During the formation of egg and sperm cells, also known as meiosis, paired chromosomes from each parent align so that similar DNA sequences from the paired chromosomes cross over one another. Crossing over results in a shuffling of genetic material and is an important cause of the genetic variation seen among offspring.
Crossing over is a biological occurrence that happens during meiosis when the paired homologs, or chromosomes of the same type, are lined up. Homologous chromosomes can exchange parts in a process called "crossing over. Purpose : Meiosis is a special version of cell division that occurs only in the testes and ovaries; the organs that produce the male and female reproductive cells; the sperm and eggs.
Why is this different? Ordinary body cells have a complete set of chromosomes. If body cells from mom and dad fused to form a baby, the fertilized egg would have twice as many chromosomes as it should.
Meiosis is sometimes called "reduction division" because it reduces the number of chromosomes to half the normal number so that, when fusion of sperm and egg occurs, baby will have the correct number.
Therefore the purpose of meiosis is to produce gametes, the sperm and eggs, with half of the genetic complement of the parent cells.
In the figures below, pink represents a genetic contribution from mom and blue represents a genetic contribution from dad. During this stage, homologous chromosomes line up on the metaphase plate and exchange genetic information. Crossing over occurs during prophase I when parts of the homologous chromosomes overlap and switch their genes. 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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Possible Answers: Metaphase I. Correct answer: Prophase I. Explanation : During prophase I homologous chromosomes will line up with one another, forming tetrads. Report an Error. What is the evolutionary purpose of cells that undergo crossing over?
Possible Answers: To produce gametes that are genetically identical. Correct answer: To increase genetic diversity. Explanation : Crossing over is a process that happens between homologous chromosomes in order to increase genetic diversity. Example Question 3 : Understanding Crossing Over. During which step of cell division does crossing over occur? Possible Answers: Metaphase II.
Explanation : When chromatids "cross over," homologous chromosomes trade pieces of genetic material, resulting in novel combinations of alleles, though the same genes are still present. Example Question 4 : Understanding Crossing Over. What structures exchange genetic material during crossing over? The crossover events are the first source of genetic variation in the nuclei produced by meiosis. A single crossover event between homologous nonsister chromatids leads to a reciprocal exchange of equivalent DNA between a maternal chromosome and a paternal chromosome.
When a recombinant sister chromatid is moved into a gamete cell it will carry some DNA from one parent and some DNA from the other parent. The recombinant chromatid has a combination of maternal and paternal genes that did not exist before the crossover. Crossover events can occur almost anywhere along the length of the synapsed chromosomes.
Different cells undergoing meiosis will therefore produce different recombinant chromatids, with varying combinations of maternal and parental genes. Multiple crossovers in an arm of the chromosome have the same effect, exchanging segments of DNA to produce genetically recombined chromosomes.
Figure 2. Crossover occurs between nonsister chromatids of homologous chromosomes. The result is an exchange of genetic material between homologous chromosomes. The key event in prometaphase I is the attachment of the spindle fiber microtubules to the kinetochore proteins at the centromeres. Kinetochore proteins are multiprotein complexes that bind the centromeres of a chromosome to the microtubules of the mitotic spindle.
Microtubules grow from microtubule-organizing centers MTOCs. In animal cells, MTOCs are centrosomes located at opposite poles of the cell. The microtubules from each pole move toward the middle of the cell and attach to one of the kinetochores of the two fused homologous chromosomes. Each member of the homologous pair attaches to a microtubule extending from opposite poles of the cell so that in the next phase, the microtubules can pull the homologous pair apart.
A spindle fiber that has attached to a kinetochore is called a kinetochore microtubule. At the end of prometaphase I, each tetrad is attached to microtubules from both poles, with one homologous chromosome facing each pole. The homologous chromosomes are still held together at the chiasmata. In addition, the nuclear membrane has broken down entirely. During metaphase I, the homologous chromosomes are arranged at the metaphase plate —roughly in the midline of the cell, with the kinetochores facing opposite poles.
