The Cell Cycle: Structure, Function, and Regulation

Chapter 12: The Cell Cycle

Introduction to the Cell Cycle

The cell cycle is a fundamental process by which cells grow, duplicate their genetic material, and divide to produce new cells. This process is essential for growth, development, and maintenance in all living organisms.

• Cell division is the process by which a parent cell gives rise to two genetically identical daughter cells.

• It is a defining characteristic that distinguishes living things from nonliving matter.

• Functions of cell division include reproduction in single-celled organisms, embryonic development in multicellular eukaryotes, and renewal and repair in mature tissues.

Cell Cycle Overview

The cell cycle consists of a series of phases that prepare the cell for division and ensure accurate distribution of genetic material.

• Interphase: The cell grows and duplicates its chromosomes in preparation for division.

• Mitosis: The duplicated chromosomes are separated and moved to opposite ends of the cell.

• Cytokinesis: The cell divides into two daughter cells, each genetically identical to the parent cell.

Functions of Cell Division

Cell division serves several critical roles in organisms:

• Reproduction: Single-celled organisms reproduce by cell division.

• Development: Multicellular eukaryotes undergo embryonic development through repeated cell divisions.

• Renewal and Repair: Cell division replaces damaged or old cells in tissues.

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

Cellular Organization of Genetic Material

Genome and Chromosomes

The genetic material of a cell is organized into a genome, which is packaged into chromosomes.

• Genome: The complete set of DNA in a cell.

• Prokaryotic genomes typically consist of a single DNA molecule; eukaryotic genomes consist of multiple DNA molecules.

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

• Each eukaryotic species has a characteristic number of chromosomes in its nucleus.

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

• Gametes: Reproductive cells with half as many chromosomes as somatic cells (haploid).

Chromosome Duplication and Distribution

Before cell division, chromosomes are duplicated so that each daughter cell receives an identical set.

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

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

Phases of the Cell Cycle

Interphase

Interphase is the longest phase of the cell cycle, during which the cell grows and prepares for division.

• Divided into three subphases:

  • G1 phase (First Gap): Cell growth.

  • S phase (Synthesis): DNA replication; chromosomes are duplicated.

  • G2 phase (Second Gap): Further growth and preparation for mitosis.

Mitosis

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

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

• Prometaphase: 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 to opposite poles of the cell.

• Telophase: Chromosomes decondense; nuclear envelope reforms; cytokinesis begins.

Cytokinesis

Cytokinesis is the division of the cytoplasm, resulting in two separate daughter cells.

• In animal cells, cytokinesis occurs via cleavage, forming a cleavage furrow.

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

Regulation of the Cell Cycle

Cell Cycle Control System

The cell cycle is regulated by a complex control system involving signaling molecules and checkpoints.

• Checkpoints are control points where the cell cycle can be halted until certain conditions are met.

• Major checkpoints occur in G1, G2, and M phases.

• If a cell does not receive a go-ahead signal at the G1 checkpoint, it enters a nondividing state called G0 phase.

Cyclins and Cyclin-Dependent Kinases (Cdks)

Two key types of regulatory proteins control progression through the cell cycle:

• Cyclins: Proteins whose concentrations fluctuate cyclically during the cell cycle.

• Cyclin-dependent kinases (Cdks): Enzymes that must bind to cyclins to be active.

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

External and Internal Signals

Cell division is influenced by both internal surveillance mechanisms and external signals.

• Growth factors: Chemical signals released by certain cells to stimulate division in others (e.g., PDGF for fibroblasts).

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

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

Cell Division in Prokaryotes: Binary Fission

Binary Fission

Prokaryotes such as bacteria reproduce by binary fission, a simpler form of cell division.

• The chromosome replicates, starting at the origin of replication.

• The two daughter chromosomes move apart as the cell elongates.

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

Loss of Cell Cycle Control and Cancer

Cancer Cells and Uncontrolled Division

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

• Cancer cells do not respond to growth factors or density-dependent inhibition.

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

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

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

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

Summary Table: Comparison of Cell Division Types

Feature	Mitosis (Eukaryotes)	Binary Fission (Prokaryotes)
Genetic Material	Multiple linear chromosomes	Single circular chromosome
Phases	Interphase, Mitosis, Cytokinesis	Chromosome replication, cell division
Regulation	Complex control system with checkpoints	Simple, less regulated
Result	Two genetically identical daughter cells	Two genetically identical daughter cells

Key Equations

• DNA content after replication: $2N \rightarrow 4N$ (where N is the number of chromosomes)

• MPF activity: $\text{MPF} = \text{Cyclin} \times \text{Cdk}$

Example

• Human somatic cells have 46 chromosomes (diploid), while human gametes have 23 chromosomes (haploid).

• During mitosis, each daughter cell receives 46 chromosomes, identical to the parent cell.

Additional info: Some details, such as the specific roles of cyclins and Cdks, and the summary table, were inferred and expanded for academic completeness.