Cell Cycle and Mitosis

Introduction to the Cell Cycle

The cell cycle is a fundamental process by which cells grow and divide, ensuring the continuity of life. All cells arise through cell division of pre-existing cells. Observations of developing organisms confirm that plants and animals:

• Start life as single-celled embryos

• Grow through a series of cell divisions

There are two main types of cell division:

• Mitosis: Produces most cells, called somatic cells

• Meiosis: Produces the gametes (egg and sperm)

Mitosis vs. Meiosis

Mitosis and meiosis are distinct processes with different outcomes and roles in organisms.

• Mitosis produces cells that are genetically identical to the original cell and to each other. Genetic material is copied and divided equally between two cells. This process produces the somatic cells that make up most of the organism, including cells for growth and replacement throughout adulthood.

• Meiosis produces cells that are genetically different from the original cell. Genetic material is divided in half into two cells. Meiosis occurs only in specialized reproductive cells and produces gametes (egg and sperm).

12.1 How Do Cells Replicate?

Phases of the Cell Cycle

Cells alternate between phases of the cell cycle:

• Interphase (G1, S, G2): Non-dividing phase

  • Cell grows and fulfills its specialized function

  • Genes on chromosomes are transcribed to make proteins the cell needs

  • Chromosomes are replicated only before proceeding to M phase

• M phase (mitotic or meiotic):

  • Cell separates chromosomes and divides to create two cells

Chromosomes and Genes

Chromosomes are the structures that carry genetic information in the form of DNA. Each chromosome contains a single long double helix of DNA wrapped around proteins called histones. Every species has a characteristic number of chromosomes in each somatic cell (e.g., humans have 46 chromosomes).

• Gene: A specific segment of DNA on a chromosome that codes for a specific mRNA, which is then used to make a specific protein.

Chromosome Replication

Before a cell divides, each chromosome is replicated during the S phase of interphase. The identical copies are called sister chromatids, which are attached to each other at the centromere. Chromosomes condense only when cells begin mitosis.

12.2 What Happens During M Phase?

Overview of Mitosis

Mitosis (M phase) is a continuous process divided into five sub-phases, followed by cytokinesis:

1. Prophase: Chromosomes condense, and spindle apparatus begins to form.

2. Prometaphase: Nuclear envelope breaks down. Microtubules contact chromosomes at the kinetochore.

3. Metaphase: Chromosomes migrate to the middle of the cell (metaphase plate).

4. Anaphase: Sister chromatids separate into daughter chromosomes, which are pulled to opposite poles of the spindle apparatus.

5. Telophase: Nuclear envelope reforms, and chromosomes de-condense.

Cytokinesis follows mitosis, dividing the cytoplasm and forming two daughter cells.

Key Structures in Mitosis

• Centrosome: Organelle that serves as the main microtubule organizing center; contains a pair of centrioles in animal cells.

• Spindle Apparatus: Structure made of microtubules that segregates chromosomes during mitosis.

• Kinetochore: Protein complex at the centromere where spindle fibers attach to pull chromatids apart.

Mechanism of Chromosome Movement

During anaphase, microtubules attached to kinetochores shorten, and motor proteins "walk" the sister chromatids toward opposite poles of the cell.

Cytokinesis

Cytokinesis is the division of the cytoplasm to form two separate daughter cells. In animal cells, a contractile ring of actin and myosin filaments pinches the plasma membrane to create a cleavage furrow, ultimately dividing the cell.

Summary Table: Mitosis Phases and Key Events

Key Terms and Definitions

• Somatic cells: All body cells except gametes

• Gametes: Reproductive cells (egg and sperm)

• Sister chromatids: Identical copies of a chromosome connected at the centromere

• Centromere: Region where sister chromatids are joined

• Kinetochore: Protein structure on chromatids where spindle fibers attach

Example: Human Cell Division

In humans, somatic cells have 46 chromosomes. During mitosis, each chromosome is replicated, and the cell divides to produce two genetically identical daughter cells, each with 46 chromosomes.

Relevant Equations

• Number of chromosomes after S phase: (where is the diploid number)

• After mitosis: Each daughter cell has chromosomes

Additional info:

• Videos and time-lapse resources are available for visualizing cell division in various organisms.