The homologous pairs orient themselves randomly at the equator. For example, if the two homologous members of chromosome 1 are labeled a and b , then the chromosomes could line up a-b or b-a. This is important in determining the genes carried by a gamete, as each will only receive one of the two homologous chromosomes. Recall that after crossing over takes place, homologous chromosomes are not identical.
They contain slight differences in their genetic information, causing each gamete to have a unique genetic makeup. The randomness in the alignment of recombined chromosomes at the metaphase plate, coupled with the crossing over events between nonsister chromatids, are responsible for much of the genetic variation in the offspring. To clarify this further, remember that the homologous chromosomes of a sexually reproducing organism are originally inherited as two separate sets, one from each parent.
Using humans as an example, one set of 23 chromosomes is present in the egg donated by the mother. The father provides the other set of 23 chromosomes in the sperm that fertilizes the egg. Every cell of the multicellular offspring has copies of the original two sets of homologous chromosomes. In prophase I of meiosis, the homologous chromosomes form the tetrads.
In metaphase I, these pairs line up at the midway point between the two poles of the cell to form the metaphase plate. Because there is an equal chance that a microtubule fiber will encounter a maternally or paternally inherited chromosome, the arrangement of the tetrads at the metaphase plate is random.
Thus, any maternally inherited chromosome may face either pole. Likewise, any paternally inherited chromosome may also face either pole. The orientation of each tetrad is independent of the orientation of the other 22 tetrads.
This event—the random or independent assortment of homologous chromosomes at the metaphase plate—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 2 n in a diploid cell, where n is the number of chromosomes per haploid set. Humans have 23 chromosome pairs, which results in over eight million 2 23 possible genetically-distinct gametes just from the random alignment of chromosomes at the metaphase plate.
This number does not include the variability that was previously produced by crossing over between the nonsister chromatids. Given these two mechanisms, it is highly unlikely that any two haploid cells resulting from meiosis will have the same genetic composition Figure. To summarize, meiosis I creates genetically diverse gametes in two ways.
First, during prophase I, crossover events between the nonsister chromatids of each homologous pair of chromosomes generate recombinant chromatids with new combinations of maternal and paternal genes. Second, the random assortment of tetrads on the metaphase plate produces unique combinations of maternal and paternal chromosomes that will make their way into the gametes.
Figure 3. 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 linked chromosomes apart. The sister chromatids remain tightly bound together at the centromere.
The chiasmata are broken in anaphase I as the microtubules attached to the fused kinetochores pull the homologous chromosomes apart Figure. In telophase, the separated chromosomes arrive at opposite poles. The remainder of the typical telophase events may or may not occur, depending on the species.
In other organisms, cytokinesis —the physical separation of the cytoplasmic components into two daughter cells—occurs without reformation of the nuclei. In nearly all species of animals and some fungi, cytokinesis separates the cell contents via a cleavage furrow constriction of the actin ring that leads to cytoplasmic division.
In plants, a cell plate is formed during cell cytokinesis by Golgi vesicles fusing at the metaphase plate. This cell plate will ultimately lead to the formation of cell walls that separate the two daughter cells.
Two haploid cells are the result of the first meiotic division of a diploid cell. The cells are haploid because at each pole, there is just one of each pair of the homologous chromosomes. Therefore, only one full set of the chromosomes is present. This is why the cells are considered haploid—there is only one chromosome set, even though each chromosome still consists of two sister chromatids. Recall that sister chromatids are merely duplicates of one of the two homologous chromosomes except for changes that occurred during crossing over.
In meiosis II, these two sister chromatids will separate, creating four haploid daughter cells. Review the process of meiosis, observing how chromosomes align and migrate, at Meiosis: An Interactive Animation. In some species, cells enter a brief interphase, or interkinesis, before entering meiosis II. Interkinesis lacks an S phase, so chromosomes are not duplicated. The two cells produced in meiosis I go through the events of meiosis II in synchrony.
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