Why Dont the Chromosomes Copy Again During Interphase Ii
Learning Objectives
By the end of this department, you lot will be able to:
- Describe the behavior of chromosomes during meiosis
- Describe cellular events during meiosis
- Explain the differences betwixt meiosis and mitosis
- Explicate the mechanisms inside meiosis that generate genetic variation amongst the products of meiosis
Sexual reproduction requires fertilization, the union of two cells from 2 individual organisms. If those ii cells each contain ane set of chromosomes, then the resulting cell contains ii sets of chromosomes. Haploid cells contain i prepare of chromosomes. Cells containing two sets of chromosomes are called diploid. The number of sets of chromosomes in a prison cell is called its ploidy level. If the reproductive cycle is to keep, then the diploid jail cell must somehow reduce its number of chromosome sets before fertilization can occur again, or there will be a continual doubling in the number of chromosome sets in every generation. So, in add-on to fertilization, sexual reproduction includes a nuclear division that reduces the number of chromosome sets.
Nigh animals and plants are diploid, containing two sets of chromosomes. In each somatic cell of the organism (all cells of a multicellular organism except the gametes or reproductive cells), the nucleus contains two copies of each chromosome, called homologous chromosomes. Somatic cells are sometimes referred to as "body" cells. Homologous chromosomes are matched pairs containing the same genes in identical locations along their length. Diploid organisms inherit 1 copy of each homologous chromosome from each parent; all together, they are considered a full set of chromosomes. Haploid cells, containing a single copy of each homologous chromosome, are found just within structures that give ascent to either gametes or spores. Spores are haploid cells that can produce a haploid organism or tin fuse with another spore to form a diploid prison cell. All animals and most plants produce eggs and sperm, or gametes. Some plants and all fungi produce spores.
The nuclear partitioning that forms haploid cells, which is chosen meiosis , is related to mitosis. As you have learned, mitosis is the part of a cell reproduction cycle that results in identical daughter nuclei that are besides genetically identical to the original parent nucleus. In mitosis, both the parent and the daughter nuclei are at the same ploidy level—diploid for nigh plants and animals. Meiosis employs many of the same mechanisms as mitosis. Yet, the starting nucleus is always diploid and the nuclei that outcome at the stop of a meiotic cell sectionalisation are haploid. To achieve this reduction in chromosome number, meiosis consists of one round of chromosome duplication and ii rounds of nuclear division. Because the events that occur during each of the sectionalization stages are analogous to the events of mitosis, the same stage names are assigned. Even so, because there are two rounds of division, the major process and the stages are designated with a "I" or a "Ii." Thus, meiosis I is the starting time round of meiotic division and consists of prophase I, prometaphase I, and and then on. Meiosis II , in which the 2nd round of meiotic division takes place, includes prophase II, prometaphase 2, and and so on.
Meiosis I
Meiosis is preceded by an interphase consisting of the G1, Southward, and Gtwo phases, which are nearly identical to the phases preceding mitosis. The G1 phase, which is also chosen the first gap stage, is the first stage of the interphase and is focused on cell growth. The South phase is the second phase of interphase, during which the Dna of the chromosomes is replicated. Finally, the One thousand2 phase, as well chosen the 2d gap phase, is the third and final phase of interphase; in this phase, the jail cell undergoes the final preparations for meiosis.
During DNA duplication in the S phase, each chromosome is replicated to produce two identical copies, called sister chromatids, that are held together at the centromere by cohesin proteins. Cohesin holds the chromatids together until anaphase II. The centrosomes, which are the structures that organize the microtubules of the meiotic spindle, also replicate. This prepares the cell to enter prophase I, the showtime meiotic phase.
Prophase I
Figure i. Early on in prophase I, homologous chromosomes come up 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.
