Mitosis and the Cell Cycle

Definition: A fundamental process wherein a single cell divides to form two new daughter cells.

Unicellular Organisms: Depend on cell division for reproduction.
Multicellular Eukaryotes: Depend on cell division for:
- Growth and development
- Repair and renewal

Definition: The life cycle of a cell from formation to its division.
Key Components of the Cell Cycle:
- Mitotic (M) phase: Includes mitosis and cytokinesis, focusing on nuclear and cytoplasmic division.
- Interphase: Involves cell growth and duplication of chromosomes in preparation for division; approximately 90% of the cycle is spent here.

Description: Most cell division results in identical daughter cells.
- Each daughter cell contains the same genetic information as the parent cell, except during meiosis.
Process:
1. Cells duplicate their DNA.
2. Allocate two copies to opposite ends of the cell.
3. Split into two daughter cells.

Definition: The complete set of genetic information of a cell, packaged as DNA.
- Prokaryotic Cells: Contain a single DNA molecule.
- Eukaryotic Cells: Contain multiple DNA molecules.
Chromatin: Long DNA molecules associated with proteins; houses approximately 100-1000 genes.
Functions of Associated Proteins: Maintain structure and control activity.

Somatic Cells: Have two sets of chromosomes (non-reproductive cells); humans possess two sets of 23 chromosomes.
Gametes: Have half as many chromosomes as somatic cells; reproductive cells (sperm and egg) contain one set of 23 chromosomes.

Definition: The division of the nucleus after DNA replication, leading to the formation of two identical nuclei.
Chromosome Condensation: Happens after DNA is replicated.
Sister Chromatids: Refer to duplicated chromosomes held together by cohesins, containing identical copies of DNA.
- The chromosomes further condense, narrowing at the centromere, which facilitates movement during mitosis.

Definition: The division of the cytoplasm that follows mitosis to form two distinct daughter cells.

Main Phases of Mitosis:
- Prophase
- Prometaphase
- Metaphase
- Anaphase
- Telophase

Sub-Phases:
- G1 Phase: First gap where cell growth occurs.
- S Phase: DNA synthesis where chromosomes are duplicated.
- G2 Phase: Second gap where further growth and preparation for mitosis occur.

Chromosomes condense and sister chromatid cohesion occurs.
The nucleolus disappears and the mitotic spindle begins to form.
Centrosomes move away from one another while forming aster arrays of microtubules.

Nuclear envelope fragments.
Kinetochore microtubules attach to kinetochores on chromosomes.
Nonkinetochore microtubules interact with those from opposite ends of the spindle.

Sister chromatids align along the metaphase plate (an imaginary line equidistant from the poles) due to movements by the spindle fibers.

Centromeres divide to separate sister chromatids.
Sister chromatids are pulled towards opposite spindle poles, resulting in an equal distribution of chromosomes.

Nuclear envelope reforms around both sets of chromosomes, which begin to de-condense.
Cytokinesis initiates, leading to the formation of two distinct cells (cleavage furrow in animals, cell plate in plants).

Definition: Composed of microtubules and associated proteins that play a critical role in chromosome movement during mitosis.
Centrosomes: Duplicate during interphase and migrate to opposite ends of the cell.
Protein Complexes: Include kinetochores that attach to spindle microtubules during prometaphase, ensuring proper alignment of chromosomes.
Actions During Mitosis:
- Chromosomes line up at the metaphase plate during metaphase.
- In anaphase, cohesins are cleaved by the enzyme separase, allowing sister chromatids to separate.

Occurs Differently in Cell Types:
- In animal cells: Cleavage furrow forms due to contractile rings of actin and myosin filaments.
- In plant cells: A cell plate forms via vesicles transporting cell wall materials to the center of the cell.

The frequency and timing of cell division varies by cell type (e.g., skin cells divide frequently, while neurons rarely divide).
Cell Cycle Control System: Functions like a clock to regulate progression through the cell cycle based on cell needs and external signals.
- Key checkpoints: G1 checkpoint, G2 checkpoint, M checkpoint.

Most critical checkpoint where a go-ahead signal prompts cell division.
A stop signal halts the cycle, leading to a non-dividing state (G0), where most cells reside.

Controls entry into mitosis based on several factors, including the presence of cyclins and cyclin-dependent kinases (Cdks), which activate enzymes necessary for mitosis.

Ensures all kinetochores are properly attached to the spindle prior to anaphase, preventing errors in chromosome separation.

Growth Factors: Stimulate division (e.g., Platelet-derived growth factor).
Density-Dependent Inhibition: Cells stop dividing when in crowded conditions, promoting communication between cells.
Anchorage Dependence: Cells must be anchored to a substratum to divide.

Defined as cells that do not adhere to normal controls of the cell cycle; they may grow and divide uncontrollably due to various mechanisms (e.g., not requiring external growth factors).
Transformation Process: Conversion of normal cells into cancerous cells, leading to tumor formation.