Why do cells divide?
The continuity of life is based upon the reproduction of cells. The single-celled amoeba divides to produce an entirely separate organism, while multicellular organisms such as humans arise from single cells via cell division. Furthermore, cells are not immortal and must be renewed, as seen in the creation of new red blood cells in bone marrow. Thus, there are 3 purposes for cell division: reproduction, growth and development, and tissue renewal or repair.
Basic overview of Mitosis
Eukaryotic cell division consists of mitosis (the division of the nucleus) and cytokinesis (the division of the cytoplasm).
Interphase: cell growth and copying of chromosomes in preparation for cell division
- First Gap (G1) represents the first part of interphase during which the cell grows and functions normally.
- DNA begins to replicate and cytoplasm increases in Synthesis (S).
- In Second Gap (G2), the cell checks for DNA damage that may have occurred during S phase and will proceed to Mitosis (M) phase once all necessary proteins are present.
- Centrosomes duplicate, each with two centrioles.
- Chromatin has not condensed yet and is not visible. The nuclear envelope and nucleolus are still present.
Prophase: The first step in Mitosis
- Chromatin fibers condense into discrete chromosomes, each with 2 sister chromatids.
- The nucleolus disappears.
- Mitotic spindle (microtubules) begins to form.
Prometaphase: Chromosomes released from the nucleus
- Nuclear envelope disintegrates.
- Microtubules from the centrosome attach to the chromosomes via the kinetochore (on the centromere).
Metaphase: Chromosomes line up
- Centrosomes are each at opposite poles.
- Chromosomes are guided by the microtubules to the metaphase plate at the equator of the cell.
Anaphase: The shortest stage of Mitosis
The two sister chromatids separate and migrate to opposite poles.
Telophase and cytokinesis: two identical daughter cells are formed
- Nuclear envelope and nucleoli reappear to enclose each group of chromosomes.
- The cell splits into two new cells.
- Cleavage furrow is formed in animal cells; cell plate in plants.
Prokaryotes (bacteria and archea) reproduce by binary fission, in which the chromosome simply replicates and the two chromosomes actively move apart to form two identical organisms. Remember, prokaryotes have no nucleus and have a single circular chromosome that contains their genetic information.
Controlling Cell division
While cell division is an essential part of life, it would be disastrous of cells had no way of regulating this process.
- The cell cycle is regulated largely by the accumulation of cyclin that combines with cyclin-dependent kinase to form MPF. The G2 checkpoint is only passed when there is a sufficient amount of MPF.
- Platelet-derived growth factor (PDGF) promotes cell division by allowing the cell to pass the G1 checkpoint and begin preparing for mitosis. This is used by platelets to clot blood.
- External factors also play a role. Density-dependent inhibition prevents cells from overcrowding, allowing them to only grow as a single layer. Cells may also exhibit anchorage dependence in which they will only divide if attached to a substrate such as tissue.
The loss of control of the cell cycle cause cells to divide abnormally, forming tumors. These cancer cells have no means of regulating cell division and will multiply rapidly.
The ability to reproduce and regulate cell division is essential for the continuity of life.
Sources: Campbell Biology 9th Edition