Does Replication Happen Again Prior to Prophase 2
Definition
During prophase II of meiosis 2, 4 important steps occur. These are the condensing of chromatin into chromosomes, disintegration of the nuclear envelope, migration of centrosomes to either pole, and the reconstruction of the spindle apparatus. Notwithstanding, centrosomes are non present in all cells.
What Happens During Prophase II?
Meiosis II occurs in both daughter cells that were formed during meiosis I. As no Deoxyribonucleic acid replication takes place in this second step of meiosis, the prison cell division process immediately begins. Prophase II prepares the jail cell for secondary meiotic division where two haploid cells eventually class 4 haploid cells, each containing one-half of the genetic information previously contained in the original, replicated diploid cell. A much less circuitous stage than prophase I, prophase II does not include the steps of leptotene, zygotene, pachytene, diplotene, and diakinesis simply more resembles the simpler process of mitotic prophase in non-gamete (somatic) cell division.
Prophase II begins immediately after cytokinesis – the splitting of the diploid gamete into two haploid girl cells. In order to prepare for the side by side division, prophase II condenses chromatin get-go into chromatids, and then more tightly into chromosomes. Simultaneously, the nuclear membrane dissolves, leaving an open area of cytoplasm in which a network of proteins (microtubules) has enough space to create pathways reaching from one side of the jail cell to the other – the spindle apparatus.
After the condensing of chromatin into chromosomes, and subsequently the disintegration of the nuclear envelope containing these chromosomes, the centrosomes migrate to either pole. Even in the absence of centrosomes, the reconstruction of the spindle apparatus used during meiosis I can be initiated. These four steps characterize prophase Two.
Building a Spindle Appliance Without Centrosomes
Electric current research is looking at the part of centrioles in human being spindle apparatus formation, as female person gametes – oocytes – do non have them. Centrioles are constructed from microtubules and, certainly in male gametes and during mitotic cell partition, play an important function in spindle appliance construction. During the normal cell cycle, centrioles replicate to form pairs that are later enveloped in pericentriolar material (PCM). From this betoken on they are referred to every bit centrosomes. Two centrosomes, each containing a centriole pair, drift to either stop of the prison cell – the poles. During mitosis and in male person gametes, centrosomes are responsible for amalgam a network of microtubules which extend from inside the centrosome towards the cell's eye. Construction of this network occurs at a afterwards phase.
Yet, female person reproductive cells have been discovered not to contain centrosomes and instead course the spindle appliance from existing microtubules within the cytoplasm. The lack of a chromosome or presence of an actress chromosome in daughter cells caused by faults during chromosome separation is known every bit nondisjunction, where resulting gametes produce abnormal embryos. Nondisjunction of chromosomes is the primary cause of infertility and miscarriage, as it is during female gamete division (meiosis I and 2) that chromosomes are most likely to exist unevenly divided betwixt daughter cells. Fifty-fifty and so, most sources list centrosomes equally the manufacturers of the microtubules which make up the spindle apparatus. This may be truthful in terms of spermatozoa and somatic cells, merely does not apply to higher plants and the ova of many other species, including the homo race.
This means that, while information technology is understood that near human cells crave at least two centrioles, the fertilized zygote contains only one which it inherited from the male gamete. Equally with Dna, male gamete centrioles are not replicated during prophase Ii. Yet, equally the zygote grows via mitosis, further cells do comprise two centrioles; it is not notwithstanding fully understood how the second centriole is formed but its advent may exist linked to precursor proteins contained either in the spermatozoa or ova.
Deviation between Prophase I and Two
Prophase I consists of five stages. These 5 stages are not represented in prophase II. Prophase II encompasses four different mechanisms; namely the tight wrapping of Dna into chromosomes, the dissolving of the nuclear membrane, migration of the centrosomes (when nowadays), and the reconstruction of the spindle apparatus.
In prophase I, the first stage is known as leptotene. This stage involves the unwinding of the DNA structure to enable an exchange of alleles between homologous chromosome pairs. No crossing over occurs in prophase II. Therefore, prophase Two does not feature leptotene.
The second stage of prophase I is called zygotene. This involves the attachment of a chromosome pair prior to crossing over. As no crossing over occurs in prophase 2, this phase is besides non included. This is also the case for subsequent phases: pachytene (crossing over), diplotene (breaking down of the crossing-over network), and diakinesis (movement of crossing-over aqueduct to chromatid arms).
Withal, some parts of prophase I and II are similar. These include the dissolution of the nuclear envelope and centrosome migration. Even then, prophase II of meiosis Two is normally compared to the prophase of mitosis, where nuclear envelope breakdown occurs together with centrosome migration, condensation of chromosomes, and the formation of the spindle apparatus. In prophase Ii, the correct term for the latter of these mechanisms is reformation, as the microtubules that fabricated up the spindle appliance previously constructed in meiosis I are all the same available.
And Why the Deviation?
When looking at the differences between primary meiotic division and secondary meiotic partition, it is always helpful to remember the goal of meiosis – to produce four dissimilar gametes (spermatozoa or ova) containing a single just complete portion of genetic information which, when combined with some other gamete, creates a prison cell (zygote) which has a consummate (double) set of genetic data from each of two parents.
Genetic Variation
To ensure that this genetic information is non exactly the same and thus contributes to the genetic variation within a species, meiosis I includes a phase in which alleles are swapped between a chromosome pair (crossing over) to produce recombinant chromosomes. This must occur before the cell divides, and takes place in the first prophase of meiosis I. Once this has occurred, the ii girl cells will comprise slightly different genes.
