Cell Cycle and Mitosis Overview

Study Notes

A human, like all sexually reproducing organisms, begins as a fertilized egg (zygote). Trillions of cell divisions, controlled, produce multicellular organisms. Single-celled organisms use cell division for reproduction.

The cell cycle is a series of events, describing a cell's life from one parent cell to two identical daughter cells. This cycle is highly regulated.

A cell's genetic information (genome) is DNA, packaged as double-stranded DNA molecules. This is called the cell's genome.
In prokaryotes, the genome is a single circular double-stranded DNA molecule within a nucleoid.
Some prokaryotes also have smaller DNA loops (plasmids) that aren't essential, but can carry beneficial genes, like antibiotic resistance. Bacteria can exchange plasmids.

DNA in eukaryotes (like humans) would measure meters if stretched out, but has to be compact for cells (approximately 10µm or 100,000 cells to equal one meter). The packaging allows for accessibility and expression of genes
DNA coils around histone proteins, forming nucleosomes. These nucleosomes coil into chromatin fibers, which further condense, becoming chromosomes.
Human somatic cells have 46 chromosomes, which are homologous pairs.
Gametes (sex cells) have 23 chromosomes each, representing one set of chromosomes (n). Thus, humans are considered 2n organisms (diploid).

The mitotic phase (M phase) includes mitosis and cytokinesis.
Mitosis is a series of phases: prophase, prometaphase, metaphase, anaphase, and telophase. Each one results in a division of the nucleus.
In animals, the division occurs at the midpoint (the metaphase plate). In plants, the new cell wall forms between daughter cells.

During interphase, cells grow and prepare for division:
- G1 phase (first gap): Cell grows, accumulating building blocks and energy for replication and protein synthesis.
- S phase (synthesis): DNA replication occurs.
- G2 phase (second gap): Cell replenishes energy and synthesizes proteins required for mitosis.

Internal controls prevent compromised cells from dividing. These checkpoints are critical control points (G1, G2, and M checkpoint) within the eukaryotic cell cycle. These checkpoints can pause or halt the progression of a cell into the next stage until certain conditions are met.
The G1 checkpoint assesses cell size and protein reserves.
The G2 checkpoint ensures all DNA has been replicated and is undamaged.
The M checkpoint checks for accurate chromosome attachment to the spindle fibers before proceeding.

Some cells, like mature muscle and nerve cells, permanently exit the cell cycle and enter a quiescent state called G0. They're not actively dividing but can be active in other ways.