Early in prophase I, earlier the chromosomes tin be seen clearly microscopically, the homologous chromosomes are attached at their tips to the nuclear envelope by proteins. As the nuclear envelope begins to intermission down, the proteins associated with homologous chromosomes bring the pair close to each other. Recall that, in mitosis, homologous chromosomes exercise not pair together. In mitosis, homologous chromosomes line upwards end-to-terminate so that when they divide, each daughter cell receives a sis chromatid from both members of the homologous pair. The synaptonemal complex , a lattice of proteins between the homologous chromosomes, beginning forms at specific locations and then spreads to embrace the entire length of the chromosomes. The tight pairing of the homologous chromosomes is chosen synapsis . In synapsis, the genes on the chromatids of the homologous chromosomes are aligned precisely with each other. The synaptonemal complex supports the substitution of chromosomal segments between non-sister homologous chromatids, a process called crossing over. Crossing over tin can be observed visually after the exchange as chiasmata (singular = chiasma) (Figure 1).
In species such equally humans, fifty-fifty though the X and Y sex activity chromosomes are not homologous (most of their genes differ), they accept a small region of homology that allows the X and Y chromosomes to pair upward during prophase I. A partial synaptonemal complex develops but between the regions of homology.
Figure 2. Crossover occurs between non-sister chromatids of homologous chromosomes. The result is an substitution of genetic textile between homologous chromosomes.
Located at intervals forth the synaptonemal circuitous are large poly peptide assemblies called recombination nodules . These assemblies marker the points of subsequently chiasmata and mediate the multistep process of crossover —or genetic recombination—between the non-sister chromatids. Near the recombination nodule on each chromatid, the double-stranded Dna is broken, the cut ends are modified, and a new connectedness is fabricated betwixt the non-sister chromatids. Every bit prophase I progresses, the synaptonemal complex begins to break downwards and the chromosomes begin to condense. When the synaptonemal complex is gone, the homologous chromosomes remain attached to each other at the centromere and at chiasmata. The chiasmata remain until anaphase I. The number of chiasmata varies according to the species and the length of the chromosome. There must be at least one chiasma per chromosome for proper separation of homologous chromosomes during meiosis I, just in that location may be every bit many equally 25. Post-obit crossover, the synaptonemal circuitous breaks down and the cohesin connection betwixt homologous pairs is besides removed. At the end of prophase I, the pairs are held together only at the chiasmata (Figure 2) and are chosen tetrads considering the 4 sister chromatids of each pair of homologous chromosomes are now visible.
The crossover events are the first source of genetic variation in the nuclei produced by meiosis. A unmarried crossover event between homologous non-sister chromatids leads to a reciprocal substitution of equivalent DNA between a maternal chromosome and a paternal chromosome. Now, when that sister chromatid is moved into a gamete cell it will carry some DNA from 1 parent of the individual and some Deoxyribonucleic acid from the other parent. The sister recombinant chromatid has a combination of maternal and paternal genes that did not exist before the crossover. Multiple crossovers in an arm of the chromosome have the same effect, exchanging segments of Deoxyribonucleic acid to create recombinant chromosomes.
Prometaphase I
The key event in prometaphase I is the zipper 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 abound from centrosomes placed at reverse poles of the jail cell. The microtubules move toward the middle of the cell and attach to one of the two fused homologous chromosomes. The microtubules adhere at each chromosomes' kinetochores. With each member of the homologous pair attached to contrary poles of the jail cell, 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 all the same held together at chiasmata. In addition, the nuclear membrane has broken down entirely.
Metaphase I
During metaphase I, the homologous chromosomes are arranged in the eye 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, equally each volition only receive one of the ii homologous chromosomes. Recall that homologous chromosomes are not identical. They incorporate slight differences in their genetic information, causing each gamete to have a unique genetic makeup.
This randomness is the physical basis for the creation of the second form of genetic variation in offspring. Consider that the homologous chromosomes of a sexually reproducing organism are originally inherited as two dissever sets, ane from each parent. Using humans as an example, one set of 23 chromosomes is present in the egg donated by the mother. The male parent 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 betwixt the ii poles of the cell to form the metaphase plate. Considering there is an equal chance that a microtubule cobweb will run into a maternally or paternally inherited chromosome, the organization of the tetrads at the metaphase plate is random. Any maternally inherited chromosome may face up either pole. Any paternally inherited chromosome may as well face up either pole. The orientation of each tetrad is independent of the orientation of the other 22 tetrads.
This event—the random (or contained) 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 organisation of the tetrads is unlike. The number of variations is dependent on the number of chromosomes making up a prepare. 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 ready. Humans have 23 chromosome pairs, which results in over eight 1000000 (2 23 ) possible genetically-distinct gametes. This number does not include the variability that was previously created in the sister chromatids past crossover. Given these two mechanisms, it is highly unlikely that any ii haploid cells resulting from meiosis will have the same genetic limerick (Effigy three).