This means that at that place is no demand for crossing over during the second cell division. Also, as crossing over only occurs between two replicated, paired chromosomes this is simply not possible in a haploid cell, which does not contain replicated pairs, just pairs.
The distinction between chromosome pairing and replication is often confusing, as when one considers the second cell division in meiosis Two, some images seem to evidence a single 10-shaped chromosome being divide in one-half to provide half a chromosome. This is very incorrect.
The human chromosome, as seen on the karyotype beneath, consists of 46 unmarried chromosomes. Human DNA holds the genetic data for the entire human torso inside these 46 chromosomes, withal this data is repeated every bit information technology is sourced from two parents. Using chromosome ane as an case, which contains approximately eight% of the genetic data required to produce a human being, we can iron out a lot of unnecessary confusion.
In the image below, 2 strands of Deoxyribonucleic acid make up chromosome 1. These single strands are usually referred to every bit chromatids, although this is technically incorrect every bit the divergence between chromatid and chromosome is more to exercise with the fashion in which the Dna molecule is packaged. Nevertheless, by calling each strand a chromatid, the replication process becomes less confusing.
1 chromatid derives from the spermatozoon of the male parent – a full set of data in the cell created during the final stages of meiosis Two. The other chromatid derives from the ovum of the mother. Again, a full set of data in each somatic prison cell similarly created during the final stages of meiosis 2. Together, both chromatids contain two sets of information with slight differences – they have the same genes at the same positions (loci), but may incorporate different alleles.
Lack of a Homologous Chromosome Pair
The default number of chromatids in any human cell (apart from the gamete) is 46. During preparations for cell partition these 46 chromatids pair up – like to like. Chromatid one of the father draws close to chromatid 1 of the mother, and so on. In this case, homologous chromosome ane is the issue. Throughout the human karyotype, 23 pairs of homologous chromosomes which are not physically attached to each other are the event. A cell which contains 23 pairs of homologous chromosomes is known as a diploid cell. Below, the left-hand image shows a pair of not-replicated homologous chromosomes (chromatids); yellow and orangish announce the genetic information provided by each parent.
During the replication process of the cell bike and prior to whatever type of cell sectionalisation, all Deoxyribonucleic acid is replicated. Chromatid one of the father is replicated, as is chromatid 1 of the mother, and so on. Each replicated chromosome is fastened to its re-create by manner of a centromere, forming the typical X-shape oft seen in textbooks. Chromosome 1 no longer consists of two split up chromatids, but of a pair of twin or 'sister' chromatids. The complete homo karyotype still consists of 23 pairs of chromosomes (one from the male parent, 1 from the mother), only each parent's single chromatid strand has been doubled to make ii sis chromatids. Whereas before replication the human being karyotype is represented past 23 homologous pairs of 46 chromatids, the replicated karyotype is represented by 23 homologous pairs of 92 chromatids. It is therefore always important to either signal or be enlightened of whether a chromosome pair is replicated or not. The right-manus prototype beneath conspicuously shows the original and replicated sister chromosomes in a single homologous chromosome pair.
In the meiotic prophase I, crossing over occurs. This procedure swaps over a variety of alleles in lodge to produce sexual activity cells (gametes) which are not clones of either parent. Once crossing over has occurred the homologous pair is known by other names – the tetrad, the bivalent or recombinant chromosomes/chromatids. It is piece of cake to meet that crossing over occurs betwixt similar alleles of each parent's chromatids in the diagram below. Genetic information is not exchanged betwixt sister chromatids of a single chromosome just between the chromatids of the homologous pair. The absence of homologous pairs in haploid cells is the reason why no farther crossing over occurs during prophase 2.
After crossing over, the tetrads (recombinant chromosome pairs) can be separated. Tetrads incorporate 23 chromosome pairs composed of 92 chromatids. Over the adjacent stages of meiosis I, each tetrad is separated from its paired partner (and is therefore no longer called a tetrad, bivalent or recombinant pair). Instead, a single replicated chromatid (X-course) moves to 1 side of the cell, and the other half of the former pair moves to the other side. As recombinant replicated chromatids, they contain a mix of alleles from both parents.
In prophase II, no crossing over occurs every bit this must take place between homologous chromosome pairs. The spindle apparatus forms in order to intermission apart the replicated chromatids. In a female person, this results in secondary polar bodies, each containing a single chromatid (it can just as hands be called a chromosome) which itself contains the unabridged human genome. One or more than of these will develop into an ovum. In males, the result is four spermatozoa. The deviation is clear in the image below, with red and blueish chromosomes or chromatids depicting the parentage, although after metaphase I pocket-sized parts of genes are crossed over.
Two Sets of Chromosomes From 2 Different Sources
The goal of meiosis is to produce offspring of the same species just with genetic variations. The following image shows recombinant chromatids in egg and sperm combining upon fertilization of the egg to produce a zygote. The zygote is described as a haploid cell as it contains chromosome pairs but these are separated by nuclei. Just during its first mitotic partitioning volition these nuclei dissolve and allow the chromosomes of both parents to align on the spindle apparatus as pairs. The resulting two embryonic cells will be diploid.
In the instance beneath, sperm meets egg to produce a zygote containing both sets of genetic information. However, this prototype is a typical example of why replication and chromosome pairs are oft disruptive or misinterpreted. The genetic data contained inside egg, sperm, and zygote is depicted here as replicated pairs. Immediately upon fertilization, egg, sperm, and zygote should not feature X-shaped chromosomes but single-strand chromatids. Just in preparation for the zygote's first partition, and after Dna replication has taken place, can this genetic data exist correctly represented by X-shaped chromosomes.
Quiz
Source: https://biologydictionary.net/prophase-2/
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