The Cell Cycle, Mitosis and Meiosis — University of Leicester
In cell biology, mitosis is a part of the cell cycle when replicated chromosomes are separated Mitosis and cytokinesis together define the mitotic (M) phase of an animal .. In plants, this structure coalesces into a cell plate at the center of the . In relation to the forms of mitosis, closed intranuclear pleuromitosis seems to be. Figure %: Telophase. Telophase is technically the final stage of mitosis. Its name derives from the latin word telos which means end. During this phase, the sister. Telophase (CYTOKINESIS OCCURS which is the pinching off of the cell membrane to In plants this structure coalesces into a cell plate at the center of the.
As the amount of cyclin increases, more and more cyclin dependent kinases attach to cyclin signaling the cell further into interphase. The peak of the cyclin attached to the cyclin dependent kinases this system pushes the cell out of interphase and into the M phase, where mitosis, meiosis, and cytokinesis occur.
The Cell Cycle, Mitosis and Meiosis
There are three transition checkpoints the cell goes through before entering the M phase. The most important being the G1-S transition checkpoint. If the cell does not pass this phase, then the cell will most likely not go through the rest of the cell division cycle. Prophase[ edit ] Prophase is the first stage of division. The nuclear envelope is broken down, long strands of chromatin condense to form shorter more visible strands called chromosomes, the nucleolus disappears, and microtubules attach to the chromosomes at the kinetochores present in the centromere.
Chromosomes will also be visible under a microscope and will be connected at the centromere. During this condensation and alignment period, homologous chromosomes may swap portions of their DNA in a process known as crossing over. Metaphase[ edit ] Metaphase is the stage in cell division when the chromosomes line up in the middle of the cell by MTOCs microtubule organizing center by pushing and pulling on centromeres of both chromatids which causes the chromosome to move to the center.
The chromosomes are still condensing and are currently at one step away from being the most coiled and condensed they will be. At this point, the chromosomes are ready to split into opposite poles of the cell towards the spindle to which they are connected.
After the chromosomes line up in the middle of the cell, the spindle fibers will pull them apart. The centrosomes move apart, the spindle forms between them, and the spindle microtubules begin to capture chromosomes.
In some organisms, the centrosomes duplicate between meiosis I and II, even though DNA is not copied during this period. The diagram above, which shows two centrosomes at the start of meiosis II, assumes that the centrosomes were duplicated between meiosis I and II. In other organisms, however, the centrosomes do not duplicate at all between meiosis I and II.
Instead, the two centrioles that make up a single centrosome separate, and each acts as a separate spindle pole during meiosis II.
Mitosis - Wikipedia
The two sister chromatids of each chromosome are captured by microtubules from opposite spindle poles. In metaphase II, the chromosomes line up individually along the metaphase plate.
In anaphase II, the sister chromatids separate and are pulled towards opposite poles of the cell. In telophase II, nuclear membranes form around each set of chromosomes, and the chromosomes decondense. Cytokinesis splits the chromosome sets into new cells, forming the final products of meiosis: In humans, the products of meiosis are sperm or egg cells.
In some cases, meiosis does produce four functional gametes: However, when meiosis takes place during oogenesis, egg cell production, in human females, only one functional egg cell is made. At the end of meiosis I, only one of the two daughter cells continues down the egg cell pathway, while the other becomes a non-egg cell called a polar body.
Similarly, of the two products of meiosis II, one will become a functional egg cell, while the other will become a second polar body.
The separated sister chromatids are now referred to as daughter chromosomes. It is the alignment and separation in metaphase and anaphase that is important in ensuring that each daughter cell receives a copy of every chromosome.
Telophase The final stage of mitosis, and a reversal of many of the processes observed during prophase. The nuclear membrane reforms around the chromosomes grouped at either pole of the cell, the chromosomes uncoil and become diffuse, and the spindle fibres disappear.
Cytokinesis The final cellular division to form two new cells. In plants a cell plate forms along the line of the metaphase plate; in animals there is a constriction of the cytoplasm. The cell then enters interphase - the interval between mitotic divisions. Meiosis Meiosis is the form of eukaryotic cell division that produces haploid sex cells or gametes which contain a single copy of each chromosome from diploid cells which contain two copies of each chromosome.
As in mitosis, meiosis is preceded by a process of DNA replication that converts each chromosome into two sister chromatids.
- The cell cycle and mitosis
- Navigation menu
Meiosis I Meiosis I separates the pairs of homologous chromosomes. In Meiosis I a special cell division reduces the cell from diploid to haploid. Prophase I is divided into five phases: Prometaphase I Spindle apparatus formed, and chromosomes attached to spindle fibres by kinetochores. Metaphase I Homologous pairs of chromosomes bivalents arranged as a double row along the metaphase plate. The arrangement of the paired chromosomes with respect to the poles of the spindle apparatus is random along the metaphase plate.
This is a source of genetic variation through random assortment, as the paternal and maternal chromosomes in a homologous pair are similar but not identical. The number of possible arrangements is 2n, where n is the number of chromosomes in a haploid set.
Human beings have 23 different chromosomes, so the number of possible combinations iswhich is over 8 million.