Mitosis
From MyMCAT
Contents |
Introduction
Unlike prokaryotes (bacteria) which reproduce through binary fission, the eukaryotic cell has two choices for how it can divide into progeny. A eukaryotic cell can either divide into two cells each identical to the original, or it can divide into four cells which each only contain half the original genetic material. This first possibility, is called mitosis and is carried it by nearly all the cells in the human body.
The process of dividing the cell into two replicates involves a series of well defined steps known as the mitosis, however prior to these steps, the DNA of the cell must be replicated as each new cell will require a complete copy of the genome. These preliminary actions are part of the general cell cycle. Upon choosing to divide, a cell will grow slightly larger (gathering material and energy necessary for the complex processes ahead), replicate its chromosomes into two sets (one for each child), and then briefly grow again to gather more energy for the complex division process. These steps are known as G1, S, and G2 phase respectively, and lead the cell into mitosis, the actual division process (sometimes referred to as M phase).
Mitosis
Prior to mitosis the cell contains double the DNA it would normally contain as each chromsome has been replicated. At the end of mitosis, the two daughter cells will each have one set of the chromosomes, thus they will contain the same DNA (and quantity) as their parent cell prior to replicating their DNA in S phase. DNA, however, exists inside the nucleus as a mess of chromatin and thus mitosis involves a series of well defined steps each designed to ensure the essential information of the cell, its genetic blueprint, is transfered to its children correctly. These steps can be broken down into prophase, metaphase, anaphase, and telophase.
Prophase
The first step of prophase involves the condensing of DNA. Normally DNA exists in the cell as chromatin (a large mess of loosely bundled DNA and protein), however for mitosis to occur, the DNA is condensed into highly ordered structures known as chromosomes. As a result of the prior DNA replication, each chromosome is in fact two identical copies anchored together at the center to form the classic "X" shape we often see in textbooks. The two halves are known as sister chromatids and are bound at the centromere.
As the chromosomes condense, two centrioles are also formed. These centrioles, a complex of microtubules, protein, and vesicles will later act to pull the cell apart into two halves. The final process occurring in prophase is the destruction of the nuclear membrane. During this process, the envelope separating the nucleus from the surround cytoplasm dissolves, releasing the contents (and the DNA) into the cytosol.
Metaphase
During metaphase, microtubles growing from the centrioles begin attaching and tugging on kinetichores (specific sequences of DNA found repeated throughout the sister chromatids). This attachment and pulling from the two centrioles aligns the chromosomes in a tug-of-war style fashion. At the peak of metaphase, all the chromosomes are roughly aligned along the center of the cell known as the "metaphase plate" with numerous spindles running towards the two centrioles.
Anaphase
When every kinetochore is attached to a cluster of microtubules and the chromosomes have lined up along the metaphase plate, the cell proceeds to anaphase. At this point, two events occur. First, the proteins that bind sister chromatids together are cleaved, allowing them to separate. These sister chromatids, which have now become distinct sister chromosomes, are pulled apart by shortening kinetochore microtubules and move toward the respective centrosomes to which they are attached. Second, some microtubles, which are not bound to kinetochores actually elongate, pushing the centrosomes apart, further separating the two halves of the cell.
At the end of anaphase, the cell has succeeded in separating identical copies of the genetic material into two distinct halves.
Telophase
Telophase is a reversal of prophase and metaphase events. It "cleans up" the after effects of mitosis. In telophase, the nonkinetochore microtubules continue to lengthen, elongating the cell even more. Corresponding sister chromosomes attach at opposite ends of the cell. A new nuclear envelope, using fragments of the parent cell's nuclear membrane, forms around each set of separated sister chromosomes. Both sets of chromosomes, now surrounded by new nuclei, unfold back into chromatin. Mitosis is complete, but cell division is not yet complete.
Occurring in conjunction, but often labeled the final phase of mitosis, is cytokinesis. Cytokinesis is the process of disconnecting the two cells. In animal cells, a cleavage furrow (pinch) containing a contractile ring develops where the metaphase plate used to be, pinching off the separated nuclei. The end of cytokinesis marks the end of the M-phase and the true separation of the cells into twin daughters.
Significance of Mitosis
All bacteria replicate themselves through binary fission, a process similar, but less complicated to mitosis. As such, both binary fission and mitosis lead to two cells which look identical to their parent. Each cell contains the EXACT same set of chromosomes and thus no genetic variation (seen in sexual reproduction) is occurring. In a complex mammal such as a human, nearly all the cells in the body are replicated through mitosis such that every cell contains the set set of genetic blueprints. These cells are known as somatic cells. Meiosis however, occurs in a few cells, known as germ-line cells, to produce the gametes (eggs and sperm in humans) which are necessary for the act of sexual reproduction and thus generating the genetic diversity of the population.
