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You are watching: What is the shortest phase of mitosis
Ibraheem Rehman; Aaishwariya Gulani; Mustafa Farooq; Brittany Simpson.Author Information
The ability to reproduce is one trait that sets living organisms apart from nonliving matter. The flow of life is based on cell division or the reproduction of cells. Cell division can play a different role in different organisms. For example, when a prokaryotic cell generally divides, it has completely reproduced because it gives rise to a new organism. However, in multicellular eukaryotes, mitotic cell division is mostly used for growth and replacement or repair of injured cells. Most cell division results in genetically identical daughter cells. First, a dividing cell replicates its DNA. After a variety of steps, the cell divides via motisis and cytokinesis. Mitosis is one part of the cell cycle, which is detailed below. <1><2><3>
The entirety of a cell’s DNA is called its genome. During cell division, the whole genome is replicated exactly and distributed to 2 daughter cells. A human cell typically has about 2 meters of DNA. Due to the enormous length, the DNA must be highly condensed to fit into the nucleus of each cell. The highly condensed packages of DNA are termed chromosomes when the cell has completed the synthesis phase and is ready to undergo mitosis. Various proteins aid the DNA in folding compactly into subunits of nucleosomes and chromatin. Human somatic cells have 2 sets of 23 chromosomes for a total of 46 chromosomes - 22 sets of autosomes and 1 set of sex chromosomes. A single set of chromsomes is inherited from each parent.<4><5><6> DNA packaging is discussed in a different StatPearls reviews - Genetics, DNA Packaging and Genetics, Histone Code.
The mitotic phase is usually the shortest part of any cell cycle. The largest portion of the cell cycle, interphase, makes up 90% of a cell"s life cycle, and is the stage for growing and performing the cellular functions specific to that cell. The interphase is further divided into two G phases- G1 and G2- and an S phase. During these phases, the cell grows by producing various proteins and cytoplasmic organelles. During the S phase, the cell replicates its genome in preparation for cell division or mitosis. Mitosis occurs during M phase, which occurs after interphase. <7>
Mitosis is conventionally divided into 5 phases, which include prophase, prometaphase, metaphase, anaphase and telophase and cytokinesis. In interphase, a nuclear envelope surrounds the nucleus, the DNA is replicated in the S phase, and the sister chromatids join together at the central portion of the chromosome - the centromere. To organize the chromsome motion in the cell to help make division efficient as well as ensure all material is present in both daughter cells, the cell has centrosomes at each pole of the cell. Centrosomes organize the fibers of the mitotic spindle during mitosis that will help pull the sister chromatids apart.
In prophase, the chromatin fibers condense into chromosomes that are visible through a light microscope, each replicated chromosome appears as two identical sister chromatids joined at their centromeres, and the mitotic spindle begins to form. Also, the centrosomes begin to move to opposite poles of the cell, and they are propelled by the lengthening microtubules between them.
In prometaphase, the nuclear envelope falls apart; microtubules can now invade the nuclear area and bind to some of the chromosomes. The microtubules bind at the kinetochores, specialized protein structures at the centromere. Not all microtubules interact with kinetochores. Some microtubules interact with microtubules extending from the other side of the cell.
In metaphase, the centrosomes have migrated to opposite poles of the cell. The chromosomes have all lined up at the metaphase plate in the middle of the cell, and all chromosomes are attached to microtubules through their kinetochores. The metaphase plate is an imaginary line equidistant from the spindle’s 2 poles.
In anaphase, the shortest stage of mitosis, the sister chromatids break apart, and the chromosomes begin moving to opposite ends of the cell. By the end of anaphase, the 2 halves of the cell have an equivalent collection of chromosomes.
In telophase, 2 daughter nuclei form. The nuclear envelope beings to reappear. DNA begins to de-condense while spindle microtubules begin to depolymerize. Mitosis, the division of one nucleus into 2, is now complete. Lastly, cytokinesis, which is the division of the cytoplasm, takes place and the cell divides into 2 separate cells. In animal cells, this is accomplished through a cleavage furrow that pinches the cell in 2.
Throughout mitosis, certain checkpoints are essential to the continuation of the process. If certain conditions are not met, mitosis halts. If any of these checkpoints are bypassed without being complete, certain pathology, such as cancer, can occur.<8><9>
There are three main checkpoints in mitosis, and those include the G1/S checkpoint, G2/M, and metaphase/ anaphase checkpoint. During the G1/S checkpoint, also known as the restriction checkpoint, primary influencers of cell cycle progression include growth factors, DNA damage, cell size, and cell nutrition. The G2/M checkpoint, also known as the DNA replication checkpoint, is influenced by improper DNA replication or DNA damage. The third checkpoint, also known as the spindle apparatus checkpoint, is inflienced by attachment of the mitotic spindle to all chromosomes. Only when all sister chromatids have been bound will mitosis proceed into anaphase.
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It is important to recognize that cells can progress through the cell cycle in different manners. Cells can withdraw from the active cell cycle, and exist in a non-proliferating or quiescent state. These cells are said to be in the G0 phase. During G1, a cells decide to remain in G1 or leave the active cell cycle and enter into the G0 phase. Different cell types are classified below on their characteristic cell cycle progressions. <11>
Labile cell types are cells that are constantly proliferating, via stem cells, in order to replace cells that have died or sloughed off. Some examples of labile cell types include skin epithelium, gastrointestinal epithelium, salivary gland tissue, and hematopoietic cell types. It is important to recognize that chemotherapy used in cancer treaments has toxic effects against healthy cells, especially cells undergoind rapid renewal.<12>
Quiescent or stable cell types are normally in a non- dividing state, but may enter the cell cycle in respons to certain stimuli. Examples of stable cell types include: lymphocytes, hepatocytes, endothelial cells, and others.
Finally, permanent cell types are unable to proliferate, and are considered non- dividing. Examples of permanent cell types include cardiac and skeletal muscle. <13>