As the new chromosomes reach the spindle during telophase I , the cytoplasm organizes itself and divides in two. There are now two cells, and each cell contains half the number of chromosomes as the parent cell. In addition, the two daughter cells are not genetically identical to each other because of the recombination that occurred during prophase I Figure 4.
At this point, the first division of meiosis is complete. The cell now rests for a bit before beginning the second meiotic division. During this period, called interkinesis , the nuclear membrane in each of the two cells reforms around the chromosomes. In some cells, the spindle also disintegrates and the chromosomes relax although most often, the spindle remains intact. It is important to note, however, that no chromosomal duplication occurs during this stage. What happens during meiosis II?
Prophase II. As prophase II begins, the chromosomes once again condense into tight structures, and the nuclear membrane disintegrates.
In addition, if the spindle was disassembled during interkinesis, it reforms at this point in time. Metaphase II. Figure 5: During metaphase II, the chromosomes align along the cell's equatorial plate. The events of metaphase II are similar to those of mitotic metaphase — in both processes, the chromosomes line up along the cell's equatorial plate, also called the metaphase plate, in preparation for their eventual separation Figure 5.
Anaphase II. Figure 6: Anaphase II involves separation of the sister chromatids. During anaphase II , microtubules from each spindle attach to each sister chromatid at the kinetochore. The sister chromatids then separate, and the microtubules pull them to opposite poles of the cell. As in mitosis, each chromatid is now considered a separate chromosome Figure 6. This means that the cells that result from meiosis II will have the same number of chromosomes as the "parent" cells that entered meiosis II.
Telophase II. Figure 7: Telophase II results in the production of four daughter cells. Finally, in telophase II , nuclear membranes reform around the newly separated chromosomes, which relax and fade from view.
As soon as the cytoplasm divides, meiosis is complete. There are now four daughter cells — two from each of the two cells that entered meiosis II — and each daughter cell has half the normal number of chromosomes Figure 7. Each also contains new mixtures of genes within its chromosomes, thanks to recombination during meiosis I. Why is meiosis important? More about meiosis.
Genes are packaged differently in mitosis and meiosis — but what is the effect of this difference? What else can go wrong with chromosomes in meiosis? Meiosis can be divided into nine stages. These are divided between the first time the cell divides meiosis I and the second time it divides meiosis II :. Illustration showing the nine stages of meiosis.
Image credit: Genome Research Limited. Cells are the basic building blocks of living things. The human body is composed of trillions of cells, all with their own specialised function. Chromosomes are bundles of tightly coiled DNA located within the nucleus of almost every cell in our body. Humans have 23 pairs of chromosomes. Mitosis is a process where a single cell divides into two identical daughter cells cell division. DNA or deoxyribonucleic acid is a long molecule that contains our unique genetic code.
Like a recipe book it holds the instructions for making all the proteins in our bodies. Cells divide and reproduce in two ways, mitosis and meiosis. Mitosis results in two identical daughter cells, whereas meiosis results in four sex cells. Below we highlight the keys differences and similarities between the two types of cell division. 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? Open survey. In: Facts In the Cell. Actually no daughter cells are produced by meiosis.
Meiosis produces four half cells or sex cells haploid , that are used in sexual reproduction. Sex cells are haploid so that when the sperm and ovum fuse, a diploid zygote is formed. None of the four half cells are identical to the "mother" cell or to each other. Meiosis in producing differences in genetic make up creates the vast variations found in populations. That is done by independent assortment and by chiasmata of non-sister chromatids.
The dividing cell may spend more than 90 percent of meiosis in Prophase I. Because this particular step includes so many events, it is further subdivided into six substages, the first of which is leptonema. 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.
0コメント