Study material_Chromosomes+Cell division_Biology by E. Solomon (8th Ed.)

In eukaryotic cells, DNA is wound around specific proteins to form chromatin, which folds to make individual chromosomes.
Cell division is a crucial part of the cell cycle; it is regulated by internal genetic programs and external signals.
Mitosis: Duplicated chromosomes are evenly distributed into two daughter nuclei.
Meiosis: Reduces chromosome sets from diploid (2n) to haploid (n) necessary for sexual reproduction and increases genetic variation.

Pre-existing cells divide to enable growth, repair, and reproduction.
Cells hold coded genetic information in DNA, organized into genes that govern cellular activities and inherited traits.
DNA must be replicated accurately during cell division.

Chromosomes are carriers of genetic information within a cell's nucleus.
Chromatin appears granular when viewed under an electron microscope and condenses during cell division.
Genes provide the information necessary for specific cell functions. Humans have approximately 25,000 genes.

Prokaryotic: Contains a single circular chromosome.
Eukaryotic: Contains multiple chromosomes organized in the nucleus. Eukaryotic DNA is largely more complex and requires proteins (histones) for packaging.

Histones: Proteins that help package DNA into nucleosomes.
- Nucleosomes consist of DNA wrapped around histones.
Condensin: Proteins that compact chromosomes further during cell division.

The cell cycle includes two main phases: Interphase (G1, S, G2) and M phase (mitosis and cytokinesis).
Mitosis ensures each daughter nucleus receives an identical set of chromosomes. It occurs in several stages:
- Prophase: Chromosomes condense and become visible; the nuclear envelope starts to disintegrate.
- Prometaphase: Nuclear envelope breakdown is complete; spindle fibers attach to chromosomes.
- Metaphase: Chromosomes align at the cell's midplane.
- Anaphase: Sister chromatids separate and move to opposite poles.
- Telophase: Chromosomes decondense, nuclear envelopes re-form; cytokinesis occurs.

Animal Cells: Involves a contractile ring that forms a cleavage furrow.
Plant Cells: Form a cell plate from vesicles originating from the Golgi apparatus.

Meiosis reduces the chromosome number and introduces genetic diversity:
- Meiosis I: Homologous chromosomes separate, with crossing over creating genetic variation.
- Meiosis II: Sister chromatids separate, like in mitosis, resulting in four haploid cells.
Meiosis includes two main stages:
- Prophase I: Homologous chromosomes pair up, chiasmata form, and crossing over occurs.
- Metaphase I: Tetrads align at the cell's midplane.
- Anaphase I: Homologous chromosomes separate and move to opposite poles.

Meiosis produces haploid cells with unique genetic combinations vital for sexual reproduction.
Genetic variation arises through crossing over and random assortment of chromosomes during metaphase I.

Asexual Reproduction: Involves unicellular division or fragmentation; offspring are clones of the parent.
Sexual Reproduction: Involves gametes (haploid) fusing to form a zygote (diploid).
Organisms possess either diploid (2n) or haploid (n) cells based on the reproductive strategy.

Controlled by checkpoints and regulatory molecules (e.g., cyclins and cyclin-dependent kinases).
Ensures proper division and managing conditions that may affect cells.

Understanding the processes of mitosis and meiosis is critical for insight into genetics, reproduction, and potential medical applications.