Offspring Closely Resemble Their Parents : In kind means that the offspring of any organism closely resemble their parent or parents. The hippopotamus gives birth to hippopotamus calves a. Joshua trees produce seeds from which Joshua tree seedlings emerge b.
Adult flamingos lay eggs that hatch into flamingo chicks c. Sexual reproduction is the production of haploid cells gametes and the fusion fertilization of two gametes to form a single, unique diploid cell called a zygote. All animals and most plants produce these gametes, or eggs and sperm. In most plants and animals, through tens of rounds of mitotic cell division, this diploid cell will develop into an adult organism. Haploid cells that are part of the sexual reproductive cycle are produced by a type of cell division called meiosis.
Meiosis employs many of the same mechanisms as mitosis. However, the starting nucleus is always diploid and the nuclei that result at the end of a meiotic cell division are haploid, so the resulting cells have half the chromosomes as the original.
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.
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. In telophase I, the separated chromosomes arrive at opposite poles. MLH1 foci are apparent on all bivalents. In contrast, in B , most bivalents have failed to synapse. Examples of bivalents with distal and interstitial regions of asynapsis are indicated by arrows and the arrowhead, respectively; in this cell, the XY bivalent is not readily apparent.
Despite the synaptic defects, MLH1 foci are visible on all bivalents. Surprisingly, the overall number and distribution of exchanges were relatively normal. That is, the mean number of MLH1 foci per cell was Additionally, the foci displayed interference, and as expected from studies of controls, distally located foci predominated data not shown.
However, although our sample contained no post-pachytene cells, a small number of atypical sperm were identified on histological examination. Thus, in at least some seminferous tubules, a small number of cells were able to escape this arrest and finish meiosis. The purpose of this study was 2-fold. First, in NOA cases in which spermatogenesis proceeded past meiosis I, we wanted to know whether defects in recombination were a common occurrence.
If so, it would imply an increased risk of aneuploidy in sperm of such individuals, because abnormalities in recombination are a well-established correlate of human meiotic non-disjunction Lamb et al. Second, we were interested in determining the proportion, if any, of cases of unexplained infertility that were due to complete breakdown of spermatogenesis during meiosis I.
The results for each of the two aims were intriguing. First, our studies of 14 cases in which post-pachytene stages were identified provided remarkably little evidence of meiotic disturbances. Synaptic defects were uncommon, recombination levels were similar to those in controls and testicular sperm were evident in all cases.
These results are somewhat surprising, because they are at variance with two recent immunostaining studies of NOA Table II. Specifically, both Gonsalves et al. The reason for the discrepancy between their reports and ours is not clear but could conceivably reflect methodological differences. We reasoned that this would prevent us from scoring cells that were just entering or exiting pachytene or cells that had severe synaptic defects and provided us the potential to detect at least one MLH1 focus per bivalent.
We continued this practice in the present study, because any changes in scoring could have introduced an artefactual difference between cases and controls. Neither Gonsalves et al. Thus, if we had taken their approach e. However, although this is possible, we think it unlikely. That is, we identified a similarly low level of synaptic defects in our cases and controls, suggesting that we would have excluded a similar proportion of cells in both subject categories.
Thus, we think it more likely that other factors e. Clearly, additional studies will be needed to resolve this discrepancy and to determine the level of impairment in recombination, if any, in individuals with NOA. Importantly, these values are similar to those of the two other recently reported immunostaining studies of NOA Table II. Specifically, Gonsalves et al.
Taken together, these studies suggest that as many as one in five cases of NOA in which germ cells are present are due to abnormalities in meiotic prophase.
This has obvious clinical implications. For example, in instances in which testicular biopsies are ascertained for diagnostic purposes, immunostaining analysis is more likely to uncover the cause of infertility than are other standard tests e. Furthermore, in instances in which a complete meiotic arrest is identified on immunostaining and in which multiple regions of the testis are examined without any evidence for spermatogenesis, it seems reasonable to consider options other than ICSI e.
Comparison of cytological findings of other investigators in individuals with non-obstructive azoospermia NOA. Eight of these 18 individuals are represented in the data set collected by Gonsalves et al. The cases of meiotic arrest are also instructive with regard to the possible mutational sources of male infertility.
That is, the three meiotic arrest phenotypes in our study were similar superficially but on closer examination were clearly distinguishable: one case exhibited no synapsis, one partial synapsis but no MLH1 localization and the third partial synapsis and MLH1 deposition. The first two individuals Sp and Sp displayed a complete meiotic arrest because there were no sperm visible in the samples. This is reminiscent of the subtle differences in meiotic phenotype observed in mice homozygous for different null mutations.
Thus, it may be that there is no single major cause of meiotic arrest in human males but rather rarely occurring mutations involving any of a number of different meiotic genes. It will be important to confirm or refute this suggestion on additional series of cases because, if true, it will complicate attempts to determine the underlying molecular causes of male infertility. Google Scholar. Oxford University Press is a department of the University of Oxford.
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Sign In. Advanced Search. Search Menu. Article Navigation. Close mobile search navigation Article Navigation. Volume Article Contents Abstract. Materials and methods. Synaptic defects at meiosis I and non-obstructive azoospermia. Daniel Topping , Daniel Topping 6. E-mail: dtopping wsu.
