Cell Division and the Cell Cycle

Chromatid: A chromatid is one of the two identical halves of a duplicated chromosome. During cell division, chromatids are separated and distributed to daughter cells.
Chromosome: A chromosome is a structure within cells that contains DNA. In humans, each cell typically contains 46 chromosomes, arranged in 23 pairs.
Homologous Chromosomes: These are pairs of chromosomes, one from each parent, that are similar in shape, size, and genetic content but not identical.
Sister Chromatids: These are identical copies of a single chromosome that are connected by a centromere, formed during the S phase of the cell cycle.

G1 Phase: The cell grows and carries out normal functions; it prepares for DNA replication.
S Phase: DNA replication occurs, resulting in two sister chromatids for each chromosome.
G2 Phase: The cell continues to grow, prepares for mitosis, and checks for DNA errors.
M Phase: This phase includes mitosis (nuclear division) and cytokinesis (cytoplasmic division). Each step of mitosis includes:
- Prophase/Prometaphase: Chromosomes condense and become visible; nuclear envelope breaks down.
- Metaphase: Chromosomes align at the metaphase plate.
- Anaphase: Sister chromatids are pulled apart toward opposite poles of the cell.
- Telophase: Nuclear envelopes reform around each set of chromosomes; chromosomes begin to de-condense.
- Cytokinesis: The cell membrane pinches inwards to form two daughter cells.

Somatic Cells: These are any body cells (not gametes) that contain 46 chromosomes in humans (diploid).
Gametes: These are sex cells (sperm and eggs) that have 23 chromosomes in humans (haploid).

Types of Cell Division:
- Mitosis: Division of somatic cells, resulting in two identical diploid daughter cells.
- Meiosis: Division of gametes, resulting in four non-identical haploid cells.
- Binary Fission: A method of cell division in prokaryotes where a single cell divides into two.

Similarities:
- Both involve DNA replication prior to division and result in cell division.
- Both involve prophase, metaphase, anaphase, and telophase.
Differences:
- Mitosis produces two diploid cells; meiosis produces four haploid cells.
- Mitosis involves one division cycle; meiosis involves two consecutive division cycles (Meiosis I and Meiosis II).

Three Checkpoints:
- G1 Checkpoint: "Is the cell large enough?" "Is the DNA intact?" If no, the cell could enter a resting state (G0).
- G2 Checkpoint: "Is the DNA replicated correctly?" If no, the cell may repair the DNA or undergo apoptosis (programmed cell death).
- M Checkpoint: "Are all chromosomes aligned and attached to the spindle?" If no, the cell cycle is halted until conditions are met.

Structure: Composed of microtubules that form during mitosis.
Purpose: It helps separate sister chromatids and move them to opposite poles of the dividing cell.

Animal Cells: In animal cells, the cleavage furrow forms as the cell membrane pinches inwards due to a contractile ring.
Plant Cells: In plant cells, a cell plate forms down the middle of the cell, developing into a new cell wall.

Effect of Growth Factors: These are proteins that stimulate cell division and growth by binding to specific receptors on the cell surface.
Density-Dependent Inhibition: Cells stop dividing when they become too crowded. Cells detect their density through cellular signaling.
Anchorage Dependence: Cells require a surface to attach to in order to divide; if detached, they will not proliferate.

Differences from Normal Cells:
- Cancer cells grow uncontrollably, ignoring the regulatory signals that normally limit cell division.
- They may evade apoptosis, allowing them to survive longer than normal cells.
- Cancer cells often exhibit abnormal numbers of chromosomes and altered metabolism.