The Cell Cycle: Structure, Function, and Regulation

The Cell Cycle

Introduction

The cell cycle is the 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. This guide covers the structure, phases, and regulation of the cell cycle, as well as its evolutionary origins and implications for diseases such as cancer.

The Key Roles of Cell Division

Functions of Cell Division

• Reproduction: Single-celled organisms (e.g., Amoeba) reproduce by cell division.

• Growth and Development: Multicellular organisms grow and develop from a single cell through repeated cell divisions (e.g., embryonic development).

• Tissue Renewal: In mature organisms, cell division replaces dead or damaged cells (e.g., bone marrow cells producing new blood cells).

Cell Division

Types of Cell Division

• Most cell division results in daughter cells with identical genetic information (DNA).

• Meiosis is a special type of division that produces gametes (sperm and egg cells) for sexual reproduction, resulting in genetic diversity.

Chromosomes

Genome Organization

• All the DNA in a cell constitutes its genome.

• Prokaryotes: Single circular DNA molecule (e.g., E. coli ~4x106 base pairs).

• Eukaryotes: Multiple linear DNA molecules (e.g., humans ~3x109 base pairs, about 1 meter if stretched out).

• DNA is compacted into chromosomes to fit inside the nucleus (diameter ~10 μm).

• Eukaryotic chromosomes consist of chromatin, a complex of DNA and proteins (mainly histones) that condenses during cell division.

Chromatin

Structure and Function

• Chromatin is the material that makes up chromosomes, consisting of DNA wrapped around histone proteins.

• During cell division, chromatin condenses to form visible chromosomes.

• This condensation is essential for the accurate segregation of DNA during mitosis and meiosis.

Chromosome Number

Somatic Cells vs. Gametes

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

• Somatic cells (nonreproductive) have two sets of chromosomes (diploid, 2n). Humans: 46 chromosomes.

• Gametes (reproductive cells: sperm and eggs) have one set (haploid, n). Humans: 23 chromosomes.

• Before cell division, DNA is replicated, but the number of chromosomes does not change; each chromosome consists of two sister chromatids.

Sister Chromatids and Centromeres

Chromosome Duplication

• Each duplicated chromosome has two sister chromatids (identical copies joined together).

• The centromere is the region where the two chromatids are most closely attached, often referred to as the "waist" of the chromosome.

Phases of the Cell Cycle

Overview of the Cell Cycle

• The cell cycle consists of two main phases:

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

  • Interphase: Accounts for 90% of the cell cycle and includes:

    • G1 phase (first gap): Cell growth and normal functions.

    • S phase (synthesis): DNA replication.

    • G2 phase (second gap): Preparation for mitosis.

Mitotic Phase

Stages of Mitosis

• Mitosis is divided into five phases:

  1. Prophase

  2. Prometaphase

  3. Metaphase

  4. Anaphase

  5. Telophase

• Cytokinesis overlaps with the latter stages of mitosis, resulting in the physical separation of the cytoplasm into two daughter cells.

Mitotic Spindle

Structure and Function

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

• It originates from the centrosome (microtubule organizing center, MTOC).

• Components include:

  • Centrosomes

  • Spindle microtubules:

    • Kinetochore microtubules – attach to chromosomes at the kinetochore.

    • Nonkinetochore microtubules – interact with microtubules from the opposite pole to elongate the cell.

  • Asters – radial arrays of short microtubules that help anchor the spindle to the cell membrane.

Detailed Stages of Mitosis

Prophase

• Chromatin condenses into visible chromosomes.

• Mitotic spindle begins to form.

Prometaphase

• Nuclear envelope fragments.

• Spindle microtubules attach to kinetochores on chromosomes.

Metaphase

• Chromosomes align at the metaphase plate (imaginary plane at the cell's equator).

• Each sister chromatid is attached to spindle fibers from opposite poles.

Anaphase

• Sister chromatids separate and move toward opposite poles.

• Kinetochore microtubules shorten by depolymerizing at their kinetochore ends.

Telophase and Cytokinesis

• Nuclear envelopes reform around the two sets of chromosomes.

• Chromosomes decondense.

• Cytokinesis divides the cytoplasm, forming two daughter cells.

• In animal cells, a cleavage furrow forms; in plant cells, a cell plate forms.

Experimental Evidence: Microtubule Shortening

Key Experiment

• Fluorescent labeling and photobleaching experiments show that kinetochore microtubules shorten at their kinetochore ends during anaphase.

• This is due to depolymerization of tubulin subunits at the kinetochore.

Comparing Cytokinesis in Animal and Plant Cells

Feature	Animal Cells	Plant Cells
Mechanism	Cleavage furrow forms, contractile ring of microfilaments pinches cell in two	Cell plate forms from vesicles, leading to new cell wall between daughter cells
Result	Two separate daughter cells	Two daughter cells separated by new cell wall

Evolution of Mitosis

Origins and Variations

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

• Mitosis likely evolved from binary fission as eukaryotes arose.

• Certain protists show intermediate forms of cell division, supporting this evolutionary link.

Cell Cycle Control

Regulation of Cell Division

• The frequency of cell division varies by cell type (e.g., epithelial cells divide often, nerve cells rarely divide in adults).

• Regulation occurs at the molecular level through a cell cycle control system with checkpoints.

• Cancer cells can evade these controls, leading to uncontrolled growth.

The Cell Cycle Control System

Checkpoints and Regulation

• The cell cycle is regulated by a control system with checkpoints (e.g., G1, G2, M).

• Cells must receive a "go-ahead" signal to proceed past each checkpoint.

• Regulation involves both internal and external signals.

The Cell Cycle Clock: Cyclins and Cdks

Molecular Mechanisms

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

• Cyclin-dependent kinases (Cdks): Enzymes activated by binding to cyclins; regulate progression through the cell cycle.

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

Internal and External Signals

Types of Signals

• Internal signals: Unattached kinetochores send signals to delay anaphase until all chromosomes are properly attached to the spindle.

• External signals: Growth factors (e.g., Platelet-Derived Growth Factor, PDGF) stimulate cell division in response to environmental cues.

External Signal Mechanisms

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

• Density-dependent inhibition: Crowded cells stop dividing; cancer cells often lose this property.

Loss of Cell Cycle Control and Cancer

Cancer Cell Characteristics

• Cancer cells do not respond to normal regulatory signals.

• They may produce their own growth factors, signal themselves to divide, or have abnormal control systems.

• This leads to uncontrolled cell division, tumor formation, and potentially metastasis (spread to other tissues).

Summary Table: Key Differences Between Normal and Cancer Cells

Property	Normal Cells	Cancer Cells
Growth Factor Dependence	Require external growth factors	May produce their own or signal without them
Density-Dependent Inhibition	Stop dividing when crowded	Continue dividing regardless of density
Anchorage Dependence	Require attachment to substrate	May divide without attachment
Cell Cycle Control	Regulated by checkpoints	Often have defective checkpoints

Key Terms

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

• Chromatin: DNA-protein complex that condenses to form chromosomes.

• Sister chromatids: Identical copies of a chromosome connected at the centromere.

• Centromere: Region where sister chromatids are joined.

• Mitotic spindle: Structure that separates chromosomes during mitosis.

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

• Cyclin/Cdk: Proteins and enzymes that regulate the cell cycle.

• Binary fission: Asexual reproduction in prokaryotes.

• Metastasis: Spread of cancer cells to other parts of the body.

Additional info: The cell cycle is a highly regulated process essential for life. Disruptions in its regulation can lead to diseases such as cancer. Understanding the molecular mechanisms of cell cycle control is crucial for advances in medicine and biotechnology.