Oxford Academic. Petrice Brown. LuAnn Judis. Stuart Schwartz. Allen Seftel. Anthony Thomas. Terry Hassold. At this point, the newly formed nuclei are both haploid. The cells produced are genetically unique because of the random assortment 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 Figure 5. Figure 5. Mitosis and meiosis are both forms of division of the nucleus in eukaryotic cells. They share some similarities, but also exhibit distinct differences that lead to very different 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 have the same number of sets of chromosomes, one set in the case of haploid cells and two sets in the case of diploid cells. In most plants and all animal species, it is typically diploid cells that undergo mitosis to form new diploid cells.
In contrast, meiosis consists of two nuclear divisions resulting in four nuclei that are usually partitioned into four new cells. The nuclei resulting from meiosis are not genetically identical and they contain one chromosome set only. This is half the number of chromosome sets in the original cell, which is diploid. The main 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 bound together with the synaptonemal complex, develop chiasmata and undergo crossover between sister chromatids, and line up 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 one. 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 analogous to a mitotic division. In this case, the duplicated chromosomes only one set of them line up on the metaphase plate with divided kinetochores attached to kinetochore fibers from opposite poles.
During anaphase II, as in mitotic anaphase, the kinetochores divide and one sister chromatid—now referred to as a chromosome—is pulled to one pole while the other sister chromatid is pulled to the other pole. If it were not for the fact that there 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 always 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 still one set of chromosomes, as there was at the end of meiosis I.
Figure 6. Meiosis and mitosis are both preceded by one round of DNA replication; however, 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.
Some characteristics of organisms are so widespread and fundamental that it is sometimes difficult to remember that they evolved like other simpler traits. Meiosis is such an extraordinarily complex series of cellular events that biologists have had trouble hypothesizing and testing how it may have evolved.
Although meiosis is inextricably entwined with sexual reproduction and its advantages and disadvantages, it is important to separate the questions of the evolution of meiosis and the evolution of sex, 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 have evolved. Meiosis and mitosis share obvious cellular processes and it makes sense that meiosis evolved from mitosis. The difficulty lies in the clear differences between meiosis I and 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 important, and that understanding how it evolved would make the evolutionary process clearer. They suggest genetic experiments that might shed light on the evolution of synapsis. There are other approaches to understanding the evolution of meiosis in progress. Different forms of meiosis exist in single-celled protists. Comparing the meiotic divisions of different protists may shed light on the evolution of meiosis.
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. Click through the steps of this interactive animation to compare the meiotic process of cell division to that of mitosis: How Cells Divide.
Sexual reproduction requires that diploid organisms produce haploid cells that can fuse during fertilization to form diploid offspring. As with mitosis, DNA replication occurs prior to meiosis during the S-phase of the cell cycle. Meiosis is a series of events that arrange 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 four nuclei and usually four daughter cells, each with half the number of chromosomes as the parent cell. The first separates homologs, and the second—like mitosis—separates chromatids into individual chromosomes.
During meiosis, variation in the daughter nuclei is introduced because 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, but have distinct outcomes. Mitotic divisions are single nuclear divisions that produce daughter nuclei that are genetically identical and have the same number of chromosome sets as the original cell.
Meiotic divisions include two nuclear divisions that produce four daughter nuclei that are genetically different and have one chromosome set instead of the two sets of chromosomes in the parent cell. The main differences between the processes occur in the first division 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 division. The second division of meiosis is more similar to a mitotic division, except that the daughter cells do not contain identical genomes because of crossover.
Describe the process that results i the formation of a tetrad. Explain how the random alignment of homologous chromosomes during metaphase I contributes to the variation in gametes produced by meiosis. 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? All of the stages of meiosis I, except possibly telophase I, are unique because homologous chromosomes are separated, not sister chromatids.
In some species, the chromosomes do not decondense and the nuclear envelopes do not form in telophase I. All of the stages of meiosis II have the same events as the stages of mitosis, with the possible exception of prophase II. In some species, the chromosomes are still condensed and there is no nuclear envelope. Other than this, all processes are the same. Skip to main content. Cell Division and Cell Cycle. Search for:. The Process of Meiosis Learning Objectives By the end of this section, you will be able to: Describe the behavior of chromosomes during meiosis Describe cellular events during meiosis Explain the differences between meiosis and mitosis Explain the mechanisms within meiosis that generate genetic variation among the products of meiosis.
Meiosis I Meiosis is preceded by an interphase consisting of the G 1 , S, and G 2 phases, which are nearly identical to the phases preceding mitosis. Prophase I Figure 1. Evolution Connection The Mystery of the Evolution of Meiosis Some characteristics of organisms are so widespread and fundamental that it is sometimes difficult to remember that they evolved like other simpler traits. Link to Learning Click through the steps of this interactive animation to compare the meiotic process of cell division to that of mitosis: How Cells Divide.
Additional Self Check Questions 1. What is the function of the fused kinetochore found on sister chromatids in prometaphase I? Answers 1.
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 attached to the nuclear envelope by their tips.
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