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Phases of meiosis I | Cells | MCAT | Khan Academy

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Meiosis I, the first meiotic division, begins with prophase I. 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.

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Phases of meiosis II | Cells | MCAT | Khan Academy

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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. The chromosomes begin moving toward the equator of the cell. During metaphase II, the centromeres of the paired chromatids align along the equatorial plate in both cells. Then in anaphase II, the chromosomes separate at the centromeres. The spindle fibers pull the separated chromosomes toward each pole of the cell. Finally, during telophase II, the chromosomes are enclosed in nuclear membranes. Cytokinesis follows, dividing the cytoplasm of the two cells. At the conclusion of meiosis, there are four haploid daughter cells that go on to develop into either sperm or egg cells.

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Cremer, T., & Cremer, C. Chromosome territories, nuclear architecture and gene regulation in mammalian cells. Nature Reviews Genetics 2, 292-301 (2001) doi:10.1038/35066075 ( link to article)

Hagstrom, K. A., & Meyer, B. J. Condensin and cohesin: More than chromosome compactor and glue. Nature Reviews Genetics 4, 520-534 (2003) doi:10.1038/nrg1110 ( link to article)

Hirano, T. At the heart of the chromosome: SMC proteins in action. Nature Reviews Molecular Cell Biology 7, 311-322 (2002) doi:10.1038/nrm1909 ( link to article)

Mitchison, T. J., & Salmon, E. D. Mitosis: A history of division. Nature Cell Biology 3, E17-E21 (2001) doi:10.1038/35050656 ( link to article)

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Fully understanding the mechanisms of mitosis remains one of the greatest challenges facing modern biologists. During mitosis, two identical copies of the genome are packaged into chromosomes that are distributed equally between two daughter nuclei by a highly dynamic spindle structure. Mitosis is truly a molecular spectacle, involving hundreds of cellular proteins in a highly regulated sequence of movements.

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Meiosis is used to make sperm and egg cells. During meiosis, a cell's chromosomes are copied once, but the cell divides twice. During mitosis, the chromosomes are copied once, and the cell divides once. For simplicity, cells are illustrated with only three pairs of chromosomes.

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Mitosis is the process in which a eukaryotic cell nucleus splits in two, followed by division of the parent cell into two daughter cells. The word "mitosis" means "threads," and it refers to the threadlike appearance of chromosomes as the cell prepares to divide. Early microscopists were the first to observe these structures, and they also noted the appearance of a specialized network of microtubules during mitosis. These tubules, collectively known as the spindle, extend from structures called centrosomes — with one centrosome located at each of the opposite ends, or poles, of a cell. As mitosis progresses, the microtubules attach to the chromosomes, which have already duplicated their DNA and aligned across the center of the cell. The spindle tubules then shorten and move toward the poles of the cell. As they move, they pull the one copy of each chromosome with them to opposite poles of the cell. This process ensures that each daughter cell will contain one exact copy of the parent cell DNA.

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Crossing over" refers to the process where genetic material is exchanged between non-sister chromatids of homologous chromosomes, and it occurs specifically during the prophase I stage of meiosis; meaning that this exchange of genetic information happens when homologous chromosomes pair up and form tetrads, creating genetic diversity in the resulting gametes.

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During metaphase I of meiosis I, tetrads that were previously formed during prophase I have spindle fibers extending from centrioles attached to opposite ends. The spindle fibers then line the tetrads up along an equatorial plane within the middle of the cell. These tetrads will later be split into their homologous pairs of chromosomes and brought to opposite ends of the cell during anaphase I. The major events of meiosis I occur during prophase I and metaphase I. During prophase I, tetrads are formed out of homologous pairs of chromosomes. The process of crossing-over then occurs, which exchanges alleles between the homologous pairs of chromosomes. During metaphase I, the tetrads line up in the middle of the cell. If spindle fibers do not attach correctly during metaphase I, chromosomes will not separate during anaphase correctly and nondisjunction can occur.

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The Principle of Independent Assortment describes how different genes independently separate from one another when reproductive cells develop. Independent assortment of genes and their corresponding traits was first observed by Gregor Mendel in 1865 during his studies of genetics in pea plants. Mendel was performing dihybrid crosses, which are crosses between organisms that differ with regard to two traits. He discovered that the combinations of traits in the offspring of his crosses did not always match the combinations of traits in the parental organisms. From his data, he formulated the Principle of Independent Assortment.

We now know that this independent assortment of genes occurs during meiosis in eukaryotes. Meiosis is a type of cell division that reduces the number of chromosomes in a parent cell by half to produce four reproductive cells called gametes. In humans, diploid cells contain 46 chromosomes, with 23 chromosomes inherited from the mother and a second similar set of 23 chromosomes inherited from the father. Pairs of similar chromosomes are called homologous chromosomes. During meiosis, the pairs of homologous chromosome are divided in half to form haploid cells, and this separation, or assortment, of homologous chromosomes is random. This means that all of the maternal chromosomes will not be separated into one cell, while the all paternal chromosomes are separated into another. Instead, after meiosis occurs, each haploid cell contains a mixture of genes from the organism's mother and father.

Another feature of of independent assortment is recombination. Recombination occurs during meiosis and is a process that breaks and recombines pieces of DNA to produce new combinations of genes. Recombination scrambles pieces of maternal and paternal genes, which ensures that genes assort independently from one another. It is important to note that there is an exception to the law of independent assortment for genes that are located very close to one another on the same chromosome because of genetic linkage.

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Anaphase in mitosis is triggered by the separation of the sister chromatids with the help of separase.

Separase breaks the cohesion that binds the sister chromatids, as the microtubules pull the sister chromatids towards the opposite plea of the cells.

The astral and interpolar microtubules play a major role in lengthening and elongating the cell which takes an oval shape.

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The anaphase of meiosis is made up of two consecutive cell divisions, i.e anaphase I and anaphase II.

In this stage of meiosis, since there is no DNA replication in between, the diploid cell with two alleles for each gene gets reduced to a haploid cell containing a single allele at each gene.

Generally, anaphase I involve separating the chromosomes from each sister chromatid to the opposite poles still attached to the microtubules of the cell while anaphase 2 involves the actual split of the sister chromatids into single chromatids.

During this phase, the kinetochore microtubules shorten thus pulling the homologous chromosomes to opposite poles of the cell.

The non-kinetochore microtubules start to lengthen thus pushing the centrosomes apart.

The cell also elongates as it prepares for dividing at the center.

The cohesins around the centromere remain protected by a protein known as Shugoshin (guardian spirit), preventing the sister chromatids from separating while the homologs are segregated.

This is the phase after metaphase 2 whereby the remaining centromeric cohesins which are not protected by the Shugoshin anymore are cleaved.

This allows the separation of the sister chromatids which are then singly referred to as sister chromosomes. They move towards the opposite poles of the cells.