The Leptotene stage starts with the chromatin fibers condensing into thread-like-fibers that resemble the formed structure at the beginning of mitosis.
The zygotene stage includes further condensation of the fibers that enables them to be distinguished as individual chromosomes. As a result of synapsis, the bivalents form when the pairs of chromosomes become tightly paired together. See figure 4. The formation of bivalent is critically important in the process of the exchange of the DNA segments containing the genetic material between the two close chromosomes in a process known as crossing over.
This process takes place during the pachytene stage. The corresponding segments of chromosomes exchange genetic information for the recombination of genes.
Want more biology facts on homologous chromosome and sister chormatids? Compacting of chromosomes to almost less than a quarter its length occurs during the pachytene stage as well. During the diplotene stage, near the centrosome, the two chromosomes of each bivalent separate from each other. However, the two chromosomes remain attached by chiasmata , which are connections present at the site where the two homologous chromosomes exchange DNA segments.
During diplotene, the transcription resumes, chromosomes decondense, and the cell stops the meiosis for a certain period of time. At the beginning of the final stage of prophase I, the diakinesis, when the chromosomes are re-condensed to their maximum state of compaction, the centrosomes move further. The chromosomes are only attached by the chiasmata. Here, the spindles form, the nucleoli disappear, and the nuclear envelope disappears.
The formation of meiotic spindle starts and the disintegration of the nucleoli are indications that meiosis prophase 1 ends and meiosis metaphase 1 begins. During this phase, the bivalents move to the equator of the spindle after attachment to the microtubules using their kinetochores. There are four chromatids in each bivalent, consequently, each bivalent contains four kinetochores as well. These kinetochores appear close to each other appearing as a single unit facing the same pole of the cell.
Such an arrangement allows the attachment of each kinetochore to the microtubules of the spindle pole on the opposite side. This arrangement is the first step that sets for the separation of the chromosomes during the following anaphase. At this stage, the bivalents are randomly arranged, accordingly, the paternal and maternal chromosomes are aligned to one pole of the cell, and therefore, each newly formed daughter cell will receive a mixture of paternal and maternal chromosomes during their movement to the opposite poles during anaphase.
The first step in anaphase includes the migration of homologous chromosomes to the spindle poles by the aid of their kinetochore. This step represents one of the main differences between meiosis and mitosis. In mitosis, the sister chromatids separate during mitosis as they are pulled to the opposite poles.
In meiosis, the two sister chromatids remain attached together and the homologous chromosomes move toward the spindle poles after separation. This results in the presence of a haploid number of chromosomes in each spindle pole at the end of meiotic anaphase I. The process of chromatid separation during mitosis is mediated by cleaving the two sister chromatids with the aid of an activated enzyme called separase.
To stop the action of separase in meiosis, the cell produces a specific protein called shugoshin that prevents the separation of chromatids by protecting the centrosomal site of the chromosome at which the cleavage process takes place. The final phase of meiosis I is telophase 1, which is characterized by the migration of chromosomes to the spindle poles. A nuclear envelope could be formed around chromosomes before cytokinesis to produce two daughter cells of haploid sets of chromosomes.
Most of the time, the chromosomes condense after the initiation of meiosis II. By the end of meiosis I, cytokinesis helps in the production of two cells, each with a haploid nucleus. The chromosomes of each haploid cell will each consist of two chromatids attached at the centromere. Interphase meiosis begins after the end of meiosis I and before the beginning of meiosis II, this stage is not associated with the replication of DNA since each chromosome already consists of two chromatids that were replicated already before the initiation of meiosis I by DNA synthesis process.
In brief, DNA is replicated before meiosis I start at one time only. The stage of meiosis II or second mitotic division has a purpose similar to that of mitosis where the two new chromatids are oriented in two new daughter cells. Therefore, the second meiotic division is sometimes referred to as separation division of meiotic division.
Prophase 2 is the stage that follows meiosis I or interkinesis, it is characterized by the nuclear envelope and nucleolus disintegration as well as the chromatids thickening and shortening in prophase II, and centrosomes replicate and migrate to the polar side. Prophase II is simpler and shorter than prophase I; it somehow resembles the mitotic prophase. On the other hand, prophase II is different from prophase I since crossing over of chromosomes occurs during prophase I only and not prophase II.
Metaphase II starts at the end of prophase II. Metaphase 2 of meiotic division is also similar to metaphase of mitotic division, however, only half the number of chromosomes are present in metaphase II, metaphase II is characterized by the chromosomal alignment in the center of the cell.