To summarize the genetic consequences of meiosis I, the maternal and paternal genes are recombined by crossover events that occur between each homologous pair during prophase I. In improver, the random assortment of tetrads on the metaphase plate produces a unique combination of maternal and paternal chromosomes that volition make their way into the gametes.
Figure 3. Random, independent assortment during metaphase I can exist demonstrated by because a cell with a set up of two chromosomes (n = 2). In this example, there are two possible arrangements at the equatorial airplane in metaphase I. The full possible number of dissimilar gametes is 2northward, where n equals the number of chromosomes in a gear up. In this case, there are four possible genetic combinations for the gametes. With n = 23 in human cells, in that location are over 8 one thousand thousand possible combinations of paternal and maternal chromosomes.
Anaphase I
In anaphase I, the microtubules pull the linked chromosomes apart. The sister chromatids remain tightly leap 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 4).
Telophase I and Cytokinesis
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 some organisms, the chromosomes decondense and nuclear envelopes course around the chromatids in telophase I. In other organisms, cytokinesis—the concrete 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 past Golgi vesicles fusing at the metaphase plate. This prison cell plate volition ultimately atomic number 82 to the formation of prison cell walls that separate the two girl cells.
Two haploid cells are the end result of the first meiotic division. The cells are haploid considering at each pole, at that place is merely one of each pair of the homologous chromosomes. Therefore, just one full set of the chromosomes is present. This is why the cells are considered haploid—in that location is only one chromosome set, even though each homolog still consists of two sister chromatids. Think that sister chromatids are but duplicates of one of the 2 homologous chromosomes (except for changes that occurred during crossing over). In meiosis II, these two sister chromatids will dissever, creating 4 haploid girl cells.
Link to Learning
Review the procedure of meiosis, observing how chromosomes align and drift, at Meiosis: An Interactive Animation.
Meiosis Two
In some species, cells enter a brief interphase, or interkinesis , before entering meiosis II. Interkinesis lacks an South phase, and so chromosomes are not duplicated. The two cells produced in meiosis I go through the events of meiosis Two in synchrony. During meiosis Ii, the sister chromatids within the two girl cells separate, forming 4 new haploid gametes. The mechanics of meiosis II is similar to mitosis, except that each dividing prison cell has only one prepare of homologous chromosomes. Therefore, each jail cell has half the number of sister chromatids to separate out as a diploid prison cell undergoing mitosis.
Prophase II
If the chromosomes decondensed in telophase I, they condense again. If nuclear envelopes were formed, they fragment into vesicles. The centrosomes that were duplicated during interkinesis movement away from each other toward opposite poles, and new spindles are formed.
Prometaphase II
The nuclear envelopes are completely cleaved down, and the spindle is fully formed. Each sister chromatid forms an individual kinetochore that attaches to microtubules from contrary poles.
Metaphase 2
The sis chromatids are maximally condensed and aligned at the equator of the prison cell.
Anaphase II
The sister chromatids are pulled autonomously by the kinetochore microtubules and move toward opposite poles. Non-kinetochore microtubules elongate the jail cell.
Figure 4. The process of chromosome alignment differs between meiosis I and meiosis II. In prometaphase I, microtubules attach to the fused kinetochores of homologous chromosomes, and the homologous chromosomes are arranged at the midpoint of the jail cell in metaphase I. In anaphase I, the homologous chromosomes are separated. In prometaphase Two, microtubules attach to the kinetochores of sis chromatids, and the sister chromatids are arranged at the midpoint of the cells in metaphase Two. In anaphase II, the sister chromatids are separated.
Telophase II and Cytokinesis
The chromosomes arrive at opposite poles and begin to decondense. Nuclear envelopes grade around the chromosomes. Cytokinesis separates the ii cells into four unique haploid cells. At this point, the newly formed nuclei are both haploid. The cells produced are genetically unique considering of the random array of paternal and maternal homologs and because of the recombining of maternal and paternal segments of chromosomes (with their sets of genes) that occurs during crossover. The entire process of meiosis is outlined in Effigy 5.
