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. If Varsity Tutors takes action in response to an Infringement Notice, it will make a good faith attempt to contact the party that made such content available by means of the most recent email address, if any, provided by such party to Varsity Tutors.
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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? Possible Answers: Egg and sperm chromosomes. Correct answer: Nonsister chromatids. Explanation : During crossing over, homologous chromosomes come together in order to form a tetrad.
Example Question 5 : Understanding Crossing Over. Crossover of homologous chromosomes in meiosis occurs during which phase? Possible Answers: Prophase II of meiosis. Correct answer: Prophase I of meiosis. Explanation : The crossing over of homologous chromosomes occurs in prophase I of meiosis. Example Question 6 : Understanding Crossing Over. During crossing over, two homologous chromosomes pair to form which of the following choices?
Possible Answers: Tetrad. During meiosis, crossing-over occurs at the pachytene stage, when homologous chromosomes are completely paired. At diplotene, when homologs separate, the sites of crossing-over become visible as chiasmata, which hold the two homologs of a bivalent together until segregation at anaphase I.
Each metaphase I bivalent will necessarily have at least one chiasma. In favorable material, such as grasshopper spermatocytes, it is possible to observe that each diplotene chiasma involves a crossover of two of the four chromatids at one site. Where two or more crossovers occur in one bivalent, they usually do not cluster together but are widely separated; this is known as chiasma interference. The occurrence of one crossover event appears to preclude the occurrence of a second crossover in the immediate vicinity.
In addition, the distribution of occurrence of chiasmata along a chromosome may be localized; the probability that a crossover will occur is higher in some chromosome segments and lower in other segments.
In general, the closer two genes are on a chromosome, that is, the more closely linked they are, the less likely it is that crossing-over will occur between them. Thus, the frequency of crossing-over between different genes on a chromosome can be used to produce an estimate of their order and distances apart; this is known as a linkage map.
See also: Genetic mapping. Since each chromatid is composed of a single deoxyribonucleic acid DNA duplex, the process of crossing-over involves the breakage and rejoining of DNA molecules. Although the precise molecular mechanisms have not been determined, it is generally agreed that the following events are necessary: 1 breaking nicking of one of the two strands of one or both nonsister DNA molecules; 2 heteroduplex hybrid DNA formation between single strands from the nonsister DNA molecules; 3 formation of a half chiasma, which is resolved by more single-strand breakages to result in either a reciprocal crossover, a noncrossover, or a nonreciprocal crossover conversion event.
Two molecular models of recombination which have gained credence are those of R. Holliday and of M. Meselson and C. Holliday's model postulates nicks in both chromatids at the initiation of crossing-over Fig.
Meselson and Radding postulate single-strand cut in only one DNA strand. Repair synthesis displaces this strand, which pairs with its complement on the other chromatid, thereby displacing and breaking the other strand of that DNA molecule.
Following pairing and ligation of the two remaining broken ends, a half chiasma is formed. Other models have been postulated in which recombination is initiated by a double-stranded break in one chromatid.
In all the above models, gene conversion can occur in the middle region of the molecules with or without outside marker crossing-over by mismatch repair of heteroduplex DNA. Pachytene, the meiotic stage at which crossing-over is considered to occur, corresponds with the period of close pairing or synapsis of homologous chromosomes. Electron microscopy has revealed that proteinaceous structures, the synaptonemal complexes Fig.
A synaptonemal complex forms during zygotene by pairing of axial elements from two homologous chromosomes. It is present along the whole length of each pachytene bivalent and disappears at diplotene. Figure 2: Near the end of metaphase I, the homologous chromosomes align on the metaphase plate.
Each chromosome looks like an elongated X-shaped structure. In the pair of chromosomes at top, the chromosome at left is mostly green, but the lower region of the right chromatid is orange. The chromosome at right is mostly orange, but the lower region of the left chromatid is green.
A second pair of chromosomes exhibiting the same pattern of coloration on their arms is shown below the topmost pair.
Mitotic spindles are located at each side of the cell. Each spindle apparatus is composed of several white lines, representing fibers, emanating from two oval-shaped structures, representing centrosomes. The fibers attach the centrosomes to the centromeres of each chromosome. Shorter fibers also emanate from the mitotic spindle but are not attached to chromosomes. At the start of metaphase I , microtubules emerge from the spindle and attach to the kinetochore near the centromere of each chromosome.
In particular, microtubules from one side of the spindle attach to one of the chromosomes in each homologous pair, while microtubules from the other side of the spindle attach to the other member of each pair. With the aid of these microtubules, the chromosome pairs then line up along the equator of the cell, termed the metaphase plate Figure 2. Anaphase I. Figure 3: During anaphase I, the homologous chromosomes are pulled toward opposite poles of the cell.
The chromosome at right is moving toward the right-hand mitotic spindle. The chromosome is mostly orange, but the lower region of the left chromatid is green.
A second pair of chromosomes exhibiting the same pattern of coloration on their arms is shown below the topmost pair, mirroring the movements of the chromosomes above. During anaphase I, the microtubules disassemble and contract; this, in turn, separates the homologous chromosomes such that the two chromosomes in each pair are pulled toward opposite ends of the cell Figure 3. This separation means that each of the daughter cells that results from meiosis I will have half the number of chromosomes of the original parent cell after interphase.
Also, the sister chromatids in each chromosome still remain connected. As a result, each chromosome maintains its X-shaped structure. Telophase I. Figure 4: Telophase I results in the production of two nonidentical daughter cells, each of which has half the number of chromosomes of the original parent cell. As the new chromosomes reach the spindle during telophase I , the cytoplasm organizes itself and divides in two. There are now two cells, and each cell contains half the number of chromosomes as the parent cell.
In addition, the two daughter cells are not genetically identical to each other because of the recombination that occurred during prophase I Figure 4. At this point, the first division of meiosis is complete. The cell now rests for a bit before beginning the second meiotic division.
During this period, called interkinesis , the nuclear membrane in each of the two cells reforms around the chromosomes. In some cells, the spindle also disintegrates and the chromosomes relax although most often, the spindle remains intact. It is important to note, however, that no chromosomal duplication occurs during this stage. What happens during meiosis II? Prophase II. As prophase II begins, the chromosomes once again condense into tight structures, and the nuclear membrane disintegrates.
In addition, if the spindle was disassembled during interkinesis, it reforms at this point in time. Metaphase II. Figure 5: During metaphase II, the chromosomes align along the cell's equatorial plate. The events of metaphase II are similar to those of mitotic metaphase — in both processes, the chromosomes line up along the cell's equatorial plate, also called the metaphase plate, in preparation for their eventual separation Figure 5.
Anaphase II. Figure 6: Anaphase II involves separation of the sister chromatids. During anaphase II , microtubules from each spindle attach to each sister chromatid at the kinetochore. The sister chromatids then separate, and the microtubules pull them to opposite poles of the cell.
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