Cell Division and the Cell Cycle

Cell division is a crucial biological process that encompasses the life cycle of a cell from its formation to its own division.
Most cell division results in daughter cells containing identical genetic information (DNA), except for meiosis, which is responsible for forming sperm and egg cells.

The cell cycle represents the stages a cell goes through as it grows and divides.
Phases of the Cell Cycle: (MIGSG)
- Mitotic (M) phase: Includes mitosis (the division of the nucleus) and cytokinesis (the division of the cytoplasm).
- Interphase: Spans approximately 90% of the cell cycle, divided into three sub-phases:
- G1 phase: First gap, focusing on cell growth.
- S phase: Synthesis phase where DNA is replicated.
- G2 phase: Second gap, continues cell growth and prepares for mitosis.

Genome: Refers to all the DNA contained within a cell.
DNA exists in the cell as chromatin, a complex of DNA and protein that condenses into chromosomes during cell division.
Somatic cells (non-reproductive) have two sets of chromosomes, while gametes (reproductive cells) have half as many chromosomes as somatic cells.

Each duplicated chromosome consists of two identical sister chromatids connected at a region called the centromere.
The centromere is described as the narrow waist of the duplicated chromosome.
Chromosome depictions are significant in understanding their structure:
- Chromosomal DNA molecules: Comprised of tightly coiled DNA that undergo condensation during cell division.

Mitosis itself is subdivided into five distinct phases:
1. Prophase: Chromatin condenses into chromosomes; nuclear envelope breaks down.
2. Prometaphase: Spindle fibers connect to kinetochores at the centromeres.
3. Metaphase: Chromosomes are aligned at the metaphase plate.
4. Anaphase: Sister chromatids are pulled apart towards opposite poles.
5. Telophase: Nuclear envelopes reform around the separated chromatids.
Cytokinesis: The division of the cytoplasm that overlaps with telophase, resulting in two daughter cells.

The mitotic spindle is a structure made of microtubules that orchestrates chromosome movement during mitosis.
Components:
- Centrosomes: Organizing centers for microtubules.
- Spindle Microtubules: Longer microtubules that chromosomes attach to during division.
- Asters: Shorter microtubules in centrosomes.
- Kinetochore: Protein complex that links centromeres of chromosomes to spindle microtubules.

Experiments demonstrate that microtubules shorten at their kinetochore ends to pull sister chromatids apart during anaphase.

Animal cells: Cytokinesis occurs through the formation of a cleavage furrow and contractile ring of microfilaments.
Plant cells: A cell plate forms during cytokinesis due to vesicles fusing at the center.
- The cell plate eventually develops into a new cell wall separating the two daughter cells.

In prokaryotes such as E. coli, chromosome replication begins at the origin of replication, continuing until two daughter cells are formed.

Mitosis is believed to be derived from the simpler process of binary fission.
Some protists exhibit cell division mechanisms that are transitional between binary fission and mitosis.

The eukaryotic cell cycle is regulated by a molecular control system incorporating chemical signals found in the cytoplasm, including checkpoints where the cycle halts until receiving specific signals.
- G1 Checkpoint: Considered the most critical checkpoint to assess whether the cell proceeds to division.
Cell Cycle Clock Components:
- Cyclins: Proteins that fluctuate in concentration during the cell cycle.
- Cyclin-dependent kinases (Cdks): Enzymes that, when activated by cyclins, drive the cell cycle progression.
- MPF (Maturation-Promoting Factor): A cyclin-Cdks complex allowing the cell to transition past the G2 phase.

Important external control signals for normal cells include growth factors, anchorage dependence, and density-dependent inhibition:
- Anchor Dependence: Requirement for cells to be attached to a substrate for growth.
- Density-Dependent Inhibition: Mechanism of canceling cell growth when a certain cell density is reached.

Cancer cells often bypass normal regulatory mechanisms and checkpoint controls, leading to uncontrolled cell proliferation.
Other characteristics of cancer cells include invasion of neighboring tissues and potential metastasis through blood and lymphatic vessels.
Tumor Development: The formation of tumors typically originates from a single mutated cell that proliferates.
- Malignant tumors have the ability to invade surrounding tissues, forming new blood vessels (angiogenesis).
Colorectal Cancer: Typically arises from a series of mutations in specific tumor-suppressor genes, increasing cancer risk.

Treatments often target rapidly dividing cells:
- Some drugs, such as Vincristine and Vinblastine, prevent microtubules from elongating by blocking polymerization.
- Other treatments, like Taxol, inhibit microtubule depolymerization, affecting the separation of chromatids.

Diagrams and illustrations help depict various phases of mitosis, including the structures involved such as chromosomes and spindle apparatus.
For instance, cells can be observed in different mitotic phases (prophase, metaphase, anaphase, telophase) in an onion root slide for practical understanding.