4.5 Cell Cycle

The Cell Cycle

Overview of the Cell Cycle

The cell cycle is a highly regulated series of events that control the growth and reproduction of cells. It consists of two major processes: interphase (where the cell grows and prepares for division) and mitosis (where the cell divides its nucleus and cytoplasm to form two daughter cells).

• Interphase: The cell grows, replicates its DNA, and prepares for division. It is subdivided into G1, S, and G2 phases.

• Mitosis: The process of nuclear division, followed by cytokinesis (division of the cytoplasm).

Key Point: The cell cycle ensures that genetic material is accurately duplicated and distributed to daughter cells.

Phases of Interphase

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

• S Phase (Synthesis): DNA is replicated, resulting in two complete sets of chromosomes.

• G2 Phase (Second Gap): Cell prepares for mitosis by producing proteins and organelles needed for cell division.

Example: A skin cell in the S phase will duplicate its DNA so that each daughter cell receives a complete genome after division.

Regulation of the Cell Cycle

The cell cycle is regulated by checkpoints that ensure each phase is completed correctly before the next begins. This regulation prevents uncontrolled cell division, which can lead to cancer.

• G1 Checkpoint: Checks for cell size, nutrients, and DNA integrity.

• G2 Checkpoint: Ensures DNA replication is complete and undamaged.

• M Checkpoint: Confirms that all chromosomes are attached to the spindle before mitosis proceeds.

Additional info: Cyclins and cyclin-dependent kinases (Cdks) are key regulatory proteins that control the progression of cells through the cell cycle.

Mitosis

Purpose and Overview

Mitosis is the process by which a eukaryotic cell divides its nucleus and genetic material to produce two genetically identical daughter cells. This process is essential for growth, tissue repair, and asexual reproduction.

Phases of Mitosis

• Prophase: Chromosomes condense and become visible. The nuclear envelope begins to break down, and spindle fibers form.

• Metaphase: Chromosomes align at the cell's equator (metaphase plate).

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

• Telophase: Chromatids reach the poles, nuclear envelopes reform, and chromosomes decondense.

Example: During metaphase, each chromosome is attached to spindle fibers from opposite poles, ensuring equal segregation of genetic material.

Cytokinesis

Cytokinesis is the division of the cytoplasm, resulting in two separate daughter cells, each with a complete set of organelles and genetic material.

• In animal cells, cytokinesis occurs via the formation of a cleavage furrow that pinches the cell in two.

• In plant cells, a cell plate forms to divide the cell.

Summary Table: Phases of the Cell Cycle and Mitosis

Phase	Main Events
G1	Cell growth, normal functions
S	DNA replication
G2	Preparation for mitosis
Prophase	Chromosomes condense, spindle forms
Metaphase	Chromosomes align at equator
Anaphase	Sister chromatids separate
Telophase	Nuclear envelopes reform
Cytokinesis	Cytoplasm divides

Key Terms and Definitions

• Chromosome: A structure of DNA and protein that contains genetic information.

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

• Spindle Apparatus: Structure made of microtubules that separates 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 to divide the cell.

Equations and Concepts

• Cell Division Rate: The rate at which cells divide can be expressed as:

$\text{Cell Division Rate} = \frac{\text{Number of cells after division}}{\text{Time}}$

Additional info: Errors in the cell cycle can lead to uncontrolled cell division and cancer.