Cell Division & Cell Cycle: Key Concepts and Processes

Cell Division & Cell Cycle

Big Picture

The cell cycle is the life cycle of a cell, encompassing growth, DNA replication, and division. In eukaryotic cells, the cell cycle consists of two main phases: interphase and mitotic phase. Cell division is essential for growth, repair, and reproduction in multicellular organisms, and for asexual reproduction in single-celled organisms.

Key Terms

• Mitosis: Process in which the nucleus of a eukaryotic cell divides.

• Prophase: First phase of mitosis; chromatin condenses into chromosomes, nuclear envelope breaks down, centrioles separate, and spindle begins to form.

• Metaphase: Second phase of mitosis; chromosomes line up at the equator of the cell.

• Anaphase: Third phase of mitosis; sister chromatids separate and move to opposite poles.

• Telophase: Last stage of mitosis; chromosomes uncoil, nuclear envelope reforms, spindle breaks down.

• Cytokinesis: Splitting of the cytoplasm to form two daughter cells.

• Chromatid: One of two identical copies of a chromosome joined at a centromere.

• Chromosome: Structure made of DNA and proteins containing genetic material.

• Centromere: Region where sister chromatids are joined.

• Homologous Chromosome: Pair of chromosomes with the same size and shape and contain the same genes.

• Mitotic Plate: The invisible line at the equator (center) of the cell where chromosomes align during metaphase.

• Cell Plate: Structure that forms during cytokinesis in plant cells, eventually leading to the plasma membrane and cell wall of two daughter cells.

Cell Cycle

Overview

For prokaryotes, the cell cycle is simple: the cell grows, DNA replicates, and the cell divides. For eukaryotes, the cell cycle is more complex, including two main phases: interphase and mitotic phase.

Interphase

Interphase includes three growth phases:

• G1 (Gap 1): Cell grows and carries out normal functions.

• S (Synthesis): DNA is replicated.

• G2 (Gap 2): Cell prepares for division by producing organelles and molecules required for mitosis.

During interphase:

• Cell grows, replicates DNA, and prepares for division.

• Most of the cell's life is spent in interphase.

• Cancer results when the cell cycle is not regulated.

Mitotic Phase

The mitotic phase consists of mitosis and cytokinesis:

• Mitosis: Multi-step process in which the nucleus divides.

• Cytokinesis: Division of the cytoplasm to form two daughter cells.

Cell Cycle Diagram

The diagram illustrates the stages of the cell cycle, including interphase (G1, S, G2) and mitotic phase (M). The resting phase (G0) is also shown, where cells may exit the cycle.

Phase	Main Events
G1	Cell growth, normal functions
S	DNA replication
G2	Preparation for mitosis
M (Mitosis)	Nuclear division
Cytokinesis	Cytoplasmic division
G0	Resting phase (non-dividing)

Example: Cell Division in Plants vs. Animals

• In animal cells, cytokinesis occurs by cleavage, forming two separate cells.

• In plant cells, a cell plate forms, eventually developing into the cell wall separating the two daughter cells.

Important Equations

• There are no specific equations for the cell cycle, but DNA replication can be represented as: $\text{DNA}_{\text{original}} \rightarrow 2 \times \text{DNA}_{\text{identical copies}}$

Additional info: Cancer is caused by uncontrolled cell division due to mutations in genes regulating the cell cycle.