Figure 5. An creature cell with a diploid number of four (2due north = iv) proceeds through the stages of meiosis to grade four haploid daughter cells.
Comparison Meiosis and Mitosis
Mitosis and meiosis are both forms of sectionalization of the nucleus in eukaryotic cells. They share some similarities, merely likewise exhibit distinct differences that lead to very dissimilar outcomes (Figure 6). Mitosis is a single nuclear division that results in two nuclei that are usually partitioned into two new cells. The nuclei resulting from a mitotic division are genetically identical to the original nucleus. They accept the same number of sets of chromosomes, ane ready in the case of haploid cells and two sets in the case of diploid cells. In most plants and all beast species, it is typically diploid cells that undergo mitosis to form new diploid cells. In contrast, meiosis consists of two nuclear divisions resulting in 4 nuclei that are usually partitioned into four new cells. The nuclei resulting from meiosis are not genetically identical and they contain 1 chromosome gear up only. This is half the number of chromosome sets in the original cell, which is diploid.
The primary differences between mitosis and meiosis occur in meiosis I, which is a very different nuclear division than mitosis. In meiosis I, the homologous chromosome pairs become associated with each other, are jump together with the synaptonemal complex, develop chiasmata and undergo crossover between sister chromatids, and line upwards 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 chiasmata resolve and the tetrad is broken up with the homologs moving to one pole or another, the ploidy level—the number of sets of chromosomes in each future nucleus—has been reduced from two to i. 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 coordinating to a mitotic division. In this case, the duplicated chromosomes (only one set of them) line upward on the metaphase plate with divided kinetochores attached to kinetochore fibers from contrary poles. During anaphase 2, as in mitotic anaphase, the kinetochores dissever and one sister chromatid—now referred to as a chromosome—is pulled to ane pole while the other sis chromatid is pulled to the other pole. If it were non for the fact that in that location had been crossover, the two products of each individual meiosis II division would be identical (like in mitosis). Instead, they are different because there has e'er 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 nevertheless one set of chromosomes, as at that place was at the end of meiosis I.
Figure 6. Meiosis and mitosis are both preceded by one round of Deoxyribonucleic acid replication; notwithstanding, 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.
Evolution Connection
The Mystery of the Development of Meiosis
Some characteristics of organisms are so widespread and primal that information technology is sometimes difficult to recall that they evolved like other simpler traits. Meiosis is such an extraordinarily complex serial of cellular events that biologists have had problem hypothesizing and testing how it may have evolved. Although meiosis is inextricably entwined with sexual reproduction and its advantages and disadvantages, it is of import to carve up the questions of the evolution of meiosis and the evolution of sex activity, because early meiosis may have been advantageous for different reasons than it is now. Thinking outside the box and imagining what the early benefits from meiosis might have been is one approach to uncovering how it may accept evolved.
Meiosis and mitosis share obvious cellular processes and it makes sense that meiosis evolved from mitosis. The difficulty lies in the clear differences betwixt meiosis I and mitosis.[1]
summarized the unique events that needed to occur for the evolution of meiosis from mitosis. These steps are homologous chromosome pairing, crossover exchanges, sister chromatids remaining attached during anaphase, and suppression of Dna replication in interphase. They argue that the first step is the hardest and most of import, and that understanding how it evolved would make the evolutionary procedure clearer. They suggest genetic experiments that might shed calorie-free on the evolution of synapsis.
At that place are other approaches to understanding the evolution of meiosis in progress. Unlike forms of meiosis exist in single-celled protists. Some announced to exist simpler or more "primitive" forms of meiosis. Comparison the meiotic divisions of different protists may shed low-cal on the evolution of meiosis.[two]
compared the genes involved in meiosis in protists to understand when and where meiosis might take evolved. Although research is still ongoing, recent scholarship into meiosis in protists suggests that some aspects of meiosis may have evolved later than others. This kind of genetic comparison can tell us what aspects of meiosis are the oldest and what cellular processes they may have borrowed from in earlier cells.
Link to Learning
Click through the steps of this interactive blitheness to compare the meiotic process of cell partitioning to that of mitosis: How Cells Separate.
