Mitosis & Meiosis Presentation

The cell cycle consists of several stages that lead to cellular division, comprising both interphase and mitotic phases:
- G1 (Gap 1 Phase): Phase where cells replicate cellular contents in preparation for division.
- S (Synthesis Phase): The phase during which DNA is synthesized, resulting in the replication of DNA, creating copies of genetic material.
- G2 (Gap 2 Phase): Final phase before mitosis; during this stage, the cell double-checks for errors in DNA and ensures that everything is ready for the next phase.
- Mitosis/Cytokinesis: The actual division of the cell into two daughter cells.
Interphase: The phase characterized by normal growth and metabolic activity of a cell, where it spends most of its life cycle.
Cell Theory: The fundamental concept in biology which states that all cells arise from pre-existing cells.

Mitosis:
- Definition: A type of cell division that occurs for growth and tissue repair.
- Outcome: Produces diploid cells that are genetically identical (e.g., two copies of each chromosome).
Meiosis:
- Definition: A specific type of cell division that creates gametes (sperm and egg).
- Outcome: Produces haploid cells that are genetically distinct from one another, incorporating genetic variability.

Somatic Cells: These cells are diploid, meaning they contain two sets of chromosomes—one from each parent. Each set contains 23 chromosomes, summing to a total of 46 chromosomes in humans.
Diploid (2N): A term indicating two copies of each chromosome, which applies to somatic cells.
Mitosis Process: Involves the creation of two identical daughter cells, taking place through four distinct stages:
1. Prophase: Chromatin condenses into visible chromosomes, nuclear membrane dissolves, and spindle fibers begin to form.
2. Metaphase: Chromosomes align at the cell's midline, attached to the spindle fibers via their kinetochore.
3. Anaphase: Sister chromatids are separated and pulled toward opposite poles of the cell.
4. Telophase: Chromatids reach the poles, nuclear membrane reforms around the separated chromatid sets, and the chromosomes decondense.

Chromosome: Structures that carry genetic information, made up of DNA and associated proteins.
Homologous Chromosomes: Pairs of chromosomes (one from each parent) that are similar in shape, size, and genetic content.
Sister Chromatids: Identical copies of a chromosome (produced during DNA replication) joined at the centromere.
Centromere: Region on a chromosome where sister chromatids are joined, crucial for proper segregation.
Centrosome: An organelle that provides the mechanical force necessary for moving chromosomes during cell division.
Kinetochore: A protein structure on the centromere where spindle fibers attach during mitosis to pull chromatids apart.

Prophase:
- Chromosomes condense, the nuclear membrane breaks down, and spindle fibers form and attach to the kinetochores on centromeres.
- Centrioles move toward opposite poles of the cell, with microtubules extending from them toward the cell midline.
Metaphase:
- Spindle fibers attach to the kinetochores of sister chromatids aligning them along the midline of the cell.
Anaphase:
- Sister chromatids are pulled apart and move toward opposite poles of the cell.
Telophase:
- The decondensing of chromosomes occurs, nuclear membranes reform around each set of separated chromatids, and spindle fibers disappear.

Definition: The process following mitosis where the cytoplasm divides to form two daughter cells.
In animal cells, a cleavage furrow forms to divide the cell, whereas in plant cells, a cell plate forms, leading to the creation of two genetically identical cells that then enter interphase.

Gametes (sperm and egg) are haploid, meaning they carry one copy of each chromosome (1N).
Meiosis Process: Meiosis results in four genetically distinct daughter cells as opposed to the two identical cells produced in mitosis.

Prophase I: Chromatin condenses, homologous chromosomes pair up (forming tetrads), and crossing over occurs where genetic material is exchanged.
- Processes like crossing over enhance genetic variation and occur at points called chiasmata.
Metaphase I: Homologous pairs align on the metaphase plate; it differs from mitosis as homologous pairs, not sister chromatids, are aligned.
Anaphase I: The homologous chromosomes are pulled apart to opposite poles.
Telophase I: Chromosomes gather into nuclei, and the cell undergoes division without re-entering interphase.

Process: DNA is not duplicated before meiosis II.
Focuses on the separation of sister chromatids rather than homologous chromosomes. Total chromosome number reduces from diploid (2N) to haploid (1N).

After meiosis I, crossing over contributes to genetic diversity.
Meiosis I and II differ fundamentally in what is being separated (homologous chromosomes vs. sister chromatids).
All four resulting haploid cells are genetically distinct due to independent assortment and crossing over.

Example: Mitosis can be observed in onion root tips, where actively dividing cells are accessible.
Recommended technique: Use a 400x high power objective to focus on individual cells at different stages of mitosis.

The key concepts of mitosis and meiosis laid out here illustrate cellular division's role in growth, repair, and genetic diversity.