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.
Here we show how to do this for mitosis, why not try to recreate this table for meiosis? CC Brian Solis, www. Licensed under the terms of CC-BY Meiosis Venn Diagram. The process of cell division is an intricate dance of molecular machinery that has fascinated researchers for hundreds of years. Advances in microscopy have had a huge impact on the field, from its humble beginnings observing metaphase chromosomes under the light microscope, to more sophisticated technologies today that can ask questions at the molecular level.
However, despite our progress, many questions still remain. While there is only one way for mitosis to go right, there are many ways for it to go wrong. For example, in early mitosis, if there are incorrect contacts between microtubules and chromosomes, chromosomes can become misaligned, which can lead to incorrect segregation of sister chromatids. In late mitosis, how is the cell certain that the time is right to perform cytokinesis? The chromosome passenger complex CPC is a molecular guardian angel that acts at many stages of mitosis to safeguard the fidelity of the process.
At the start of mitosis, the CPC localises all over the chromosomes and acts to modify chromatin, during mitosis it moves to the chromosome centromeres to prevent incorrect microtubule attachments and before cytokinesis the CPC finds its way to the central spindle. Therefore, a question of ongoing research is how does the CPC elegantly re-localise throughout mitosis to save the day?
The chromosomal passenger complex: guiding Aurora-B through mitosis. The Journal of cell biology, 6 , A mitosis-specific and R loop—driven ATR pathway promotes faithful chromosome segregation. Science, , You might remember from above that it is the protein cohesin that holds together sister chromatids in metaphase of mitosis and metaphase II of meiosis. However, in meiosis I homologous chromosomes must be held together in metaphase I, before these ties are swiftly broken during anaphase I.
This feat is performed by a miraculous cellular zipper called the synaptonemal complex SC. This zipper must be strong enough to hold chromosomes together, but it must also be disassembled equally efficiently, otherwise homologous chromosomes will not accurately segregate in anaphase I, leading to a potentially disastrous genetic inequality in the daughter cells.
How exactly this zipper disassembles is a hot topic of research. Polo is not solo in meiosis. Cell Cycle, 17 3 , Zipping and unzipping: protein modifications regulating synaptonemal complex dynamics. Trends in Genetics. The time and duration of meiosis.
B, , How long does it take a cell to divide? Cytometry Part A, 87 5 , During leptonema, the diffuse chromatin starts condensing into chromosomes. Each of these chromosomes is double stranded, consisting of two identical sister chromatids which are held together by a centromere; this arrangement will later give each chromosome a variation on an X-like shape, depending on the positioning of the centromere. In the next substage, zygonema, there is further condensation of the chromosomes.
As they come into closer contact, a protein compound called the synaptonemal complex forms between each pair of double-stranded chromosomes. As Prophase I continues into its next substage, pachynema, the homologous chromosomes move even closer to each other as the synaptonemal complex becomes more intricate and developed.
This process is called synapsis, and the synapsed chromosomes are called a tetrad. The tetrad is composed of four chromatids which make up the two homologous chromosomes. During pachynema and the next substage, diplonema, certain regions of synapsed chromosomes often become closely associated and swap corresponding segments of the DNA in a process known as chiasma.
At this point, while still associated at the chiasmata, the sister chromatids start to part from each other although they are still firmly bound at the centromere; this creates the X-shape commonly associated with condensed chromosomes.
The nuclear membrane starts to dissolve by the end of diplonema and the chromosomes complete their condensation in preparation for the last substage of prophase I, diakinesis. During this part, the chiasmata terminalize move toward the ends of their respective chromatids and drift further apart, with each chromatid now bearing some newly-acquired genetic material as the result of crossing over.
Simultaneously, the centrioles, pairs of cylindrical microtubular organelles, move to opposite poles and the region containing them becomes the source for spindle fibers. These spindle fibers anchor onto the kinetochore, a macromolecule that regulates the interaction between them and the chromosome during the next stages of meiosis. The kinetochores are attached to the centromere of each chromosome and help move the chromosomes to position along a three-dimensional plane at the middle of the cell, called the metaphase plate.
The cell now prepares for metaphase I, the next step after prophase I. During metaphase I, the tetrads finish aligning along the metaphase plate, although the orientation of the chromosomes making them up is random. The chromosomes have fully condensed by the point and are firmly associated with the spindle fibers in preparation for the next step, anaphase I. During this third stage of meiosis I, the tetrads are pulled apart by the spindle fibers, each half becoming a dyad in effect, a chromosome or two sister chromatids attached at the centromere.
Assuming that nondisjunction failure of chromosomes to separate does not occur, half of the chromosomes in the cell will be maneuvered to one pole while the rest will be pulled to the opposite pole. This migration of the chromosomes is followed by the final and brief step of meiosis I, telophase I, which, coupled with cytokinesis physical separation of the entire mother cell , produces two daughter cells. Each of these daughter cells contains 23 dyads, which sum up to 46 monads or single-stranded chromosomes.
Meiosis II follows with no further replication of the genetic material. The chromosomes briefly unravel at the end of meiosis I, and at the beginning of meiosis II they must reform into chromosomes in their newly-created cells. This brief prophase II stage [isEmbeddedIn] is followed by metaphase II, during which the chromosomes migrate toward the metaphase plate. During anaphase II, the spindle fibers again pull the chromosomes apart to opposite poles of the cell; however, this time it is the sister chromatids that are being split apart, instead of the pairs of homologous chromosomes as in the first meiotic step.
A second round of telophase this time called telophase II and cytokinesis splits each daughter cell further into two new cells. Each of these cells has 23 single-stranded chromosomes, making each cell haploid possessing 1N chromosomes. As mentioned, sperm and egg cells follow roughly the same pattern during meiosis , albeit a number of important differences. Google Classroom Facebook Twitter. Video transcript - [Voiceover] Before we go in-depth on meiosis, I want to do a very high level overview comparing mitosis to meiosis.
So, in mitosis, this is all a review, if you've watched the mitosis video, in mitosis, we start with a cell, that has a diploid number of chromosomes. I'll just write 2n to show it has a diploid number. For human beings, this would be 46 chromosomes.
Now after the process of mitosis happens and you have your cytokinesis and all the rest, you end up with two cells that each have the same genetic information as the original. So you now have two cells that each have the diploid number of chromosomes. So, 2n and 2n. And now each of these cells are just like this cell was, it can go through interphase again. It grows and it can replicate its DNA and centrosomes and grow some more then each of these can go through mitosis again. And this is actually how most of the cells in your body grow.
This is how you turn from a single cell organism into you, or for the most part, into you. So that is mitosis. It's a cycle. After each of these things go through mitosis, they can then go through the entire cell cycle again. Let me write this a little bit neater. Mitosis, that s was a little bit hard to read.
Now what happens in meiosis? What happens in meiosis? I'll do that over here. In meiosis, something slightly different happens and it happens in two phases. You will start with a cell that has a diploid number of chromosomes.
So you will start with a cell that has a diploid number of chromosomes.
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