We start with the cell, because so much of what happens to us when we get sick, and how we get healthy again, can be explained by what happens on a cellular level. The cell is extremely complex and I will only touch on the basics in these posts, but at least we can have a rudimentary understanding.
We have discussed cell membranes (May 24), as well as cell organelles (May 31). On June 7, we discussed the cellular transport mechanisms and on June 14 the cell nucleus.As we near the end of this series on the cell, we get to one of the most interesting parts: cell division.
One of the features of a cell is it's ability to reproduce independently. In somatic cell division, a cell undergoes a nuclear division called mitosis. Reproductive cell division is the mechanism that produces gametes. This process consists of a two step division called meiosis. In this post we will look at the process of mitosis. Each of us began life as one cell, a fertilized egg. Each of us now consists of billions of cells produced by the process of mitosis. In mitosis one cell with the diploid number of chromosomes (46, except in chromosomal abnormalities) divides into two identical daughter cells, each with the diploid number of chromosomes. Mitosis is a continuous process compromised of a series of events. This series of events are divided into different stages called: prophase, metaphase, anaphase, telophase and cytokinesis. Prior to these events interphase occurs, which is divided into 3 phases of its own.
Interphase is the period in which the cell spends most of its time performing unique functions.
- G1: Metabolically active cell duplicates organelles and cytosolic components.
- S: The DNA within the nucleus replicates.
- G2: Cell growth, enzyme and protein synthesis continues.
The phases of mitosis are as follows:
- Prophase: The chromosomes coil up and become visible as short rods. Each chromosome is really 2 chromatids (original DNA plus its copy) still attached at a region called a centromere. The nucleus and nucleolus disappear. The centrioles organize the spindle fibers.
- Metaphase: The pairs of chromatids line up. Chromatids are the copies of the chromosonal threads. 92 chromosomes in 46 pairs exist at the equator of the cell.
- Anaphase: Each chromatid is now considered a separate chromosome. The spindle fibers contract and pull the chromosomes, one set toward each pole of the cell.
- Telophase: The chromosomes uncoil and nucleus and nucleolus form. The spindle apparatus is dismantled and a nuclear membrane re-forms around each set of chromosomes.
- Cytokinesis: Cleavage furrow is formed by contracting microfilaments; the cell's cytoplasm is divided by cleavage.
How does this affect you?
Cardiac myocytes' cell cycles stop soon after birth. That means that the heart cannot repair itself after injury. Research is looking at ways to "restart" the cell cycle. For more on this, go to Physiological Reviews.
