Prophase (from Ancient Greek προ- (pro-) 'before' and φάσις (phásis) 'appearance') is the first stage ofcell division in bothmitosis andmeiosis. Beginning afterinterphase,DNA has already been replicated when thecell enters prophase. The main occurrences in prophase are the condensation of thechromatin reticulum and the disappearance of thenucleolus.[3]
Microscopy can be used to visualize condensedchromosomes as they move throughmeiosis andmitosis.[4]
Various DNAstains are used to treat cells such that condensingchromosomes can be visualized as the move through prophase.[4]
ThegiemsaG-banding technique is commonly used to identifymammalianchromosomes, but utilizing the technology onplant cells was originally difficult due to the high degree of chromosome compaction in plant cells.[5][4]G-banding was fully realized for plant chromosomes in 1990.[6] During bothmeiotic andmitotic prophase,giemsa staining can be applied to cells to elicitG-banding inchromosomes.[2] Silver staining, a more modern technology, in conjunction withgiemsa staining can be used to image thesynaptonemal complex throughout the various stages ofmeiotic prophase.[7] To performG-banding,chromosomes must be fixed, and thus it is not possible to perform on living cells.[8]
Fluorescent stains such asDAPI can be used in both liveplant andanimal cells. These stains do not bandchromosomes, but instead allow for DNA probing of specific regions andgenes. Use offluorescent microscopy has vastly improvedspatial resolution.[9]
Prophase is the first stage ofmitosis inanimal cells, and the second stage ofmitosis inplant cells.[10] At the start of prophase there are two identical copies of eachchromosome in the cell due to replication ininterphase. These copies are referred to assister chromatids and are attached byDNA element called thecentromere.[11] The main events of prophase are: the condensation ofchromosomes, the movement of thecentrosomes, the formation of themitotic spindle, and the beginning ofnucleoli break down.[3]
DNA that wasreplicated ininterphase is condensed from DNA strands with lengths reaching 0.7 μm down to 0.2-0.3 μm.[3] This process employs thecondensin complex.[11] Condensed chromosomes consist of twosister chromatids joined at thecentromere.[12]
During prophase inanimal cells,centrosomes move far enough apart to be resolved using alight microscope.[3]Microtubule activity in eachcentrosome is increased due to recruitment ofγ-tubulin. Replicatedcentrosomes frominterphase move apart towards opposite poles of the cell, powered bycentrosome associated motor proteins.[13] Interdigitated interpolarmicrotubules from eachcentrosome interact with each other, helping to move thecentrosomes to opposite poles.[13][3]
Microtubules involved in theinterphase scaffolding break down as the replicatedcentrosomes separate.[3] The movement ofcentrosomes to opposite poles is accompanied inanimal cells by the organization of individual radialmicrotubule arrays (asters) by each centriole.[13] Interpolarmicrotubules from bothcentrosomes interact, joining the sets ofmicrotubules and forming the basic structure of themitotic spindle.[13] Plant cells do not have centrosomes and thechromosomes cannucleatemicrotubule assembly into themitotic apparatus.[13] Inplant cells,microtubules gather at opposite poles and begin to form thespindle apparatus at locations called foci.[10] Themitotic spindle is of great importance in the process ofmitosis and will eventually segregate thesister chromatids inmetaphase.[3]
Thenucleoli begin to break down in prophase, resulting in the discontinuation of ribosome production.[3] This indicates a redirection of cellular energy from general cellular metabolism tocellular division.[3] Thenuclear envelope stays intact during this process.[10]
Meiosis involves two rounds ofchromosome segregation and thus undergoes prophase twice, resulting in prophase I and prophase II.[12] Prophase I is the most complex phase in all of meiosis becausehomologous chromosomes must pair and exchangegenetic information.[3]: 98 Prophase II is very similar tomitotic prophase.[12]
Prophase I is divided into five phases: leptotene, zygotene, pachytene, diplotene, and diakinesis. In addition to the events that occur inmitotic prophase, several crucial events occur within these phases such as pairing ofhomologous chromosomes and the reciprocalexchange of genetic material between thesehomologous chromosomes. Prophase I occurs at different speeds dependent onspecies andsex. Many species arrestmeiosis in diplotene of prophase I untilovulation.[3]: 98 In humans, decades can pass asoocytes remain arrested in prophase I only to quickly complete meiosis I prior toovulation.[12]
In the first stage of prophase I, leptotene (from the Greek for "delicate"),chromosomes begin to condense. Each chromosome is in adiploid state and consists of twosister chromatids; however, thechromatin of thesister chromatids is not yet condensed enough to be resolvable inmicroscopy.[3]: 98 Homologous regions withinhomologous chromosome pairs begin to associate with each other.[2]
