In-depth Notes on the Cell Cycle

Definition: The cell cycle encompasses the life of a cell from its formation until it divides, integral to processes like reproduction, growth, and repair of tissues.

DNA organization occurs before cell division to ensure effective replication and separation.
Histones: Proteins that DNA wraps around to form nucleosomes.
Nucleosomes: Basic unit of DNA packaging.
Chromatin: A form of DNA that is less condensed when the cell is not dividing.
Chromosomes: Formed from condensed chromatin post-DNA replication, allowing easier organization for cell division.

Sister Chromatids: Two identical copies of a chromosome connected at the centromere.
- Centromere: Region where sister chromatids are joined most tightly.
- Kinetochore: Protein complex that attaches sister chromatids to the mitotic spindle.

Genome: Total genetic material within a cell.
- Prokaryotes: Contain one circular DNA molecule.
- Eukaryotes: Have multiple linear chromosomes (e.g., Humans: 46 chromosomes).
Homologous Chromosomes: Chromosomes with the same structure and gene sequence, one inherited from each parent.

Somatic Cells: Body cells that are diploid (2n) carrying two sets of chromosomes (e.g., Humans: 2n=46).
Gametes: Reproductive cells that are haploid (n) with one set of chromosomes (e.g., Humans: n=23).

The cell cycle consists of alternating interphase and mitotic phase (M phase).
Interphase: 90% of the cycle, with three phases:
- G1: Gap phase, normal functions and growth.
- S: DNA synthesis and chromosome duplication.
- G2: Second gap for final growth and preparation for mitosis.
M Phase: Includes mitosis (nuclear division) and cytokinesis (cytoplasm division), resulting in two diploid daughter cells.

Prophase: Chromatin condenses, nucleoli disappear, and mitotic spindle forms.
Prometaphase: Nuclear envelope breaks down; microtubules attach to kinetochores.
Metaphase: Chromosomes align at the metaphase plate; microtubules are attached to kinetochores.
Anaphase: Sister chromatids separate and move to opposite poles; cell elongates.
Telophase/Cytokinesis: Nuclei form at each pole, nucleoli reappear; chromosomes de-condense; cytokinesis occurs (cleavage furrow in animals, cell plate in plants).

Chromosome Count: Human cells (2n=46) track chromatid arrangements through various mitosis phases:
- Prophase: 46 chromosomes, 92 chromatids
- Metaphase: 46 chromosomes, 92 chromatids
- Anaphase: 92 chromosomes, 92 chromatids
- Telophase: 46 chromosomes (in each daughter cell).

Checkpoints: Ensure proper progression through the cell cycle, reacting to internal and external signals.
G1 Checkpoint: Assesses cell size, growth factors, and DNA integrity. A "go" signal allows the cycle to continue; a "stop" signal leads to quiescence (G0 phase).
- G0 Phase: Cells can remain inactive indefinitely or be re-activated.
G2 Checkpoint: Verifies DNA replication and repairs DNA damage before mitosis.
M (Spindle) Checkpoint: Checks microtubule attachment at kinetochores; ensures the cell only progresses through mitosis if all chromosomes are properly attached.

Cyclins: Vary in concentration across the cell cycle and regulate cyclin-dependent kinases (CDKs).
CDKs: Consistent concentrations that become active only in combination with specific cyclins, phosphorylating target proteins to advance cell cycle events.

Growth Factors: Hormones stimulating cell growth, activating CDK pathways.
Contact Inhibition: Prevents the cell cycle from progressing if cells are in contact with each other.
Anchorage Dependence: Cells require attachment to other cells or the extracellular matrix to divide.

Cancer arises from mutations in DNA affecting growth regulation, often leading to uncontrolled division and evasion of apoptosis.
- Tumors: Abnormal cell masses; benign (localized) vs. malignant (invading other tissues).
Prevention: Healthy lifestyle, avoiding carcinogens, and regular screenings.

Vinblastine: Chemotherapeutic agent that inhibits microtubule formation, impacting mitosis phases and effectively arresting the cell cycle.