Ch. 12 The Cell Cycle: Mechanisms and Regulation

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.

• In unicellular organisms, cell division is a means of reproduction.

• In multicellular organisms, it enables growth, development, tissue renewal, and repair.

Cell Division and Its Functions

Roles of Cell Division

Cell division is essential for the continuity of life and serves several key functions:

• Reproduction: Single-celled organisms reproduce by dividing into two new individuals.

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

• Tissue Renewal and Repair: Cell division replaces dead or damaged cells in mature organisms.

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

Molecular Organization of Genetic Material

Chromosomes and Chromatin

The genetic material of a cell is organized into structures called chromosomes, which are composed of DNA and associated proteins (chromatin).

• Genome: The complete set of genetic material in a cell.

• Prokaryotic genomes usually consist of a single circular DNA molecule.

• Eukaryotic genomes are divided among multiple linear chromosomes.

• Chromatin: The complex of DNA and proteins that makes up eukaryotic chromosomes.

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

• Gametes: Reproductive cells (sperm and egg) with one set of chromosomes (haploid).

Distribution of Chromosomes During Eukaryotic Cell Division

Chromosome Duplication and Segregation

Before a cell divides, it duplicates its chromosomes so that each daughter cell receives an identical set.

• Each duplicated chromosome consists of two sister chromatids joined at a region called the centromere.

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

• Cell division in somatic cells involves two main processes: mitosis (division of the nucleus) and cytokinesis (division of the cytoplasm).

The Cell Cycle Phases

Main Phases of the Cell Cycle

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

• Interphase: Period of cell growth and DNA replication, subdivided into three phases:

  • G1 phase (First Gap): Cell grows and carries out normal functions.

  • S phase (Synthesis): DNA is replicated.

  • G2 phase (Second Gap): Cell prepares for division.

• Mitotic (M) phase: Includes mitosis and cytokinesis, resulting in two genetically identical daughter cells.

Mitosis: Steps and Mechanisms

Phases of Mitosis

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

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

• Prometaphase: Nuclear envelope fragments; 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 toward opposite poles of the cell.

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

• Cytokinesis: Division of the cytoplasm, resulting in two separate daughter cells. In animal cells, this occurs via cleavage furrow; in plant cells, a cell plate forms.

The Mitotic Spindle

Structure and Function

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

• Composed of centrosomes, spindle microtubules, and asters.

• Centrosomes duplicate during interphase and move to opposite poles during mitosis.

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

Binary Fission in Bacteria

Prokaryotic Cell Division

Prokaryotes (bacteria and archaea) divide by a process called binary fission, which is simpler than mitosis.

• Chromosome replication begins at the origin of replication.

• The two copies of the chromosome move to opposite ends of the cell.

• The plasma membrane pinches inward, dividing the cell into two genetically identical daughter cells.

Regulation of the Eukaryotic Cell Cycle

Cell Cycle Control System

The cell cycle is regulated by a molecular control system that ensures proper timing and order of events.

• 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)

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 are active only when bound to cyclins.

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

External Signals and Cell Cycle Regulation

Growth Factors and Environmental Cues

Cell division is also influenced by external factors:

• Growth factors: Proteins released by certain cells that stimulate others to divide (e.g., Platelet-Derived Growth Factor, PDGF).

• Density-dependent inhibition: Crowded cells stop dividing when they touch each other.

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

Loss of Cell Cycle Controls in Cancer

Cancer and Uncontrolled Cell Division

Cancer cells escape normal cell cycle controls, leading to uncontrolled growth and division.

• Cancer cells may produce their own growth factors, ignore external signals, or have abnormal control systems.

• Cells that divide indefinitely are said to have undergone transformation.

• Benign tumors: Abnormal cells remain at the original site and usually do not cause serious problems.

• Malignant tumors: Invade surrounding tissues and can spread (metastasize) to other parts of the body, forming secondary tumors.

Summary Table: Key Differences in Cell Division

Feature	Eukaryotic Mitosis	Prokaryotic Binary Fission
Genetic Material	Multiple linear chromosomes	Single circular chromosome
Process	Mitosis (prophase, metaphase, anaphase, telophase) + cytokinesis	Binary fission (DNA replication, segregation, division)
Spindle Apparatus	Present	Absent
Regulation	Complex checkpoints, cyclins, Cdks	Simpler, less regulated