Mitosis and Meiosis: Cell Division in Eukaryotes

Mitosis

Introduction to Mitosis

Mitosis is a type of cell division that produces two genetically identical daughter cells from a single parent cell. It is essential for growth, tissue repair, and asexual reproduction in multicellular organisms.

• Purpose: To produce new cells that are genetically identical to the original cell.

• Occurs in: Somatic (body) cells of eukaryotes.

• Phases: Prophase, Metaphase, Anaphase, Telophase, followed by Cytokinesis.

Phases of Mitosis

Mitosis consists of four main phases, each with distinct events that ensure accurate division of the cell's genetic material.

• Prophase: Chromosomes condense and become visible. The mitotic spindle begins to form, and the nucleolus disappears.

• Prometaphase (Late Prophase): The nuclear envelope breaks down. Spindle fibers attach to chromosomes at the kinetochore.

• Metaphase: Chromosomes align at the metaphase plate (center of the cell). Spindle fibers ensure each sister chromatid is attached to opposite poles.

• Anaphase: Sister chromatids separate and are pulled to opposite poles of the cell by spindle fibers.

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

• Cytokinesis: Division of the cytoplasm, resulting in two separate daughter cells. In animal cells, a cleavage furrow forms; in plant cells, a cell plate develops.

Key Structures in Mitosis

• Chromosome: A DNA molecule with associated proteins, visible during cell division.

• Centromere: The region where sister chromatids are most tightly connected.

• Kinetochore: Protein structure on the centromere where spindle fibers attach.

• Mitotic Spindle: Microtubule-based structure that separates chromosomes.

Summary Table: Mitosis Phases and Key Events

Phase	Main Events
Prophase	Chromosomes condense, spindle forms, nucleolus disappears
Prometaphase	Nuclear envelope breaks down, spindle attaches to kinetochores
Metaphase	Chromosomes align at metaphase plate
Anaphase	Sister chromatids separate and move to opposite poles
Telophase	Nuclear envelopes reform, chromosomes decondense
Cytokinesis	Cytoplasm divides, two daughter cells form

Example: Mitosis in Animal vs. Plant Cells

• Animal Cells: Cytokinesis occurs via a cleavage furrow that pinches the cell in two.

• Plant Cells: A cell plate forms, eventually developing into a new cell wall.

Meiosis

Introduction to Meiosis

Meiosis is a specialized form of cell division that reduces the chromosome number by half, producing four genetically unique haploid cells (gametes). It is essential for sexual reproduction.

• Purpose: To produce gametes (sperm and egg cells) with half the chromosome number of the parent cell.

• Occurs in: Germ cells of sexually reproducing organisms.

• Phases: Two sequential divisions: Meiosis I and Meiosis II.

Phases of Meiosis

Meiosis consists of two rounds of division, each with its own phases:

• Meiosis I: Homologous chromosomes separate, reducing chromosome number by half.

• Meiosis II: Sister chromatids separate, similar to mitosis.

Meiosis I

• Prophase I: Chromosomes condense, homologous chromosomes pair up (synapsis), and crossing over occurs, exchanging genetic material.

• Metaphase I: Homologous pairs align at the metaphase plate.

• Anaphase I: Homologous chromosomes are pulled to opposite poles; sister chromatids remain attached.

• Telophase I: Chromosomes arrive at poles, and the cell divides (cytokinesis), forming two haploid cells.

Meiosis II

• Prophase II: Chromosomes condense again, new spindle forms in each haploid cell.

• Metaphase II: Chromosomes align at the metaphase plate.

• Anaphase II: Sister chromatids separate and move to opposite poles.

• Telophase II: Chromatids reach poles, nuclear envelopes reform, and cytokinesis produces four haploid cells.

Key Features of Meiosis

• Crossing Over: Exchange of genetic material between homologous chromosomes during prophase I, increasing genetic diversity.

• Independent Assortment: Random orientation of homologous pairs during metaphase I leads to genetic variation in gametes.

Summary Table: Comparison of Mitosis and Meiosis

Feature	Mitosis	Meiosis
Number of Divisions	1	2
Number of Daughter Cells	2	4
Genetic Identity	Identical to parent	Genetically unique
Chromosome Number	Diploid (2n)	Haploid (n)
Role	Growth, repair, asexual reproduction	Sexual reproduction (gamete formation)

Genetic Variation in Meiosis

• Crossing Over: Occurs during prophase I, resulting in new combinations of alleles.

• Random Orientation: Homologous pairs align randomly at metaphase I, leading to many possible combinations of chromosomes in gametes.

Key Terms

• Homologous Chromosomes: Chromosome pairs, one from each parent, that are similar in shape, size, and genetic content.

• Sister Chromatids: Identical copies of a chromosome, connected by a centromere.

• Synapsis: Pairing of homologous chromosomes during prophase I of meiosis.

• Chiasma (plural: chiasmata): Site where crossing over occurs between homologous chromosomes.

Equations

• Number of possible gamete combinations due to independent assortment:

$2^n$

where n is the haploid number of chromosomes.

Example: Human Gametes

• Humans have 23 pairs of chromosomes, so the number of possible combinations due to independent assortment is $2^{23}$, which is over 8 million combinations, not including variation from crossing over.

Additional info: The notes also include diagrams and visual aids to illustrate the stages of mitosis and meiosis, as well as the process of crossing over and independent assortment, which are essential for understanding genetic diversity.