Mitosis & The Cell Cycle: Structure, Phases, and Regulation

Mitosis & The Cell Cycle

Overview of the Cell Cycle

The cell cycle is the ordered sequence of events that leads to cell growth and division. It is essential for development, tissue renewal, and reproduction in multicellular organisms. The cell cycle consists of interphase (G1, S, G2) and the M phase (mitosis and cytokinesis).

• G1 phase: Cell grows and carries out normal functions.

• S phase: DNA is replicated.

• G2 phase: Cell prepares for division.

• M phase: Includes mitosis (nuclear division) and cytokinesis (cytoplasmic division).

Example: Human cells typically complete the cell cycle in 10–30 hours, depending on cell type.

Chromosome Structure

Chromosomes are structures that contain genetic material (DNA) and associated proteins. Their organization is crucial for accurate cell division.

• Chromatin: DNA-protein complex; loosely packed in non-dividing cells.

• Chromosome: Highly condensed chromatin visible during cell division.

• Chromatid: Each chromosome consists of two sister chromatids after DNA replication.

• Centromere: Constricted region where sister chromatids are joined; site of kinetochore formation.

• Kinetochore: Protein structure at the centromere; attachment site for spindle microtubules.

Example: Humans have 23 pairs of chromosomes in somatic cells.

Mitosis

Mitosis is the process by which a eukaryotic cell divides its nucleus and genetic material to produce two genetically identical daughter cells. It consists of five continuous phases:

1. Prophase: Chromosomes condense; nucleolus disappears; spindle forms.

2. Prometaphase: Nuclear envelope breaks down; spindle fibers attach to kinetochores.

3. Metaphase: Chromosomes align at the metaphase plate; each chromatid attached to spindle fibers from opposite poles.

4. Anaphase: Cohesion proteins are cleaved; sister chromatids separate and move to opposite poles.

5. Telophase: Nuclear envelope re-forms; chromosomes decondense; spindle disassembles.

Example: Mitosis ensures each daughter cell receives an identical set of chromosomes.

Cytokinesis

Cytokinesis is the division of the cytoplasm, resulting in two separate daughter cells. The mechanism differs between animal and plant cells.

• Animal cells: Formation of a cleavage furrow that pinches the cell in two.

• Plant cells: Formation of a cell plate from vesicles, which develops into a new cell wall.

Example: Cytokinesis typically follows telophase but may overlap with late stages of mitosis.

Cell Cycle Regulation

Cell division is tightly regulated by molecular mechanisms to ensure proper timing and sequence of events. Disruption of regulation can lead to uncontrolled cell division (cancer).

• Checkpoints: Control points where the cell cycle can be halted if conditions are not met (e.g., DNA damage, incomplete replication).

• Growth factors: Proteins that stimulate cell division (e.g., PDGF).

• Cyclins and CDKs: Regulatory proteins that control progression through the cell cycle.

• Cancer: Results from loss of cell cycle control; cells divide uncontrollably, forming tumors.

Example: The G1 checkpoint ensures DNA integrity before replication; the M checkpoint ensures proper chromosome attachment before separation.

Binary Fission (Prokaryotes)

Prokaryotic cells divide by binary fission, a simpler process than mitosis. The cell grows, replicates its DNA, and splits into two identical cells.

• Step 1: Cell grows to appropriate size.

• Step 2: DNA is replicated.

• Step 3: Cell divides, producing two daughter cells.

Example: Bacteria reproduce rapidly by binary fission.

Key Terms Table

Term	Definition	Role in Cell Cycle
Chromosome	Condensed DNA-protein structure	Genetic material distribution
Centromere	Constricted region joining chromatids	Kinetochore formation, spindle attachment
Kinetochore	Protein complex at centromere	Spindle microtubule attachment
Spindle	Microtubule structure	Chromosome movement during mitosis
Cytokinesis	Division of cytoplasm	Formation of two daughter cells
Checkpoint	Control point in cell cycle	Ensures proper progression

Important Equations

• Chromosome Number After Replication:

  • Before S phase: $n$ chromosomes, each with 1 chromatid

  • After S phase: $n$ chromosomes, each with 2 chromatids

• Cell Cycle Duration:

  • Total time = G1 + S + G2 + M

Summary Table: Mitosis Phases

Phase	Main Events
Prophase	Chromosomes condense, spindle forms
Prometaphase	Nuclear envelope breaks down, spindle attaches
Metaphase	Chromosomes align at metaphase plate
Anaphase	Sister chromatids separate
Telophase	Nuclear envelope reforms, chromosomes decondense

Additional info:

• Mutations in chromosome number (e.g., trisomy 21) can result from errors in cell division.

• Polyploidy is the presence of more than two sets of chromosomes, common in plants.

• Meiosis, not covered here, is the process for producing gametes (egg and sperm).