It is the stage that comes after metaphase II, in this phase, the sister chromatids separate and move towards the poles of the cell. Anaphase II is similar to mitotic anaphase, where both involve the separation of the chromatids.
The kinetochore shortening leads to the movement of sister chromatids to the two ends of the cell. Telophase is the final step of meiosis, during telophase II, four haploid cells are produced from the two cells produced during meiosis I, nuclear membranes of the newly formed cells are fully developed, and the cells are completely separated at the end of this phase. However, during spermatogenesis in humans and other animals, the sperms are not fully functioning at the end of the telophase II since they need to develop flagella in order to function properly.
Four haploid cells are produced after telophase II and cytokinesis, each daughter cell contains only one chromosome of the two homologous pairs. The produced haploid cells contain a mixture of genetic information from the maternal and paternal chromosomes. These cells contribute to the genetic diversity among individuals of the same species as well as the evolutionary process of organisms.
Where does meiosis occur? Meiosis is not restricted to one species, it is included in the life cycle of various organisms such as fungi, plants, algae, animals, and humans.
What is the purpose of meiosis? Meiosis may produce spores or gametes depending on the species where in humans and other animals meiosis produces gametes sperm cells and egg cells while in plants and algae meiosis is responsible for the production of spores. Plants and algae are multicellular organisms that exhibit both haploid and diploid forms of cells in their life cycle. This phenomenon is called alternation of generations where the haploid spores are produced by meiosis.
This is also why it is called sporic meiosis in plants and algae. The formed spores germinate and undergo mitotic division giving rise to a haploid plant or a haploid alga.
The gametes are produced by mitotic division from the already existing haploid cells; therefore, the haploid form is called gametophyte. The gametes fuse during fertilization to produce the diploid form of cells. The spores are formed from the diploid form by meiosis. Therefore, the diploid form is called the sporophyte. Fungi also have asexual and sexual phases in their life cycle. The mycelium , in particular, may enter either the sexual phase or the asexual phase.
When it enters the sexual phase, the haploid mycelia undergoes plasmogamy the fusion of the two protoplasts and karyogamy the fusion of two haploid nuclei. Thus, following karyogamy is the formation of the diploid zygote.
The zygote grows to a stalked sporangium , which by then, will form haploid spores by meiosis. The spores produced by meiosis are called meiospores in contrast to mitospores that are produced via mitosis. These haploid spores reproductive cells will be released from the sporangium and each will eventually germinate into a new mycelium.
Thus, in fungi, meiosis is the third step in the sequential stages of the sexual phase where plasmogamy is the first followed by karyogamy. Meiosis is crucial in restoring the haploid state of the fungus. See the figure below. How does meiosis work in humans?
Meiosis produces haploid gametes in humans and other animals. It is a crucial part of gametogenesis. As the name implies, gametogenesis is the biological process of creating gametes.
In humans and other animals, there are two forms of gametogenesis: spermatogenesis formation of male gamete, i. Early on in meiosis, during prophase I, homologous chromosomes pair up. Homologous chromosomes have similar genes with other homologous chromosomes: one chromosome came from the mother and one came from the father. During meiosis, they look for each other and stick together length-wise.
During this time, they exchange parts of their arms with each other, like combing two deck of cards, shuffling, and then equally separating the two decks. The results in paired homologous chromosomes that now have regions of DNA that were formerly on the other chromosome. The second way that meiosis generates genetic diversity is that each individual chromosome goes into one of four different gametes: a sperm or an egg cell.
Meiosis in a normal human cell that has 46 chromosomes produces four gametes that each have 23 chromosomes. Each of these chromosomes goes into a separate gamete cell. The third way that meiosis generates genetic variation happens after meiosis occurs.
In sexually reproducing organisms, such as humans, a sperm from the male must fertilize the egg from the female. Human males produce many sperm, each with 23 chromosomes that have been shuffled, that have a unique combination of genes compared to the many other sperm. The egg also has this shuffled genetic diversity. So when one unique sperm fuses with one unique egg, a cell with 46 chromosome forms. Second, that recombination at meiosis plays an important role in the repair of genetic defects in germ line cells.
Third, that it is essential, at least in animals, for the reprogramming of gametes which give rise to the fertilized egg. Fourth, that it helps maintain the immortality of the germ line, possible by a process of rejuvenation involving the removal of faulty RNA and protein molecules, or by the elimination of defective meiocytes. A unified hypothesis is proposed which attempts to link these diverse functions. Evidence is now available which strongly indicates that the control of gene activity in higher organisms depends in part on the pattern of cytosine methylation in DNA, and that this pattern is inherited through the activity of a maintenance methylase.
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