In the second phase of prophase I, zygotene (from the Greek for "conjugation"), all maternally and paternally derivedchromosomes have found theirhomologous partner.[3]: 98 The homologous pairs then undergo synapsis,a process by which thesynaptonemal complex (a proteinaceous structure) aligns corresponding regions ofgenetic information on maternally and paternally derived non-sisterchromatids ofhomologous chromosome pairs.[3]: 98 [12] The paired homologous chromosome bound by thesynaptonemal complex are referred to asbivalents or tetrads.[10][3]: 98 Sex (X and Y) chromosomes do not fully synapse because only a small region of the chromosomes are homologous.[3]: 98
Thenucleolus moves from a central to a peripheral position in thenucleus.[14]
The third phase of prophase I, pachytene (from the Greek for "thick"), begins at the completion of synapsis.[3]: 98 Chromatin has condensed enough thatchromosomes can now be resolved inmicroscopy.[10] Structures called recombination nodules form on thesynaptonemal complex ofbivalents. These recombination nodules facilitategenetic exchange between the non-sister chromatids of thesynaptonemal complex in an event known ascrossing-over or genetic recombination.[3]: 98 Multiple recombination events can occur on each bivalent. In humans, an average of 2-3 events occur on each chromosome.[13]: 681
In the fourth phase of prophase I, diplotene (from the Greek for "twofold"),crossing-over is completed.[3]: 99 [10]Homologous chromosomes retain a full set of genetic information; however, thehomologous chromosomes are now of mixed maternal and paternal descent.[3]: 99 Visible junctions called chiasmata hold thehomologous chromosomes together at locations where recombination occurred as thesynaptonemal complex dissolves.[12][3]: 99 It is at this stage where meiotic arrest occurs in manyspecies.[3]: 99
In the fifth and final phase of prophase I, diakinesis (from the Greek for "double movement"), full chromatin condensation has occurred and all foursister chromatids can be seen inbivalents withmicroscopy. The rest of the phase resemble the early stages of mitoticprometaphase, as the meiotic prophase ends with thespindle apparatus beginning to form, and thenuclear membrane beginning to break down.[10][3]: 99
Prophase II ofmeiosis is very similar to prophase ofmitosis. The most noticeable difference is that prophase II occurs with ahaploid number ofchromosomes as opposed to thediploid number in mitotic prophase.[12][10] In bothanimal andplant cells chromosomes may de-condense duringtelophase I requiring them to re-condense in prophase II.[3]: 100 [10] If chromosomes do not need to re-condense, prophase II often proceeds very quickly as is seen in themodel organismArabidopsis.[10]
Female mammals and birds are born possessing all the oocytes needed for future ovulations, and theseoocytes are arrested at the prophase I stage ofmeiosis.[15] In humans, as an example, oocytes are formed between three and four months ofgestation within the fetus and are therefore present at birth. During this prophase I arrested stage (dictyate), which may last for decades, four copies of thegenome are present in the oocytes. The adaptive significance of prophase I arrest is still not fully understood. However, it has been proposed that the arrest of oocytes at the four genome copy stage may provide the informational redundancy needed torepair damage in the DNA of thegermline.[15] The repair process used appears to behomologous recombinational repair.[15][16] Prophase arrested oocytes have a high capability for efficient repair ofDNA damages.[16] DNA repair capability appears to be a key quality control mechanism in the female germ line and a critical determinant offertility.[16]
The most notable difference between prophase inplant cells andanimal cells occurs because plant cells lackcentrioles. The organization of thespindle apparatus is associated instead with foci at opposite poles of the cell or is mediated by chromosomes. Another notable difference ispreprophase, an additional step in plantmitosis that results in formation of thepreprophase band, a structure composed ofmicrotubules. Inmitotic prophase I of plants, this band disappears.[10]
Prophase I inmeiosis is the most complex iteration of prophase that occurs in bothplant cells andanimal cells.[3] To ensure pairing ofhomologous chromosomes andrecombination of genetic material occurs properly, there arecellular checkpoints in place. The meiotic checkpoint network is aDNA damage response system that controlsdouble strand break repair,chromatin structure, and the movement and pairing ofchromosomes.[17] The system consists of multiple pathways (including themeiotic recombination checkpoint) that prevent the cell from enteringmetaphase I with errors due to recombination.[18]
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