Mitosis is also known as "karyokinesis. The "-kinesis" part of "karyokinesis" comes from the same roots as "kinetic" and refers to movement. Thus, mitosis is the movement of the nucleus.
Packing of the DNA occurs in prophase of mitosis so that it's easier to move rather than having to move the loose chromatin. Think of moving forty-six strands of hundreds of yards of yarn—we would want it to be tightly coiled to make it manageable.
Meiosis is the process by which a haploid cell is formed from a diploid cell. The difference between haploid cells and diploid cells is that haploid cells contain one complete set of chromosomes, whereas diploid cells contain two complete sets of chromosomes. Meiosis involves the division of a diploid 2n parent cell.
The chromosomes are duplicated, but carry out two consecutive divisions. The result is four haploid n cells, each with half the number of chromosomes as the parent cell due to the separation of homologous pairs in meiosis I.
In contrast, mitosis is the process by which a diploid parent cell produces two diploid daughter cells. In meiosis I, the homologous chromosomes have already been duplicated in S phase of interphase. The sister chromatids are identical at this stage. Homologous chromosomes pair in prophase I, forming tetrads. The tetrads then cross over, exchanging genetic material. Then, the genetically-mixed tetrads line up on the metaphase plate and are separated in anaphase I. Note that after the first meiotic division, the two daughter cells are nonidentical and are haploid.
Meiosis involves two divisions and results in four unique daughter cells called gametes. Meiosis begins with one parent cell, after the first division there are two daughter cells, and then those each split, resulting in a total of four daughter cells.
In prophase I chromosomes become compact and homologous chromosomes pair up. Also during prophase I, the nuclear membrane begins to break down and the spindle apparatus begins to form. In metaphase I, homologous chromosomes line up along the center of the cell in order to be pulled apart. Recall that during meiosis I, homologous chromosomes pair, cross over, and separate. Meiosis II is when the sister chromatids are separated. Chromatid disjunction occurs in anaphase II after the chromosomes line up along the equator during metaphase II.
The chromosomes are then pulled apart, with one chromatid moving north, and one moving south. The next steps are telophase, and cytokinesis, which upon completion, will result in genetically distinct haploid gametes. If you've found an issue with this question, please let us know. With the help of the community we can continue to improve our educational resources.
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To achieve this reduction in chromosomes, meiosis consists of one round of chromosome duplication and two rounds of nuclear division. Because the events that occur during each of the division stages are analogous to the events of mitosis, the same stage names are assigned. In meiosis I, the first round of meiosis, homologous chromosomes exchange DNA and the diploid cell is divided into two haploid cells. Meiosis is preceded by an interphase consisting of three stages. The G 1 phase also called the first gap phase initiates this stage and is focused on cell growth.
The S phase is next, during which the DNA of the chromosomes is replicated. This replication produces two identical copies, called sister chromatids, that are held together at the centromere by cohesin proteins. The centrosomes, which are the structures that organize the microtubules of the meiotic spindle, also replicate. Finally, during the G 2 phase also called the second gap phase , the cell undergoes the final preparations for meiosis.
During prophase I, chromosomes condense and become visible inside the nucleus. As the nuclear envelope begins to break down, homologous chromosomes move closer together.
The synaptonemal complex, a lattice of proteins between the homologous chromosomes, forms at specific locations, spreading to cover the entire length of the chromosomes.
The tight pairing of the homologous chromosomes is called synapsis. In synapsis, the genes on the chromatids of the homologous chromosomes are aligned with each other. The synaptonemal complex also supports the exchange of chromosomal segments between non-sister homologous chromatids in a process called crossing over.
The crossover events are the first source of genetic variation produced by meiosis. A single crossover event between homologous non-sister chromatids leads to an exchange of DNA between chromosomes.
Following crossover, the synaptonemal complex breaks down and the cohesin connection between homologous pairs is also removed. At the end of prophase I, the pairs are held together only at the chiasmata; they are called tetrads because the four sister chromatids of each pair of homologous chromosomes are now visible. Crossover between homologous chromosomes : Crossover occurs between non-sister chromatids of homologous chromosomes.
The result is an exchange of genetic material between homologous chromosomes. Synapsis holds pairs of homologous chromosomes together : Early in prophase I, homologous chromosomes come together to form a synapse. The chromosomes are bound tightly together and in perfect alignment by a protein lattice called a synaptonemal complex and by cohesin proteins at the centromere. The key event in prometaphase I is the formation of the spindle fiber apparatus where spindle fiber microtubules attach to the kinetochore proteins at the centromeres.
Microtubules grow from centrosomes placed at opposite poles of the cell. The microtubules move toward the middle of the cell and attach to one of the two fused homologous chromosomes at the kinetochores. At the end of prometaphase I, each tetrad is attached to microtubules from both poles, with one homologous chromosome facing each pole. In addition, the nuclear membrane has broken down entirely.
During metaphase I, the tetrads move to the metaphase plate with kinetochores facing opposite poles. The homologous pairs orient themselves randomly at the equator. This event is the second mechanism that introduces variation into the gametes or spores. In each cell that undergoes meiosis, the arrangement of the tetrads is different. The number of variations is dependent on the number of chromosomes making up a set. There are two possibilities for orientation at the metaphase plate.
The possible number of alignments, therefore, equals 2n, where n is the number of chromosomes per set. Given these two mechanisms, it is highly unlikely that any two haploid cells resulting from meiosis will have the same genetic composition.
In this case, there are two possible arrangements at the equatorial plane in metaphase I. The total possible number of different gametes is 2n, where n equals the number of chromosomes in a set.
In this example, there are four possible genetic combinations for the gametes. In anaphase I, the microtubules pull the attached chromosomes apart. 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. Further Exploration Concept Links for further exploration cell division replication metaphase anaphase telophase linkage chromosome cytokinesis haploid prometaphase principle of segregation principle of independent assortment spindle fibers gamete DNA chromatin nucleus cytoplasm eukaryote prophase recombination principle of segregation Principles of Inheritance.
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