In Biology, meiosis is the process that transforms one diploid cell into four haploid cells in eukaryotes in order to redistribute the diploid's cell's Genome.
Meiosis forms the basis of Sexual reproduction and can only occur in eukaryotes. In meiosis, the diploid cell's genome, which is composed of ordered structures of coiled DNA called chromosomes, is replicated once and split twice, producing four sets of haploid cells each containing half of the original cell's chromosomes. These resultant haploid cells will fertilize with other haploid cells of the opposite gender to form a diploid cell again. The cyclical process of splitting by meiosis and Genetic recombination through fertilization is called the life cycle. The result is that the offspring produced during Germination after meiosis will have a slightly different blueprint which has instructions for the cells to work, contained in the DNA. This allows sexual reproduction to occur.
Biochemically, meiosis uses many similar processes Mitosis uses in order to manipulate the redistribution of chromosomes, but with a vastly different outcome.
The organism phase of the life cycle can occur between the haploid to diploid transition or the diploid to haploid transition. Some species are diploid, grown from a diploid cell called the Zygote. Others are haploid instead, spawned by the proliferation and differentiation of a single haploid cell called the Gamete. Humans, for example, are diploid creatures. Human stem cells undergo meiosis to create haploid gametes, which are sperm cells for males or ova for females. These gametes then fertilize in the Uterus of the female, producing a diploid zygote. The zygote undergoes progressive stages of mitosis and Differentiation to create an Embryo, the early stage of human life.
There are three types of life cycles that utilise sexual reproduction, differentiated by the location of the organism stage.
In the gametic life cycle, of which humans are a part, the living organism is diploid in nature. Here, we will generalize the example of human reproduction stated previously. The organism's diploid germ-line stem cells undergo meiosis to create haploid gametes, which fertilize to form the zygote. The diploid zygote undergoes repeated cellular division by Mitosis to grow into the organism. Mitosis is a related process to meiosis that creates two cells that are genetically identical to the parent cell. The general principle is that mitosis creates somatic cells and meiosis creates germ cells.
In the zygotic life cycle, the living organism is haploid. Two organisms of opposing gender contribute their haploid germ cells to form a diploid zygote. The zygote undergoes meiosis immediately, creating four haploid cells. These cells undergo Mitosis to create the organism. Fungi and many Protozoa are members of the zygotic life cycle.
Finally, in the sporic life cycle, the living organism alternates between haploid and diploid states. Consequently, this cycle is also known as the Alternation of generations. The diploid organism's germ-line cells undergo meiosis to produce gametes. The gametes proliferate by mitosis, growing into a haploid organism. The haploid organism's germ cells then combine with another haploid organism's cells, creating the zygote. The zygote undergoes repeated mitosis and differentiation to become the diploid organism again. The sporic life cycle can be considered a fusion of the gametic and zygotic life cycles, and indeed its diagram supports this conclusion.
Before division, the genome is replicated. Each chromosome now contains two identical sister chromatids joined together by a region of DNA called the Centromere. Meiosis I, the first round of division, separates homologous chromosomes. Meiosis II, the second round of division, separates sister chromatids. There are four haploid cells produced at the conclusion of meiosis.
This description suggests that two out of four gametes will contain the maternal set of chromosomes, while the other two will contain the paternal set. In reality, however, the gametes are genetically varied, containing a mix of both paternal and maternal genetic information. This is accomplished in two processes. During meiosis I, the concept of Independent assortment is used to redistribute information. Homologous chromosomes will eventually part ways into separate cells. However, homologous chromosomes are oriented independently of their companions. That means that each daughter cell has a fifty-fifty chance of receiving the maternal chromosome or the paternal chromosome. At the same time during meiosis I, when the chromosomes are pairing up together for a short time before being split apart during Synapsis, Chromosomal crossover occurs. During this time, nonsister chromatids of homologous chromosomes may exchange segments at random locations called chiasmata. The chromosome that is subjected to crossing over is then called a recombinant chromosome.
