3.5|Cell Growth and Division

Most somatic cells in the body undergo regular division, while a few specific cells (e.g., gametes, red blood cells, most neurons, and some muscle cells) do not.
Somatic Cells: General term for body cells; include all human cells except for germ cells (cells that produce eggs and sperm).
Somatic cells contain two copies of each chromosome—one from each parent.
Homologous Pair of Chromosomes: Two copies of a single chromosome found in each somatic cell.
Humans are diploid organisms with 23 homologous pairs of chromosomes (total 46 chromosomes).
The condition of having pairs of chromosomes is known as Diploidy.
Cells are continuously replaced throughout a person's lifetime; for instance, cells lining the gastrointestinal tract must frequently be replaced due to wear from food movement.

Definition: The sequence of events in a cell's life from its creation until it divides.
The cell cycle consists of two general phases:
- Interphase: The period when the cell is not dividing; it includes G1, S, and G2 phases.
- Mitosis and Cytokinesis: The phase where cell division occurs.

G1 Phase (Gap 1): The first growth phase where the cell grows and performs normal metabolic functions.
S Phase (Synthesis): The period in which DNA is replicated; each chromosome is duplicated, resulting in sister chromatids.
- After DNA replication, a human cell contains 92 chromatids (46 × 2).
G2 Phase (Gap 2): The second growth phase where the cell continues to grow and prepares for mitosis.
G0 Phase: A resting phase where cells cease dividing temporarily or permanently (e.g., neurons).

Each chromosome consists of two identical sister chromatids connected at the Centromere. The structure is essential for proper distribution during cell division.
The existence of homologous pairs (inherited from each parent) must not be conflated with pairs of chromatids during mitosis.

Total duration of mitosis ranges from 1 to 2 hours and consists of the following stages:
- Prophase: Chromatin condenses into visible chromosomes, nucleolus disappears, and nuclear envelope disintegrates. Centrosomes (pairs of centrioles) move apart, the mitotic spindle forms, and microtubules attach to centromeres.
- Metaphase: Sister chromatids align at the metaphase plate. Microtubules are attached and ready to separate the sister chromatids.
- Anaphase: Sister chromatids are pulled apart, forming individual chromosomes again. They are drawn to opposite cell ends.
- Telophase: New daughter nuclei form around genetic material, chromatids uncoil back to chromatin, nucleoli reappear, and the mitotic spindle breaks apart.
Cytokinesis: The division of cytoplasm occurs; a cleavage furrow forms through the action of microfilaments, splitting the cell into two new cells, one of which remains a stem cell while the other becomes a functional cell of the tissue.

A complex regulation system controls the cell’s progression throughout the cell cycle and mitosis, involving internal and external signals that provide “stop” and “advance” cues.
Checkpoint: A point in the cell cycle that can signal the cycle to either move forward or stop. Specific molecules provide these signals.
Cyclins: Primary classes of cell cycle control molecules that determine progression at checkpoints.
Cyclin-dependent kinase (CDK): Enzymes that work alongside cyclins to facilitate progression through checkpoints.
Specific checkpoints include:
- G1 Checkpoint: Cell readiness for DNA synthesis.
- G2 Checkpoint: Cell readiness for mitosis.
- Metaphase Checkpoint: Ensures all sister chromatids are properly attached to the spindle prior to separation.

Cancer is largely caused by abnormal cells that divide continuously due to failures of cell cycle control mechanisms. These can lead to tissue damage, metastasis, and ultimately death.
Cell cycle regulation prevents continuous, uncontrolled division. Factors such as genetic mutations and environmental influences can disrupt regulatory mechanisms, exacerbating cancer risk.
Benign Tumor: A tumor that does not pose a threat to surrounding tissues and can be easily removed.
Malignant Tumor: A cancerous tumor capable of damaging surrounding tissues; diagnosed as cancer.

Cancer results from genetic mutations that affect normal cell cycle control systems, disrupting the balance of signals that regulate progression through the cell cycle.
Proto-oncogenes: Genes that signal progression in the cell cycle; when mutated, they can turn into oncogenes which propel unwanted cell growth.
Tumor Suppressor Genes: Genes that send stop signals, halting cell division when necessary.
A delicate homeostatic balance exists between proto-oncogenes (accelerators) and tumor suppressor genes (brakes). Disruption of this balance leads to aberrant cell division and potential cancer development.