Time for “The Talk”
Cellular growth and division is a simple fact of life itself. Controlled at the molecular level and via secreted materials, growth and division are usually maintained with precision, unless a cell or group of cells begins to divide out of control, which leads to the formation of a tumor.
Cell Cycle
A typical cell will spend the majority of its cell cycle—the process of growth and replication—providing an essential function to the tissues and organs where it resides. This stage of the cell cycle is called interphase. In this beginning stage, the cell grows to its final size and may remain in this static, functional state until it prepares to divide again. At this stage, called mitosis, the cell divides its chromosomes and nucleus. The cell cycle finishes with cytokinesis, the division of the cytoplasm, resulting in 2 daughter cells identical to the single parent cell from which they were produced. The resulting cells begin the process of interphase and go through the cell cycle all over again.
Interphase: G1
Once mitosis and cytokinesis are complete, each daughter cell enters the first phase of interphase: the gap 1 (G1) phase. Here, most cells increase in size, replicate essential organelles, and move the nucleus more toward the center of the cell. At the end of the G1 phase, the cell checks to be sure that the process of replication has begun without errors. If there is a problem, cell division will be halted and the cell will attempt to repair the problem.
Interphase: S
The synthesis (S) phase follows G1 and is the period when the chromosomes are duplicated so that each daughter cell can have a complete set of chromosomes.
Interphase: G2
In the gap 2 (G2) phase, the cell begins to prepare for mitosis. During this time, the cell produces and organizes all of the structures and materials essential for mitosis. The most important point is the G2-M transition. Here, the cell size, DNA replication, and DNA damage are checked before the cell continues the process of replication.
Mitosis: M
During the M, or mitotic phase, a parent cell is cloned into 2 daughter cells. Each human parent cells possess 46 chromosomes in 23 pairs. Each resulting daughter cell will be a clone of the parent with exactly the same 46 chromosomes.
Anatomy of a Word
mitosis
Mitosis is strictly defined as the process by which the chromosomes in a cell are duplicated and separated into their own nuclei.
During prophase, the first phase of mitosis, the sister chromatids (a chromatid is one of a pair of duplicated chromosomes), which were formed during the S phase of interphase, are condensed and become more coiled in preparation for cell division. Additional changes occur in the cell during this phase, such as the nuclear membrane beginning to disintegrate.
An intermediate period, prometaphase, is often considered late prophase. During this time, microtubules called spindle fibers, which stretch from each pole of the cell toward the opposite pole, are organized. The spindle fibers serve two critical functions during cell division. First, fibers from each pole connect to each side of the sister chromatids to move the chromatids to the middle of the cell. Other spindle fibers function to push the poles of the cell farther apart in preparation for the division of the cytoplasm in the last stage of mitosis.
The most often illustrated phase of mitosis, metaphase, is the point when chromosomes become attached to the spindle fibers that were previously produced. It is recognizable because of the alignment of all 46 sister chromatids at the equator of the cell.
The shortest of the phases of mitosis, anaphase is characterized by the pulling apart of the sister chromatids into 46 individual and identical chromosomes that are being moved in opposite directions.
Anaphase continues until the chromosomes arrive at the poles, which signals the start of telophase. At this time, the chromosomes begin to rapidly relax and uncoil, the nuclear membrane begins to re-form, and many of the spindle fibers disappear. This concludes the division of the nucleus, which coincides with the initiation of cytoplasmic division, or cytokinesis.
Cytokinesis
Near the end of telophase, proteins called actin begin to form a belt that extends around the equator of the cell. As cytokinesis continues, the actin ring becomes smaller and smaller, resulting in a narrowing at the waist of the cell called the cleavage furrow. This constriction continues until the 2 resulting daughter cells are pinched away from each other into independent yet identical cells.
Meiosis
Not all human cells divide by mitosis. During sexual reproduction, sperm and egg cells must be divided in half, so that each cell contains only 23 chromosomes rather than 46. This process of division is called meiosis. Thus, while mitosis is often referred to as cloning, meiosis is termed a reduction division.
Why do sperm and egg cells contain only 23 chromosomes?
When a sperm cell containing 23 chromosomes unites with an egg cell containing 23 chromosomes, they combine to form 46 chromosomes. The resulting new individual will have the final complete amount of genetic material. If each sperm and egg cell had the full complement of 46 chromosomes, the combined genetic material would result in an individual with 92 chromosomes.
To complete this reduction of genomic material (“genome” meaning “the genetic material of a cell”) the cell undergoes 2 divisions, each given a name followed by the division number (e.g., prophase I, metaphase I). In meiosis I, the 46 chromosomes are divided so that each half now has 23 chromosomes.
The second division is identical to mitosis in that the daughter cells from the first division will now have their sister chromatids pulled apart. The only difference between this division and mitosis is that the starting material for meiosis II are 23 sister chromatids (in mitosis there are 46). Thus after the completion of 2 divisions, 4 daughter cells will result.