The diagram shown above summarizes the distribution of the chromosomes. Chromosomes which are the same size (one light blue and one red to show parentage) are homologous to each other. They are replicated before meiosis so that each chromosome contains two genetically identical sister chromatids (the vertical bars of the H-like structure). Crossing over occurs between nonsister chromatids of the two homologous chromosomes. Homologous chromosomes are separated in meiosis I. In this case, each daughter cell receives one recombinant mother chromosome and recombinant father chromosome. Meiosis II splits up the sister chromatids. At conclusion, four genetically varied gametes are produced.
Interphase is divided into three phases:
It is immediately followed by meiosis I, which divides one diploid cell into two haploid cells by separation of homologous chromosomes, and meiosis II, which divides two haploid cells into four haploid cells by separation of sister chromatids. Meiosis I and II are both divided into Prophase, Metaphase, Anaphase, and Telophase subphases, similar in purpose to their analogous subphases in the mitotic cell cycle. Therefore, meiosis encompasses the interphase (G1, S, G2), meiosis I (prophase I, metaphase I, anaphase I, telophase I), and meiosis II (prophase II, metaphase II, anaphase II, telophase II).
The zygotene stage sees the completion of synapsis. The combined homologous chromosomes are said to be bivalent, a reference to the two homologous chromosomes. They may also be referred to as a tetrad, a reference to the four sister chromatidsa. During this stage, one percent of DNA that wasn't replicated during S phase is replicated. The significance of this cleanup act is unclear.
The pachytene stage heralds crossing over. Nonsister chromatids of homologous chromosomes exchange segments of genetic information. Because the chromosomes cannot be distinguished in the synaptonemal complex, the actual act of crossing over is not perceivable through the microscope.
During the diplotene stage, the synaptonemal complex degrades. Homologous chromosomes fall apart and begin to repel each other. The chromosomes themselves uncoil a bit, allowing some transcription of DNA. They are held together by virtue of recombination nodules, betraying the sites of previous crossing over, the chiasmata.
Chromosomes recondense during the diakinesis stage. Sites of crossing over entangle together, effectively overlapping, making chismata clearly visible. In general, every chromosome will have crossed over at least once. The nucleoli disappears and the nuclear membrane disintegrates into vesicles.
During these stages, centrioles are migrating to the two poles of the cell. These centrioles, which were duplicated during interphase, function as microtubule coordinating centers. Centrioles sprout microtubules, essentially cellular ropes and poles, during crossing over. They invade the nuclear membrane after it disintegrates, attaching to the chromosomes at the Kinetochore. The kinetochore functions as a motor, pulling the chromosome along the attached microtubule toward the originating centriole, like a train on a track. There are two kinetochores on each tetrad, one for each centrosome.
Microtubulesa that attach to the kinetochores are known as kinetochore microtubules. Other microtubules will interact with microtubules from the opposite centriole. These are called nonkinetochore microtubules.
Cells enter a period of rest known as interkinesis or interphase II. No DNA replication occurs during this stage. Note that many plants skip telophase I and interphase II, going immediately into prophase II.
In metaphase II, the centromeres contain two kinetochores, organizing fibers from the centrosomes on each side. This is followed by anaphase II, where the centromeres are cleaved, allowing the kinetochores to pull the sister chromatids apart. The sister chromatids by convention are now called sister chromosomes, and they are pulled toward opposing poles.
The process ends with telophase II, which is similar to telophase I, marked by uncoiling, lengthening, and disappearance of the chromosomes occur as the disappearance of the microtubules. Nuclear envelopes reform; cleavage or cell wall formation eventually produces a total of four daughter cells, each with an haploid set of chromosomes. Meiosis is complete.
Most importantly, however, meiosis produces genetic variety in gametes that propagate to offspring. By crossing over and independent assortment, the gene pool of the species is dynamic and easily adaptable to changing environments and situations. Without genetic variation, progeny would be identical in traits to their parents, a dangerous weakness in a world where survival of the fittest is very much in effect.
In males, meiosis occurs in precursor cells known as spermatogonia that divide twice to become sperm. These cells continuously divide without arrest in the seminiferous tubules of the testicles. Sperm is produced at a steady pace. The process of meiosis in males is called Spermatogenesis.