Cell Cycle and Division Overview

Study Notes

Cell cycle is the entire sequence of events a cell goes through, from one division to the next.
The cycle includes growth, DNA replication, and preparation for division.
The cycle is made up of interphase (G1, S, G2 phases) and mitotic phase (M phase, which includes mitosis and cytokinesis).

Interphase is the longest part of the cell cycle, lasting 95% of the cycle and has three distinct stages: G1, S, and G2.
G1 Phase: Cell grows, produces proteins, builds energy reserves, performs regular functions, and prepares for DNA replication.
S Phase: DNA replication occurs, chromosomes duplicate to form sister chromatids, and centrosomes replicate.
G2 Phase: Cell continues to grow and produces proteins needed for cell division. Checkpoints ensure DNA replication accuracy and repairs are made. Cell is fully prepared to enter mitosis.

Mitosis is the nuclear division, with four stages.
Prophase: Chromosomes condense, nuclear envelope breaks down, and spindle forms.
Metaphase: Chromosomes align at the center, spindle fibers attach to centromeres.
Anaphase: Sister chromatids are pulled apart to opposite poles.
Telophase: New nuclear envelopes form, chromosomes decondense.
Cytokinesis is the cytoplasmic division where the cytoplasm splits, completing cell division, and forming two identical daughter cells.

Cell differentiation is the process where a less specialized cell becomes a more specialized cell type.
Differentiation is guided by:
- Gene Expression: Specific genes are turned on or off, directing cells to produce proteins that define their structure and function.
- Signaling Molecules: External signals (growth factors, hormones, neighboring cells) influence the differentiation path.
- Epigenetic Modifications: Chemical changes to DNA or histones regulate gene activity without altering the genetic code.

Cells progressively become more specialized.
A stem cell might become a precursor cell (e.g., hematopoietic stem cell). This can lead to fully differentiated cell types such as a red blood cell.

Totipotent (fertilized egg): Can become any cell type in the body, including extra-embryonic tissues.
Pluripotent (embryonic stem cells): Can become many types of cells, but not all cells including extra-embryonic tissues.
Multipotent (hematopoietic stem cells, neural stem cells, mesenchymal stem cells): Can differentiate into a limited range of cell types related to their tissue of origin.
Unipotent (T cell): Can only differentiate into one cell type.

Embryonic Stem Cells (ESCs): Derived from early-stage embryos (blastocysts). They are pluripotent and can become any cell type in the body.
Adult (Somatic) Stem Cells: Found in specific tissues (bone marrow, skin). They are multipotent, differentiating into a limited range of cell types depending on the tissue of origin.
Induced Pluripotent Stem Cells (iPSCs): Created by reprogramming adult cells to an embryonic-like state. They are pluripotent and can become any cell type, similar to ESCs but with ethical advantages.