In humans, tissues such as the skin and blood contain cells that are actively dividing , whilst other tissues such as fat contain cells that expand good if you need energy for winter, bad if you are trying to fit into some expensive jeans. Other cells, such as neurons, will never divide again once they are terminally differentiated; they are post-mitotic. In the process of replicating themselves, cells have another choice: do they want to make an identical copy and be left with two cells?
This choice is the choice between mitosis and meiosis. This article will explore the characteristics of both kinds of cell division, shining a light on how they are similar and in which aspects they are crucially distinct. Meiosis and mitosis both have a prophase , metaphase , anaphase , telophase and cytokinesis. In meiosis, prophase, metaphase, anaphase and telophase occur twice.
The first round of division is special, but the second round is more like mitosis. Chromosomes condense and the centrosomes begin to form an early spindle. In metaphase II of meiosis, and metaphase of mitosis, chromosomes line up along the metaphase plate due to the action of microtubule spindle fibres emanating from the centrosomes located at opposite cell poles.
These fibres are attached to the chromosomes by kinetochores at the centromeres of the chromosomes. In anaphase, chromosomes are split to opposite poles of the cell. A nuclear membrane reforms around the newly separated chromosomes, which begin to uncoil, becoming less condense.
The spindle microtubules disassociate. Each daughter cell will inherit one centrosome. The cell plasma membrane pinches , to leave two daughter cells with separate plasma membranes.
Introduction In order for organisms to grow, cells have two options: they must either replicate themselves to create more cells, or the cells themselves must expand in volume. Difference Between Mitosis and Meiosis This article will explore the characteristics of both kinds of cell division, shining a light on how they are similar and in which aspects they are crucially distinct. Mitosis vs. Meiosis: Overview and commonly asked questions.
Setting the scene for mitosis vs. Just like your day has a routine from day to night, cells have routines of their own. The cell cycle is generally described as consisting of four main phases: G1, S phase, G2 and mitosis or meiosis.
Cells can also take a break from the grind of the cell cycle, in a state called G0 or senescence note that some cells are permanently in G0. External growth factors can stimulate cells in G1 or G0 to proceed through the rest of the cycle, an example is Nerve Growth Factor NGF , which promotes neuron growth. There are also internal signals that tell the cell to progress, these proteins are called cyclins and the cyclin that promotes mitosis is called cyclin B.
The stages of mitosis are interphase, prophase, metaphase, anaphase and telophase, sometimes followed by cytokinesis. See our detailed explanation below:. Another way to understand the progression of mitosis and meiosis is by thinking about what is happeningto the chromosomes, centrosomes, nuclear membrane and cell plasma membrane at each stage of the process.
The key event in prometaphase I is the formation of the spindle fiber apparatus where spindle fiber microtubules attach to the kinetochore proteins at the centromeres. Microtubules grow from centrosomes placed at opposite poles of the cell. The microtubules move toward the middle of the cell and attach to one of the two fused homologous chromosomes at the kinetochores.
At the end of prometaphase I, each tetrad is attached to microtubules from both poles, with one homologous chromosome facing each pole. In addition, the nuclear membrane has broken down entirely. During metaphase I, the tetrads move to the metaphase plate with kinetochores facing opposite poles.
The homologous pairs orient themselves randomly at the equator. This event is the second mechanism that introduces variation into the gametes or spores. In each cell that undergoes meiosis, the arrangement of the tetrads is different. The number of variations is dependent on the number of chromosomes making up a set. There are two possibilities for orientation at the metaphase plate. The possible number of alignments, therefore, equals 2n, where n is the number of chromosomes per set.
Given these two mechanisms, it is highly unlikely that any two haploid cells resulting from meiosis will have the same genetic composition. In this case, there are two possible arrangements at the equatorial plane in metaphase I. The total possible number of different gametes is 2n, where n equals the number of chromosomes in a set. In this example, there are four possible genetic combinations for the gametes. In anaphase I, the microtubules pull the attached chromosomes apart.
The sister chromatids remain tightly bound together at the centromere. The chiasmata are broken in anaphase I as the microtubules attached to the fused kinetochores pull the homologous chromosomes apart. In telophase I, the separated chromosomes arrive at opposite poles. In some organisms, the chromosomes decondense and nuclear envelopes form around the chromatids in telophase I. Then cytokinesis, the physical separation of the cytoplasmic components into two daughter cells, occurs without reformation of the nuclei.
In nearly all species of animals and some fungi, cytokinesis separates the cell contents via a cleavage furrow constriction of the actin ring that leads to cytoplasmic division. In plants, a cell plate is formed during cell cytokinesis by Golgi vesicles fusing at the metaphase plate. This cell plate will ultimately lead to the formation of cell walls that separate the two daughter cells. Two haploid cells are the end result of the first meiotic division. The cells are haploid because at each pole there is just one of each pair of the homologous chromosomes.
Therefore, only one full set of the chromosomes is present. Although there is only one chromosome set, each homolog still consists of two sister chromatids. During meiosis II, the sister chromatids within the two daughter cells separate, forming four new haploid gametes.
Meiosis II initiates immediately after cytokinesis, usually before the chromosomes have fully decondensed. In contrast to meiosis I, meiosis II resembles a normal mitosis.
In some species, cells enter a brief interphase, or interkinesis, before entering meiosis II. Interkinesis lacks an S phase, so chromosomes are not duplicated. The two cells produced in meiosis I go through the events of meiosis II together. The mechanics of meiosis II is similar to mitosis, except that each dividing cell has only one set of homologous chromosomes.
If the chromosomes decondensed in telophase I, they condense again. If nuclear envelopes were formed, they fragment into vesicles. The centrosomes that were duplicated during interphase I move away from each other toward opposite poles and new spindles are formed. The nuclear envelopes are completely broken down and the spindle is fully formed.
Meiosis is a process where a single cell divides twice to produce four cells containing half the original amount of genetic information. These cells are our sex cells — sperm in males, eggs in females. Cells are the basic building blocks of living things. The human body is composed of trillions of cells, all with their own specialised function.
A stem cell is a cell with the unique ability to develop into specialised cell types in the body. In the future they may be used to replace cells and tissues that have been damaged or lost due to disease. If you have any other comments or suggestions, please let us know at comment yourgenome.
Can you spare minutes to tell us what you think of this website? If nuclear envelopes were formed, they fragment into vesicles. The centrosomes that were duplicated during interkinesis move away from each other toward opposite poles, and new spindles are formed. The nuclear envelopes are completely broken down, and the spindle is fully formed.
Each sister chromatid forms an individual kinetochore that attaches to microtubules from opposite poles. The sister chromatids are pulled apart by the kinetochore microtubules and move toward opposite poles.
Non-kinetochore microtubules elongate the cell. Figure 1. The process of chromosome alignment differs between meiosis I and meiosis II.
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