More than one celled organisms grow by way of mitosis and the cytoplasmic division of body cells. On the other hand, meiosis occurs only in germ cells, which are put aside for the formation of gametes (sperm and egg). Reproduction by meiosis allows for species survival and it increases genetic variability. The process, during which the germ cells are generated is called meiosis. It represents nature’s solution to the problem of chromosome doubling that would occur, if two diploid cells, i. e. two cells with a double set of chromosomes would fuse.
Accordingly does meiosis produce haploid germ cells, with maternal and paternal germ cell fusing at fertilization and thus generating a diploid fusion product, the zygote. Meiosis is made up by two subsequent processes, both of which resemble mitosis. In the first process are the homologous chromosomes separated. It has an unusually long prophase that is subdivided into different stages They are followed by metaphase, anaphase and telophase. Two nuclei fuse upon fertilization, so that the number of chromosomes does necessarily double.
If this thought is spun further, would an greater growth of the number of chromosomes from generation to generation have to be expected. This is not the case, because the chromosomes are reduced to half their normal number in germ cell production. This process is called meiosis. It consists of two successive mitosis-like divisions: in the first division is the number of chromosomes reduced to their half, the second is a normal mitosis. Each germ cell contains a complete set of chromosomes, a haploid set.
Accordingly are the cells haploid and zygotes and the body cells that stem from them are diploid, because they contain two equal sets of chromosomes, one from the mother and one from the father. They exist, especially in plants. At the beginning of meiosis, in prophase 1 the plate breaksdown and chromosomes become visible as in mitosis (1). The chromosomes have replicated but individual chromatids are not visible. Instead of lining up on a metaphase, as in mitosis, chromosomes come together in pairs (2).
Each chromosome in a pair is similar in structure (homologous), but would have come originally from different parents. Later in prophase the homologous pairs twist round each other and chromatids may cross over (3). Breaks occur at these crossovers or chiasmata, and pieces of chromatid are exchanged (4). Chromosome pairs line up across the equator of the spindle at metaphase I (5). In anaphase I the chromosomes separate and travel to opposite ends of the spindle. The chromosomes migrate to the equators of two new spindles for metaphase 2 (7).
Next the chromatids are pulled apart in anaphase 2 to form four clusters of chromosomes in telophase 2. The nuclear envelopes reform around four haploid nuclei that will give rise gamete The leptotene. This phase differs only slightly from the early stages of mitosis. Usually are the cells and nuclei of meiotic tissues bigger than that of their neighbouring tissues and often do they seem to be longer and are longitudinally structured. At regular intervals can thickenings be found, like beads on a string: the chromomeres. Their number, size and positioning is constant in each species.
The zygotene. During this phase begins the pairing of homologous chromosomes. It is also called synapsis and the resulting structure synaptic complex. Directly after initiation of the process does the pairing spread like a zipper across the whole length of the chromosome. The pachytene. During the pachytene does the pairing stabilize. The number of synaptic complexes corresponds to the number of chromosomes in a haploid set of the respective species. The pairs are also called bivalents. The diplotene. The bivalents separate again.
During this does it become visible that each chromosome is built of two chromatids, so that the whole complex stands still, four strands during the separation. Normally is the separation not accomplished, but the homologous chromosomes stick together at certain points, the chaismata. This state is marked by the formation of cross-like structures, single or multipleloops. The diakinesis is the continuation of the diplotene. It is usually difficult to mark an exact limit between both states. The chromosomes condense and become more compact. The metaphase. From now on do the processes resemble that of mitosis again.
The nuclear membrane is completely dissolved, the mitotic spindle fully developed. The homologous chromosomes do still stay together. The anaphase. During anaphase are the homologous chromosomes separated from each other. It is followed by the telophase, then by interkinesis. Shortly afterwards occurs the second mitotic division with the usual states of prophase, metaphase, anaphase and telophase. Here are the chromatids separated from each other. As a result of the meiosis of a diploid cell form four haploid cells, of which one or all can develop into gametes.
Mitosis is the process that facilitates the equal partitioning of replicated chromosomes into two identical groups. Before the breakdown can occur, the chromosomes must become aligned so that the separation process can occur in an orderly fashion. The alignment of replicated chromosomes and their separation into two groups. Both the alignment and separation processes are the consequence of the chromosomes interacting with structures, known as microtubules. The microtubules become organized into an array known as a spindle, which forms early in mitosis, and then breaksdown as mitosis nears completion.
Mitotic spindles are visible in living cells with the polarizing light microscope. Some of the spindle microtubules become attached to the chromosomes at sites known as kinetochores. They reside near the place on the chromosome known as its centromere, which can be observed with the light microscope. There are two kinetochores on each replicated chromosome (one on each chromatid), and when the replicated chromosome splits apart at its centromere at the onset of anaphase, each daughter chromosome possesses one centromere and one kinetochore.
The linkages between kinetochores and microtubules are thought to be central in controlling both the positioning of the replicated chromosome at the central portion of the spindle during the alignment phase, and in moving the daughter chromosomes apart after they split at their centromeres. The separation of daughter cells from each other is a process known as cytokinesis, and is separate from mitosis. In cytokinesis, animal and plant cells differ considerably from each other. These differences are the consequence of having or not having a cell wall.
During prophase, the replicated chromosomes undergo extensive condensation. The chromosomes are greatly thickened and shortened but are still contained within the nuclear envelope. Late in prophase, within some minutes of the nuclear envelope breakdown, the mitotic spindle begins to grow, and two zones become visible, with one on each side of the nucleus. With continued spindle expansion, the nucleus quickly becomes compressed and appears circular. Prophase ends with the sudden dispersion of the nuclear envelope, and the chromozone mass is no longer occupying a discrete, circular shape zone in the cell.
Once the nuclear envelope has broken down, the spindle microtubules and the chromosomes are no longer separated by a membrane limit. The microtubules begin to interact with the chromosomes, and the chromosomes undergo a progressional movement, where they end up with their centromeres all situated in middle of the spindle, at a site known as the metaphase plate. Each kinetochore of the replicated chromosome is pointed toward one side of the spindle; later, in anaphase, each kinetochore moves to one of the two spindle pole regions as the daughter chromosome.
The progress of chromosomes and the alignment of centromeres on the metaphase plate represent essential prerequisites for the orderly separation of the replicated genome into two equal parts. The replicated chromosomes converge toward the center of the spindle, and once they get there, all movements cease. On either side of each centromere are sites for microtubule attachment to the chromosome; electron microscopists called these plate-like structures kinetochores. The kinetochores are not visible with the light microscope. At several points during metaphase, the chromatid arms may unwind from each other.
This unwinding is especially apparent late in metaphase, just 1 or 2 minutes before the chromatids will split apart at their centromeres, with each replicated chromosome giving rise to two daughter chromosomes. Anaphase commences with the initial splitting of sister chromatids at their centromeres. These daughter chromosomes then begin to separate from each other, each moving away from the metaphase plate and toward one of the two spindle pole regions. The mechanisms that control chromosome separation clearly involve the interactions between microtubules and components in or near the kinetochore.
Sometime after anaphase onset, the chromosomes have moved close to the spindle pole regions, and the spindle middle begins to clear. In this middle region of the spindle, a thin line of vesicles begins to grow. The vesicle aggregation event is a harbinger to the assembly of a new cell wall that will be positioned midway along the length of the original cell. It will form the boundary between the newly separating daughter cells. This basket shaped structure forms in late anaphase or early telophase and breaksdown about the time that the vesicles begin to grow.