Section Summary
Sexual reproduction requires that diploid organisms produce haploid cells that tin can fuse during fertilization to form diploid offspring. As with mitosis, Deoxyribonucleic acid replication occurs prior to meiosis during the S-phase of the cell cycle. Meiosis is a series of events that suit and separate chromosomes and chromatids into daughter cells. During the interphases of meiosis, each chromosome is duplicated. In meiosis, there are two rounds of nuclear division resulting in iv nuclei and usually 4 daughter cells, each with half the number of chromosomes as the parent prison cell. The first separates homologs, and the 2d—like mitosis—separates chromatids into private chromosomes. During meiosis, variation in the girl nuclei is introduced considering of crossover in prophase I and random alignment of tetrads at metaphase I. The cells that are produced by meiosis are genetically unique.
Meiosis and mitosis share similarities, merely have singled-out outcomes. Mitotic divisions are single nuclear divisions that produce girl nuclei that are genetically identical and take the same number of chromosome sets as the original cell. Meiotic divisions include two nuclear divisions that produce four daughter nuclei that are genetically unlike and have one chromosome set instead of the two sets of chromosomes in the parent cell. The master differences between the processes occur in the showtime sectionalization of meiosis, in which homologous chromosomes are paired and exchange non-sister chromatid segments. The homologous chromosomes separate into different nuclei during meiosis I, causing a reduction of ploidy level in the first sectionalisation. The 2nd division of meiosis is more than similar to a mitotic sectionalisation, except that the girl cells do non contain identical genomes because of crossover.
Additional Self Bank check Questions
ane. Describe the process that results i the formation of a tetrad.
2. Explain how the random alignment of homologous chromosomes during metaphase I contributes to the variation in gametes produced by meiosis.
3. What is the function of the fused kinetochore institute on sister chromatids in prometaphase I?
4. In a comparison of the stages of meiosis to the stages of mitosis, which stages are unique to meiosis and which stages have the same events in both meiosis and mitosis?
Answers
ane. During the meiotic interphase, each chromosome is duplicated. The sister chromatids that are formed during synthesis are held together at the centromere region by cohesin proteins. All chromosomes are fastened to the nuclear envelope by their tips. Equally the jail cell enters prophase I, the nuclear envelope begins to fragment, and the proteins holding homologous chromosomes locate each other. The four sister chromatids align lengthwise, and a protein lattice called the synaptonemal complex is formed between them to bind them together. The synaptonemal complex facilitates crossover betwixt non-sister chromatids, which is observed as chiasmata along the length of the chromosome. As prophase I progresses, the synaptonemal complex breaks downwardly and the sister chromatids become complimentary, except where they are attached past chiasmata. At this stage, the 4 chromatids are visible in each homologous pairing and are chosen a tetrad.
2. Random alignment leads to new combinations of traits. The chromosomes that were originally inherited by the gamete-producing individual came every bit from the egg and the sperm. In metaphase I, the duplicated copies of these maternal and paternal homologous chromosomes line up across the center of the prison cell. The orientation of each tetrad is random. There is an equal run a risk that the maternally derived chromosomes will be facing either pole. The aforementioned is true of the paternally derived chromosomes. The alignment should occur differently in almost every meiosis. As the homologous chromosomes are pulled apart in anaphase I, any combination of maternal and paternal chromosomes volition move toward each pole. The gametes formed from these two groups of chromosomes will have a mixture of traits from the individual's parents. Each gamete is unique.
3. In metaphase I, the homologous chromosomes line up at the metaphase plate. In anaphase I, the homologous chromosomes are pulled apart and motion to opposite poles. Sister chromatids are not separated until meiosis Two. The fused kinetochore formed during meiosis I ensures that each spindle microtubule that binds to the tetrad will attach to both sister chromatids.
4. All of the stages of meiosis I, except peradventure telophase I, are unique because homologous chromosomes are separated, not sister chromatids. In some species, the chromosomes do non decondense and the nuclear envelopes practice not grade in telophase I. All of the stages of meiosis II accept the same events as the stages of mitosis, with the possible exception of prophase II. In some species, the chromosomes are withal condensed and there is no nuclear envelope. Other than this, all processes are the same.
Source: https://courses.lumenlearning.com/suny-biology1/chapter/the-process-of-meiosis/