The Cell Cycle

Key Roles of Cell Division

Cell division is fundamental to life, enabling organisms to reproduce, grow, and repair tissues. The continuity of life depends on the accurate reproduction of cells, ensuring genetic consistency across generations.

• Reproduction: Unicellular organisms reproduce by cell division, creating new individuals.

• Growth and Development: Multicellular organisms develop from a single cell through repeated cell divisions.

• Tissue Renewal: Cell division replaces worn-out or damaged cells in mature organisms.

• Genetic Consistency: Daughter cells are typically genetically identical to the parent cell, maintaining organismal integrity.

Cellular Organization of Genetic Material

All the DNA in a cell constitutes its genome. DNA molecules are packaged into chromosomes, which carry genes. Eukaryotic chromosomes consist of chromatin, a complex of DNA and protein that condenses during cell division.

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

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

Chromosome Duplication and Distribution

During cell division, chromosomes are duplicated and distributed to daughter cells. Each duplicated chromosome consists of two sister chromatids joined at the centromere. Upon separation, chromatids become individual chromosomes.

• Chromosome duplication: Occurs during the S phase of interphase.

• Separation: Sister chromatids are separated during mitosis and distributed to daughter cells.

Phases of the Cell Cycle

Overview of the Cell Cycle

The cell cycle consists of two main phases: interphase and the mitotic (M) phase. Interphase is subdivided into G1, S, and G2 phases, during which the cell grows and prepares for division. The M phase includes mitosis and cytokinesis.

• G1 phase: Cell growth and metabolic activity.

• S phase: DNA synthesis and chromosome duplication.

• G2 phase: Further growth and preparation for division.

• M phase: Mitosis (nuclear division) and cytokinesis (cytoplasmic division).

Mitosis: Stages and Mechanisms

Mitosis is conventionally divided into five stages: prophase, prometaphase, metaphase, anaphase, and telophase. Each stage is characterized by specific structural changes and movements of chromosomes.

• Prophase: Chromatin condenses, nucleoli disappear, spindle forms.

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

• Metaphase: Chromosomes align at the metaphase plate.

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

• Telophase: Nuclear envelopes reform, chromosomes decondense.

• Cytokinesis: Division of cytoplasm, forming two daughter cells.

The Mitotic Spindle: Structure and Function

The mitotic spindle is a structure made of microtubules that controls chromosome movement during mitosis. It includes centrosomes, spindle microtubules, and asters. The spindle ensures accurate chromosome segregation.

• Centrosomes: Microtubule-organizing centers that replicate and migrate to opposite poles.

• Kinetochore: Protein complex at the centromere where spindle microtubules attach.

• Metaphase plate: Imaginary plane where chromosomes align during metaphase.

Mechanisms of Chromosome Movement

During anaphase, sister chromatids are separated by the action of motor proteins and microtubule depolymerization. Chromosomes are "walked" along microtubules, which shorten at the kinetochore ends.

• Pac-man mechanism: Motor proteins move chromosomes along microtubules, which depolymerize at the kinetochore.

• Reeling-in mechanism: Chromosomes are pulled toward spindle poles by motor proteins.

Cytokinesis: Animal vs. Plant Cells

Cytokinesis differs between animal and plant cells. In animal cells, a cleavage furrow forms and pinches the cell in two. In plant cells, a cell plate forms, eventually becoming the new cell wall.

• Cleavage furrow: Contractile ring of actin filaments pinches the cell.

• Cell plate: Vesicles fuse to form a plate that develops into the cell wall.

Binary Fission in Prokaryotes

Mechanism of Binary Fission

Prokaryotes reproduce by binary fission, a simpler process than mitosis. The chromosome replicates, and the cell divides into two genetically identical daughter cells.

• Origin of replication: Site where chromosome replication begins.

• Active movement: Daughter chromosomes move apart as the cell elongates.

Regulation of the Cell Cycle

Cell Cycle Control System

The cell cycle is regulated by a molecular control system, involving internal and external signals. Specific checkpoints ensure proper progression and division.

• Checkpoints: G1, G2, and M phase checkpoints monitor cell readiness.

• G0 phase: Nondividing state entered if the cell does not receive a go-ahead signal.

Cyclins and Cyclin-Dependent Kinases (Cdks)

Two types of regulatory proteins, cyclins and cyclin-dependent kinases (Cdks), control the cell cycle. Cyclins fluctuate in concentration, activating Cdks to trigger cell cycle events.

• MPF (Maturation-Promoting Factor): Cyclin-Cdk complex that triggers entry into mitosis.

Internal and External Signals

Cell cycle progression is influenced by internal surveillance mechanisms and external factors such as growth factors.

• Growth factors: Chemical signals that stimulate cell division (e.g., PDGF).

• Density-dependent inhibition: Cells stop dividing when crowded.

• Anchorage dependence: Cells must be attached to a substratum to divide.

Loss of Cell Cycle Controls in Cancer

Cancer cells evade normal cell cycle regulation, leading to uncontrolled growth and tumor formation. Malignant tumors can invade tissues and metastasize to other parts of the body.

• Transformation: Cells acquire the ability to divide indefinitely.

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

• Malignant tumor: Cells invade tissues and spread via metastasis.

• Chemotherapy: Drugs target the cell cycle in rapidly dividing cells.

Summary Table: Cell Cycle Phases and Key Events

Key Terms

• Genome: All genetic material in a cell

• Chromatin: DNA-protein complex in eukaryotic chromosomes

• Chromosome: DNA molecule carrying genes

• Sister chromatids: Duplicated chromosomes joined at the centromere

• Centromere: Region joining sister chromatids

• Kinetochore: Protein complex at centromere for spindle attachment

• Mitotic spindle: Microtubule structure for chromosome movement

• Binary fission: Prokaryotic cell division

• Cyclin: Regulatory protein fluctuating in concentration

• Cdk: Cyclin-dependent kinase, triggers cell cycle events

• MPF: Maturation-promoting factor, cyclin-Cdk complex

• Metastasis: Spread of cancer cells to other body parts