Mitosis and the Eukaryotic Cell Cycle: Division of Somatic Cells

3.1 Mitosis Divides Somatic Cells

Introduction to Mitosis

Mitosis is a fundamental process in eukaryotic organisms, responsible for the division of somatic (non-reproductive) cells. It is tightly regulated to ensure proper growth, development, and maintenance of tissues and organs. Errors in mitosis can lead to abnormal development or diseases such as cancer.

• Definition: Mitosis is the process by which a cell divides its nucleus and genetic material to produce two genetically identical daughter cells.

• Importance: Ensures faithful transmission of genetic information to successive generations of cells.

• Regulation: Proper regulation is essential; too little division leads to underdevelopment, while excessive division can cause abnormal growths.

• Applications: Tissue repair, growth, and asexual reproduction in multicellular organisms.

The Eukaryotic Cell Cycle

Phases of the Cell Cycle

The cell cycle is the series of events that cells go through as they grow and divide. It consists of two main phases: M phase (mitosis and cytokinesis) and interphase (the period between cell divisions).

• M phase: The stage where the cell divides its nucleus (mitosis) and cytoplasm (cytokinesis).

• Interphase: The longer period between M phases, subdivided into three stages:

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

  • S phase (Synthesis): DNA replication occurs, doubling the genetic material.

  • G2 phase (Gap 2): Cell prepares for mitosis by synthesizing proteins and organelles.

Cell Cycle Table

Phase	Main Events
G1 (Gap 1)	Cell growth, normal metabolism, preparation for DNA synthesis
S (Synthesis)	DNA replication, formation of sister chromatids
G2 (Gap 2)	Preparation for mitosis, synthesis of mitotic proteins
M (Mitosis & Cytokinesis)	Nuclear division (mitosis), cytoplasmic division (cytokinesis)

Cell Cycle Regulation and Variations

Cells may spend varying amounts of time in each phase depending on their type and function. Some cells, such as nerve cells, may exit the cycle and enter a quiescent state called G0, where they no longer divide.

• G0 phase: Non-dividing state; cells perform specialized functions.

• Cell cycle genes: Many genes regulate progression through the cell cycle, ensuring proper timing and fidelity.

DNA Replication and Chromosome Structure

S Phase and Sister Chromatids

During the S phase, DNA replication results in the formation of sister chromatids, which are identical copies of each chromosome joined at the centromere.

• DNA Replication: Each DNA strand serves as a template for the synthesis of a new strand.

• Sister Chromatids: Paired, identical copies of a chromosome, joined at the centromere.

• Result: The nucleus contains twice the normal amount of DNA after S phase.

Equation for DNA Doubling:

$ext{DNA}_{ ext{after S phase}} = 2 imes ext{DNA}_{ ext{before S phase}}$

Mitosis: Stages and Key Processes

Stages of Mitosis

Mitosis is divided into five principal stages, each with distinct events that ensure accurate chromosome segregation and cell division.

• Prophase: Chromosomes condense and become visible; spindle apparatus begins to form.

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

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

• Anaphase: Sister chromatids separate and move toward opposite poles of the cell.

• Telophase: Chromatids reach the poles; nuclear envelopes reform around each set of chromosomes.

Functions of Mitosis

• Karyokinesis: Division of the nucleus and equal partitioning of chromosomes.

• Cytokinesis: Division of the cytoplasm, resulting in two separate daughter cells.

Stages of Mitosis Table

Stage	Main Events
Prophase	Chromosome condensation, spindle formation
Prometaphase	Nuclear envelope breakdown, spindle attachment
Metaphase	Chromosomes align at metaphase plate
Anaphase	Sister chromatids separate to opposite poles
Telophase	Nuclear envelopes reform, chromosomes decondense

Chromosome Condensation and Spindle Formation

During interphase, chromosomes are diffuse and not easily visible. In prophase, chromosomes condense, becoming visible under a microscope. The spindle apparatus forms, facilitating chromosome movement during mitosis.

• Centrosomes: Organelles that organize spindle fibers.

• Spindle fibers: Microtubules that attach to chromosomes and help segregate them.

• Chromosome condensation: Essential for accurate segregation and prevention of DNA damage.

Significance of Mitosis

Genetic Fidelity and Cell Lineages

Mitosis ensures that each daughter cell receives an exact copy of the genetic material, maintaining genetic stability across generations of cells. This is crucial for development, tissue maintenance, and repair.

• Genetic Information: Faithfully transmitted from a single founder cell to all progeny.

• Applications: Growth, wound healing, and asexual reproduction.

Key Terms and Definitions

• Mitosis: Division of the nucleus resulting in two genetically identical daughter cells.

• Somatic cells: All body cells except reproductive cells.

• Interphase: Period between cell divisions, including G1, S, and G2 phases.

• Karyokinesis: Division of the nucleus.

• Cytokinesis: Division of the cytoplasm.

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

• Centrosome: Organelle that organizes spindle fibers during mitosis.

• Spindle apparatus: Structure that segregates chromosomes during mitosis.

Example: Mitosis in Skin Cells

Skin cells regularly undergo mitosis to replace cells lost due to abrasion or injury, ensuring the integrity and function of the skin barrier.

Additional info: The cell cycle and mitosis are highly conserved processes across eukaryotes, with similar regulatory mechanisms found in plants, animals, and fungi.