If the chromosomes decondensed in telophase I, they condense again. If nuclear envelopes were formed, they fragment into vesicles. The centrosomes that were duplicated during interphase I move away from each other toward opposite poles and new spindles are formed. The nuclear envelopes are completely broken down and the spindle is fully formed. Each sister chromatid forms an individual kinetochore that attaches to microtubules from opposite poles.
The sister chromatids are pulled apart by the kinetochore microtubules and move toward opposite poles. Non-kinetochore microtubules elongate the cell. Meiosis I vs. In prometaphase I, microtubules attach to the fused kinetochores of homologous chromosomes, and the homologous chromosomes are arranged at the midpoint of the cell in metaphase I. In anaphase I, the homologous chromosomes are separated.
In prometaphase II, microtubules attach to the kinetochores of sister chromatids, and the sister chromatids are arranged at the midpoint of the cells in metaphase II. In anaphase II, the sister chromatids are separated. The chromosomes arrive at opposite poles and begin to decondense. Nuclear envelopes form around the chromosomes.
Cytokinesis separates the two cells into four unique haploid cells. At this point, the newly-formed nuclei are both haploid. The cells produced are genetically unique because of the random assortment of paternal and maternal homologs and because of the recombining of maternal and paternal segments of chromosomes with their sets of genes that occurs during crossover.
Mitosis and meiosis share some similarities, but also some differences, most of which are observed during meiosis I. Mitosis and meiosis are both forms of division of the nucleus in eukaryotic cells. They share some similarities, but also exhibit distinct differences that lead to very different outcomes. The purpose of mitosis is cell regeneration, growth, and asexual reproduction,while the purpose of meiosis is the production of gametes for sexual reproduction.
Mitosis is a single nuclear division that results in two nuclei that are usually partitioned into two new daughter cells. The nuclei resulting from a mitotic division are genetically identical to the original nucleus. They have the same number of sets of chromosomes, one set in the case of haploid cells and two sets in the case of diploid cells. In most plants and all animal species, it is typically diploid cells that undergo mitosis to form new diploid cells.
In contrast, meiosis consists of two nuclear divisions resulting in four nuclei that are usually partitioned into four new haploid daughter cells. The nuclei resulting from meiosis are not genetically identical and they contain one chromosome set only. This is half the number of chromosome sets in the original cell, which is diploid.
Comparing Meiosis and Mitosis : Meiosis and mitosis are both preceded by one round of DNA replication; however, meiosis includes two nuclear divisions.
The four daughter cells resulting from meiosis are haploid and genetically distinct. The daughter cells resulting from mitosis are diploid and identical to the parent cell. The main differences between mitosis and meiosis occur in meiosis I.
In meiosis I, the homologous chromosome pairs become associated with each other and are bound together with the synaptonemal complex. Chiasmata develop and crossover occurs between homologous chromosomes, which then line up along the metaphase plate in tetrads with kinetochore fibers from opposite spindle poles attached to each kinetochore of a homolog in a tetrad. All of these events occur only in meiosis I.
When the tetrad is broken up and the homologous chromosomes move to opposite poles, the ploidy level is reduced from two to one. For this reason, meiosis I is referred to as a reduction division. There is no such reduction in ploidy level during mitosis. Meiosis II is much more similar to a mitotic division. In this case, the duplicated chromosomes only one set, as the homologous pairs have now been separated into two different cells line up on the metaphase plate with divided kinetochores attached to kinetochore fibers from opposite poles.
During anaphase II and mitotic anaphase, the kinetochores divide and sister chromatids, now referred to as chromosomes, are pulled to opposite poles. The two daughter cells of mitosis, however, are identical, unlike the daughter cells produced by meiosis.
They are different because there has been at least one crossover per chromosome. Meiosis II is not a reduction division because, although there are fewer copies of the genome in the resulting cells, there is still one set of chromosomes, as there was at the end of meiosis I. Meiosis II is, therefore, referred to as equatorial division. Privacy Policy. Skip to main content. Meiosis and Sexual Reproduction. Search for:. The Process of Meiosis. Introduction to Meiosis Meiosis is the nuclear division of diploid cells into haploid cells, which is a necessary step in sexual reproduction.
Learning Objectives Describe the importance of meiosis in sexual reproduction. Key Takeaways Key Points Sexual reproduction is the production of haploid cells and the fusion of two of those cells to form a diploid cell. So most animal cells and plant cells are diploid. Then they're diploid in part because they got one chromosome from their mother and one chromosome from their father, therefore making them diploid. A haploid cell only has one set of chromosomes, and most of the time that refers to the so-called sex cells, either eggs or sperm.
And these are [a] critical transition from a diploid cell to a haploid cell to allow normal reproduction to occur, so that when these two haploid cells come together with a single set of genetic information--single chromosomes--they can come together into a so-called zygote made of when the egg cell and the sperm cell come together that then reconstitutes a diploid cell, which can then become a new individual.
Christopher P. A diploid brewer's yeast cell has two sets of biological information. This collection of genes is organized on two more or less identical sets of chromosomes and DNA molecules. The informational content of these genes is normally available to cell all the time.
However, the cell must have at least one undamaged, working gene for each function it requires in order to survive. Maintaining two sets of chromosomes, DNA and genes is relatively "expensive" for the cell. The DNA molecules have to be repaired, organized and controlled on a moment to moment basis. Despite the fact that having two genes coding for each separate protein and function is a good insurance policy, like all insurance, it comes at some cost.
It is not surprising, therefore, that when times become hard the nutrient media begins to run out of supplies , diploid brewer's yeast cells sacrifice the benefits of being diploid, and undergo as special sort of cell division that results in the halving of their biological information content. This is their form of meiosis. During meiosis, two things must happen; the biological information must be accurately sorted out so all the daughter cells receive "one of everything", and the original cell must divide it's contents in such as way as to provide for each of the resulting haploid offspring, what ever they may turn out to be.
Sorting out the chromosomes, and the genetic information they carry, is clearly a critical process in which no mistakes can be tolerated. Each of the haploid offspring must get at least one copy of all the genes it needs for survival. The sets of chromosomes are scattered in a similar manner to a dozen pair of different socks all randomly mixed up in a drawer. The solution to this problem is fairly straight forward.
Before the original cell undergoes any kind of nuclear division, the pairs of chromosomes each itself doubled find each other in the nucleus and pair together.
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