The Cell Cycle: Mechanisms and Regulation

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The Cell Cycle

Introduction to the Cell Cycle

The cell cycle is the series of events that cells go through as they grow and divide. It is fundamental to the continuity of life, enabling organisms to grow, develop, and repair tissues. The cell cycle ensures that genetic material is accurately duplicated and distributed to daughter cells.

Cell division distinguishes living organisms from nonliving matter.
It is essential for reproduction, growth, development, and tissue repair.
Most cell division produces genetically identical daughter cells, except during meiosis.

Early embryonic cell divisions showing two and four cells

Key Roles of Cell Division

Single-celled organisms reproduce by cell division.
Multicellular organisms use cell division for development, growth, and repair.
Accurate distribution of genetic material is crucial for maintaining genetic continuity.

Diagram showing how one parent cell gives rise to two genetically identical daughter cells

Cellular Organization of Genetic Material

Chromosomes and Chromatin

All the DNA in a cell constitutes its genome. In eukaryotes, DNA is organized into multiple linear chromosomes, which are composed of chromatin—a complex of DNA and proteins (mainly histones).

Somatic cells have two sets of chromosomes (diploid).
Gametes (sperm and eggs) have one set (haploid).
Each duplicated chromosome consists of two sister chromatids joined at a centromere.

Diagram of chromosome, chromatin, and DNA structure Electron micrograph of a duplicated chromosome with sister chromatids and centromeres

Distribution of Chromosomes During Eukaryotic Cell Division

During cell division, sister chromatids separate and are distributed to two daughter cells.
Once separated, each chromatid is considered an individual chromosome.

Diagram showing chromosome duplication and separation of sister chromatids

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 accounts for about 90% of the cycle and includes cell growth and DNA replication. The M phase includes mitosis and cytokinesis.

Interphase is subdivided into:
- G1 phase (first gap): Cell growth
- S phase (synthesis): DNA replication
- G2 phase (second gap): Preparation for division
M phase includes:
- Mitosis: Division of the nucleus
- Cytokinesis: Division of the cytoplasm

Pie chart of the cell cycle showing interphase and mitotic phase

Mitosis: Stages and Mechanisms

Mitosis is conventionally divided into five stages: prophase, prometaphase, metaphase, anaphase, and telophase. Each stage is characterized by specific events that ensure accurate chromosome segregation.

Prophase: Chromosomes condense, spindle begins to form.
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.

Stages of mitosis: G2 of interphase, prophase, prometaphase Stages of mitosis: metaphase, anaphase, telophase and cytokinesis

The Mitotic Spindle

The mitotic spindle is a structure made of microtubules that orchestrates chromosome movement during mitosis. In animal cells, spindle assembly begins at the centrosomes, which replicate and migrate to opposite poles.

Spindle includes centrosomes, spindle microtubules, and asters.
Each sister chromatid has a kinetochore where spindle fibers attach.
At metaphase, chromosomes align at the metaphase plate.

Diagram of mitotic spindle with centrosomes, kinetochores, and microtubules

Chromatid Separation and Cytokinesis

During anaphase, cohesins are cleaved by separase, allowing sister chromatids to separate.
Chromatids move toward opposite poles as microtubules shorten.
Cytokinesis divides the cytoplasm, forming two daughter cells.
In animal cells, cytokinesis occurs by cleavage (formation of a cleavage furrow).
In plant cells, a cell plate forms to divide the cell.

Diagram showing breakdown of cohesin and chromatid separation Diagram of anaphase and telophase/cytokinesis Cleavage in animal cells and cell plate formation in plant cells

Cell Division in Prokaryotes: Binary Fission

Binary Fission

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

Replication begins at the origin of replication.
Two copies of the chromosome move to opposite ends.
The plasma membrane pinches inward, dividing the cell.

Diagram of binary fission in bacteria

Regulation of the Eukaryotic Cell Cycle

Cell Cycle Control System

The cell cycle is regulated by a molecular control system with checkpoints at critical stages. These checkpoints ensure that each phase is completed accurately before the next begins.

Checkpoints are found in G1, G2, and M phases.
The G1 checkpoint is often the most important; cells that do not pass enter a nondividing state called G0 phase.

Experiment showing cell fusion and cell cycle phase control

Cyclins and Cyclin-Dependent Kinases (Cdks)

Cyclins are proteins whose concentrations fluctuate cyclically during the cell cycle.
Cyclin-dependent kinases (Cdks) are enzymes that, when bound to cyclins, phosphorylate target proteins to drive cell cycle progression.
The MPF (maturation-promoting factor) is a cyclin-Cdk complex that triggers passage from G2 to M phase.

Internal and External Signals

Internal signals include surveillance mechanisms that monitor DNA integrity and spindle attachment.
External signals include growth factors (e.g., PDGF) and physical factors such as density-dependent inhibition and anchorage dependence.

Loss of Cell Cycle Control and Cancer

Cancer Cells

Cancer cells do not respond to normal cell cycle controls.
They may divide without growth factors, produce their own, or have abnormal signaling pathways.
Cells that divide indefinitely have undergone transformation.
Benign tumors remain localized; malignant tumors invade tissues and can metastasize.
Treatments include surgery, radiation, and chemotherapy, often targeting rapidly dividing cells.

Summary Table: Key Terms and Concepts

Term	Definition
Chromosome	DNA molecule with associated proteins, carrying genetic information
Sister Chromatids	Identical copies of a duplicated chromosome, joined at the centromere
Centromere	Region where sister chromatids are most closely attached
Mitotic Spindle	Structure of microtubules that separates chromosomes during mitosis
Cytokinesis	Division of the cytoplasm, forming two daughter cells
Binary Fission	Prokaryotic cell division mechanism
Cyclin	Regulatory protein with cyclic concentration changes
Cdk	Cyclin-dependent kinase, enzyme regulating cell cycle transitions
MPF	Maturation-promoting factor, cyclin-Cdk complex for G2/M transition
G0 Phase	Nondividing state of a cell

Key Equations and Concepts

DNA Replication: Each chromosome duplicates to form two sister chromatids before mitosis.
Cell Cycle Checkpoints:
- G1: DNA damage, cell size, nutrients
- G2: DNA replication completion, DNA damage
- M: Chromosome attachment to spindle

Example: If a human somatic cell (2n = 46) undergoes mitosis, each daughter cell will have 46 chromosomes, identical to the parent cell.

Additional info: The cell cycle is tightly regulated to prevent uncontrolled cell division, which can lead to cancer. Modern cancer therapies increasingly target specific molecules involved in cell cycle regulation.