The Eukaryotic Cell Cycle and Mitosis

The Cell Cycle

Overview of the Cell Cycle

The cell cycle is an ordered series of events involving cell growth and division that produces two new daughter cells. It consists of precisely timed and carefully regulated stages of growth, DNA replication, and division. The cell cycle has two major phases: interphase and the mitotic phase (M phase). During interphase, the cell grows and DNA is replicated. During the mitotic phase, the replicated DNA and cytoplasmic contents are separated and the cell divides.

• Interphase: Cell grows, performs normal functions, and duplicates DNA.

• Mitotic Phase (M phase): Division of the nucleus (mitosis) and cytoplasm (cytokinesis).

Interphase

Subphases of Interphase

Interphase is the period during which the cell undergoes normal processes while also preparing for cell division. It is divided into three stages: G1 phase, S phase, and G2 phase.

G1 Phase (First Gap)

• The cell is active and accumulating building blocks of chromosomal DNA and associated proteins.

• It is also gathering energy reserves needed for DNA replication.

• Little visible change occurs, but the cell is metabolically active.

S Phase (Synthesis Phase)

• DNA replication occurs, resulting in the formation of two identical copies of each chromosome (sister chromatids).

• Centrosomes are also duplicated, which are important for chromosome movement during mitosis.

G2 Phase (Second Gap)

• The cell replenishes energy stores and synthesizes proteins necessary for chromosome manipulation.

• Some cell organelles are duplicated, and the cytoskeleton is dismantled to provide resources for mitosis.

• Final preparations for the mitotic phase are completed.

G0 Phase (Quiescent Stage)

• Some cells exit the cell cycle and enter a resting state called G0.

• Cells in G0 are not actively preparing to divide; some may remain in G0 permanently (e.g., mature muscle and nerve cells).

The Mitotic Phase

Mitosis

Mitosis is the process by which the nucleus divides, resulting in two genetically identical daughter nuclei. It is divided into several phases: prophase, prometaphase, metaphase, anaphase, and telophase.

• Prophase: Chromosomes condense and become visible; spindle fibers begin to form.

• Prometaphase: Nuclear envelope breaks down; spindle fibers attach to kinetochores on chromosomes.

• Metaphase: Chromosomes align at the metaphase plate (center of the cell).

• Anaphase: Sister chromatids are pulled apart toward opposite poles of the cell.

• Telophase: Chromosomes decondense; nuclear envelopes reform around the two sets of chromosomes.

Example: In animal cells, mitosis ensures that each daughter cell receives an identical set of chromosomes, maintaining genetic continuity.

Cytokinesis

Cytokinesis is the division of the cytoplasm, which usually begins during or after telophase. The process differs between animal and plant cells:

• Animal Cells: A contractile ring forms a cleavage furrow that pinches the cell in two.

• Plant Cells: Vesicles from the Golgi apparatus coalesce at the center of the cell to form a cell plate, which develops into a new cell wall separating the two daughter cells.

Control of the Cell Cycle

Regulation at Internal Checkpoints

The cell cycle is regulated by internal checkpoints that ensure each phase is completed accurately before the next begins. These checkpoints help prevent mutations and ensure the fidelity of cell division.

Checkpoint	Main Function	Location in Cell Cycle
G1 Checkpoint	Determines if conditions are favorable for cell division and if DNA is undamaged	End of G1 phase, before S phase
G2 Checkpoint	Ensures all chromosomes have been replicated and DNA is not damaged	End of G2 phase, before mitosis
M Checkpoint (Spindle Checkpoint)	Ensures all sister chromatids are correctly attached to spindle microtubules	Near end of metaphase, before anaphase

Summary of Checkpoints

• G1 Checkpoint: Cell commits to division if conditions are right; otherwise, it may enter G0.

• G2 Checkpoint: Cell checks for DNA damage and completion of replication.

• M Checkpoint: Cell ensures all chromosomes are properly attached to the spindle before proceeding with chromatid separation.

Key Terms and Concepts

• Chromosome: A DNA molecule with part or all of the genetic material of an organism.

• Sister Chromatids: Two identical copies of a chromosome connected by a centromere.

• Centrosome: Organelle that serves as the main microtubule organizing center and is important for spindle formation.

• Spindle Fibers: Microtubules that separate chromosomes during mitosis.

• Cleavage Furrow: Indentation that begins the process of cytokinesis in animal cells.

• Cell Plate: Structure that forms in plant cells during cytokinesis, leading to the formation of a new cell wall.

Equations and Diagrams

• DNA Replication: Each chromosome duplicates to form two sister chromatids: $\text{1 chromosome} \xrightarrow{\text{S phase}} \text{2 sister chromatids}$

Additional info: The above notes expand on the cell cycle, mitosis, and checkpoint regulation, providing definitions, examples, and a summary table for clarity.