoptosis (programmed cell death) if damage is irreparable.

• Mutations in the p53 gene can lead to uncontrolled cell division and cancer.

Control of Cell Division and Cancer

Cancer is a family of diseases characterized by uncontrolled cell division and the ability to invade other tissues.

• Malignant tumors are invasive and can spread (metastasize) to other parts of the body.

• Benign tumors are non-invasive and do not spread.

• Cancer cells often have defects in cell-cycle checkpoints and ignore signals that regulate division.

• Growth factors in serum are required for normal cells to pass the G1 checkpoint, but cancer cells can divide without these signals.

Apoptosis Pathway

Apoptosis is a programmed cell death mechanism that removes damaged or unnecessary cells.

• Cells that fail to undergo apoptosis can contribute to tumor formation.

• Apoptosis involves inactivation of mitochondria, which normally produce ATP.

• Chemicals such as dichloroacetate (DCA) can reactivate mitochondrial activity and induce apoptosis in cancer cells.

Summary Table: Cell Cycle Phases and Checkpoints

Phase	Main Events	Checkpoint Criteria
G1	Cell growth, preparation for DNA synthesis	Size, nutrients, growth signals, DNA integrity
S	DNA replication	None (progresses if G1 passed)
G2	Preparation for mitosis, error checking	DNA replication complete, no damage
M	Mitosis and cytokinesis	Chromosome alignment, spindle attachment

Example: p53 and Cancer

• Many lung cancers are caused by mutations in the p53 gene, often due to carcinogens in cigarette smoke.

• Loss of p53 function is implicated in about half of all cancers.

Additional info: The notes have been expanded to include definitions, examples, and a summary table for clarity and completeness.