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 growth, development, and maintenance of all living organisms. The cycle ensures that genetic material is accurately duplicated and distributed to daughter cells.

• 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

Chromosomes and Genome

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 DNA in a cell.

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

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

• Somatic cells (non-reproductive cells) have two sets of chromosomes (diploid), while gametes (sperm and egg cells) have one set (haploid).

Phases of the Cell Cycle

Main Phases

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

• 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.

• Mitotic (M) phase: Includes mitosis (division of the nucleus) and cytokinesis (division of the cytoplasm).

Steps of Mitosis

Mitosis is conventionally divided into five stages, ensuring equal distribution of genetic material:

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

2. Prometaphase: Nuclear envelope fragments; spindle microtubules attach to kinetochores on chromosomes.

3. Metaphase: Chromosomes align at the metaphase plate (cell's equator).

4. Anaphase: Sister chromatids are separated and pulled toward opposite poles of the cell.

5. Telophase: Chromosomes arrive at poles; nuclear envelopes reform; chromosomes decondense.

• Cytokinesis usually overlaps with telophase, dividing the cytoplasm and forming two daughter cells.

Summary Table: Phases of the Cell Cycle

Mechanisms of Chromosome Movement

The Mitotic Spindle

The mitotic spindle is a structure made of microtubules that orchestrates the movement of chromosomes during mitosis.

• Composed of centrosomes, spindle microtubules, and asters.

• Each sister chromatid has a kinetochore where spindle fibers attach.

• Microtubules shorten by depolymerization, pulling chromatids apart.

Cytokinesis

• In animal cells, cytokinesis occurs via cleavage, forming a cleavage furrow that pinches the cell in two.

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

Cell Division in Prokaryotes: Binary Fission

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

• The single, circular chromosome replicates at the origin of replication.

• 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 may enter a non-dividing state called G0 phase.

Cyclins and Cyclin-Dependent Kinases (Cdks)

• Cyclins: Proteins whose concentrations fluctuate cyclically in the cell.

• Cyclin-dependent kinases (Cdks): Enzymes that, when bound to cyclins, phosphorylate target proteins to advance the cell cycle.

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

External Signals and Growth Factors

• Growth factors are proteins released by certain cells that stimulate other cells to divide (e.g., Platelet-Derived Growth Factor, PDGF).

• Density-dependent inhibition: Crowded cells stop dividing.

• Anchorage dependence: Most animal cells must be attached to a substrate to divide.

Loss of Cell Cycle Control and Cancer

Cancer Cells

Cancer results from the loss of normal cell cycle control, leading to uncontrolled cell division.

• Cancer cells may not require external growth factors to divide.

• They may produce their own growth signals or have abnormal signaling pathways.

• 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: Benign vs. Malignant Tumors

Key Terms and Definitions

• Chromosome: A DNA molecule with associated proteins, carrying genetic information.

• Chromatin: The complex of DNA and proteins that forms chromosomes.

• Centromere: The region where sister chromatids are joined.

• Kinetochore: Protein structure on chromatids where spindle fibers attach.

• Mitotic spindle: Structure that separates chromosomes during mitosis.

• Binary fission: Asexual reproduction in prokaryotes, producing two identical cells.

• Checkpoints: Control points in the cell cycle where stop and go-ahead signals regulate progression.

Key Equations

• Number of chromosomes in daughter cells after mitosis:

• DNA content doubles during S phase:

Example: Application of Cell Cycle Concepts

• Example: In human somatic cells (46 chromosomes), after DNA replication in S phase, there are still 46 chromosomes, but each consists of two sister chromatids (92 chromatids total). After mitosis and cytokinesis, each daughter cell receives 46 chromosomes.