Chromosomes, each with two chromatids, move to separate poles. Each of the daughter cells is now haploid 23 chromosomes , but each chromosome has two chromatids. Telophase I Nuclear envelopes may reform, or the cell may quickly start meiosis II. Cytokinesis Analogous to mitosis where two complete daughter cells form. Meiosis I Animation kb. Meiosis II Animation kb. In meiosis I, chromosomes in a diploid cell resegregate, producing four haploid daughter cells. It is this step in meiosis that generates genetic diversity.
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. Before sexual reproduction can occur, the number of chromosomes in a diploid cell must decrease by half.
Meiosis produces cells with half the number of chromosomes as the original cell. Haploid cells used in sexual reproduction, gametes, are formed during meiosis, which consists of one round of chromosome replication and two rounds of nuclear division.
Meiosis I is the first round of meiotic division, while meiosis II is the second round. Key Terms haploid : of a cell having a single set of unpaired chromosomes gamete : a reproductive cell, male sperm or female egg , that has only half the usual number of chromosomes diploid : of a cell, having a pair of each type of chromosome, one of the pair being derived from the ovum and the other from the spermatozoon.
Meiosis I In meiosis I, the first round of meiosis, homologous chromosomes exchange DNA and the diploid cell is divided into two haploid cells. Learning Objectives Describe the stages and results of meiosis I. During prophase I, the homologous chromosomes condense and become visible as the x shape we know, pair up to form a tetrad, and exchange genetic material by crossing over.
In metaphase I, the tetrads line themselves up at the metaphase plate and homologous pairs orient themselves randomly. In anaphase I, centromeres break down and homologous chromosomes separate. In telophase I, chromosomes move to opposite poles; during cytokinesis the cell separates into two haploid cells.
Key Terms crossing over : the exchange of genetic material between homologous chromosomes that results in recombinant chromosomes tetrad : two pairs of sister chromatids a dyad pair aligned in a certain way and often on the equatorial plane during the meiosis process chromatid : either of the two strands of a chromosome that separate during meiosis.
Meiosis II During meiosis II, the sister chromatids within the two daughter cells separate, forming four new haploid gametes. Key Takeaways Key Points During prophase II, chromsomes condense again, centrosomes that were duplicated during interphase I move away from each other toward opposite poles, and new spindles are formed.
During prometaphase II, the nuclear envelopes are completely broken down, and each sister chromatid forms an individual kinetochore that attaches to microtubules from opposite poles.
During metaphase II, sister chromatids are condensed and aligned at the equator of the cell. During anaphase II sister chromatids are pulled apart by the kinetochore microtubules and move toward opposite poles. During telophase II and cytokinesis, chromosomes arrive at opposite poles and begin to decondense; the two cells divide into four unique haploid cells. Key Terms meiosis II : the second part of the meiotic process; the end result is production of four haploid cells from the two haploid cells produced in meiosis I.
Comparing Meiosis and Mitosis Mitosis and meiosis share some similarities, but also some differences, most of which are observed during meiosis I. Learning Objectives Compare and contrast mitosis and meiosis. Key Takeaways Key Points For the most part, in mitosis, diploid cells are partitioned into two new diploid cells, while in meiosis, diploid cells are partitioned into four new haploid cells.
In mitosis, the daughter cells have the same number of chromosomes as the parent cell, while in meiosis, the daughter cells have half the number of chromosomes as the parent. During prophase I, the complex of DNA and protein known as chromatin condenses to form chromosomes.
The pairs of replicated chromosomes are known as sister chromatids, and they remain joined at a central point called the centromere. A large structure called the meiotic spindle also forms from long proteins called microtubules on each side, or pole, of the cell.
Between prophase I and metaphase I, the pairs of homologous chromosome form tetrads. Within the tetrad, any pair of chromatid arms can overlap and fuse in a process called crossing-over or recombination. Recombination is a process that breaks, recombines and rejoins sections of DNA to produce new combinations of genes. In metaphase I, the homologous pairs of chromosomes align on either side of the equatorial plate.
Then, in anaphase I, the spindle fibers contract and pull the homologous pairs, each with two chromatids, away from each other and toward each pole of the cell. During telophase I, the chromosomes are enclosed in nuclei. The cell now undergoes a process called cytokinesis that divides the cytoplasm of the original cell into two daughter cells. Each daughter cell is haploid and has only one set of chromosomes, or half the total number of chromosomes of the original cell.
Meiosis II is a mitotic division of each of the haploid cells produced in meiosis I. During prophase II, the chromosomes condense, and a new set of spindle fibers forms.
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