Editing Prophase

{{short description|First phase of cell division in both mitosis and meiosis}}
[[File:Prophase eukaryotic mitosis.svg|thumb|300px|Prophase is the first step of cell division in mitosis. As it occurs after G2 of interphase, DNA has been already replicated when prophase begins.<ref name="Nussbaum 2016 12–20">{{cite book |title=Thompson & Thompson Genetics in Medicine |vauthors=Nussbaum RL, McInnes RR, Huntington F |publisher=[[Elsevier]] |year=2016 |isbn=9781437706963 |location=Philadelphia |pages=12–20}}</ref>]]
[[File:3D-SIM-3 Prophase 3 color.jpg|thumb|right|200px|[[Fluorescence microscope]] image of two mouse cell nuclei in prophase (scale bar is 5&nbsp;μm).<ref name=":0">{{cite journal |vauthors=Schermelleh L, Carlton PM, Haase S, Shao L, Winoto L, Kner P, Burke B, Cardoso MC, Agard DA, Gustafsson MG, Leonhardt H, Sedat JW |display-authors=6 |title=Subdiffraction multicolor imaging of the nuclear periphery with 3D structured illumination microscopy |journal=[[Science (journal)|Science]] |volume=320 |issue=5881 |pages=1332–36 |date=June 2008 |pmid=18535242 |pmc=2916659 |doi=10.1126/science.1156947 |bibcode=2008Sci...320.1332S}}</ref>]]

'''Prophase''' ({{etymology|grc|''[[wikt:προ-|προ-]]'' ([[wikt:pro-|pro-]])|before||''[[wikt:|φάσις]]'' (phásis)|appearance}}) is the first stage of [[cell division]] in both [[mitosis]] and [[meiosis]]. Beginning after [[interphase]], [[DNA]] has already been replicated when the [[Cell (biology)|cell]] enters prophase. The main occurrences in prophase are the condensation of the [[chromatin]] reticulum and the disappearance of the [[nucleolus]].<ref name=":2" />

== Staining and microscopy ==
[[Microscopy]] can be used to visualize condensed [[chromosome]]s as they move through [[meiosis]] and [[mitosis]].<ref name=":6">{{Cite book|title=Plant Cytogenetics | edition = Third| vauthors = Singh RJ |publisher=CBC Press, Taylor & Francis Group|year=2017|isbn=9781439884188|location=Boca Raton, FL|pages=19}}</ref>

Various DNA [[Staining|stains]] are used to treat cells such that condensing [[chromosome]]s can be visualized as the move through prophase.<ref name=":6" />

The [[Giemsa stain|giemsa]] [[G banding|G-banding]] technique is commonly used to identify [[mammal]]ian [[chromosome]]s, but utilizing the technology on [[plant cell]]s was originally difficult due to the high degree of chromosome compaction in plant cells.<ref>{{Cite journal| vauthors = Wang HC, Kao KN |date=1988|title=G-banding in plant chromosomes|journal=Genome|volume=30|pages=48–51|via=ResearchGate|doi=10.1139/g88-009|s2cid=83823255 }}</ref><ref name=":6" /> [[G banding|G-banding]] was fully realized for plant chromosomes in 1990.<ref>{{cite journal | vauthors = Kakeda K, Yamagata H, Fukui K, Ohno M, Fukui K, Wei ZZ, Zhu ES | title = High resolution bands in maize chromosomes by G-banding methods | journal = Theoretical and Applied Genetics | volume = 80 | issue = 2 | pages = 265–72 | date = August 1990 | pmid = 24220906 | doi = 10.1007/BF00224397 | s2cid = 6600449 }}</ref> During both [[Meiosis|meiotic]] and [[Mitosis|mitotic]] prophase, [[giemsa stain]]ing can be applied to cells to elicit [[G banding|G-banding]] in [[chromosome]]s.<ref name=":0" /> Silver staining, a more modern technology, in conjunction with [[Giemsa stain|giemsa staining]] can be used to image the [[synaptonemal complex]] throughout the various stages of [[Meiosis|meiotic]] prophase.<ref>{{cite journal | vauthors = Pathak S, Hsu TC | title = Silver-stained structures in mammalian meiotic prophase | journal = Chromosoma | volume = 70 | issue = 2 | pages = 195–203 | date = January 1979 | pmid = 85512 | doi = 10.1007/bf00288406 | s2cid = 27763957 }}</ref> To perform [[G banding|G-banding]], [[chromosome]]s must be fixed, and thus it is not possible to perform on living cells.<ref>{{cite journal | vauthors = Sumner AT | title = The nature and mechanisms of chromosome banding | journal = Cancer Genetics and Cytogenetics | volume = 6 | issue = 1 | pages = 59–87 | date = May 1982 | pmid = 7049353 | doi = 10.1016/0165-4608(82)90022-x }}</ref>

[[Fluorescence microscope|Fluorescent stains]] such as [[DAPI]] can be used in both live [[Plant cell|plant]] and [[Cell (biology)|animal cells]]. These stains do not band [[chromosome]]s, but instead allow for DNA probing of specific regions and [[gene]]s. Use of [[Fluorescence microscope|fluorescent microscopy]] has vastly improved [[spatial resolution]].<ref>{{cite journal | vauthors = de Jong H | title = Visualizing DNA domains and sequences by microscopy: a fifty-year history of molecular cytogenetics | journal = Genome | volume = 46 | issue = 6 | pages = 943–6 | date = December 2003 | pmid = 14663510 | doi = 10.1139/g03-107 }}</ref>

== Mitotic prophase ==
Prophase is the first stage of [[mitosis]] in [[Cell (biology)|animal cells]], and the second stage of [[mitosis]] in [[plant cell]]s.<ref name=":1">{{Cite book|title=Plant Physiology and Development| vauthors = Taiz L, Zeiger E, Moller IM, Murphy A |publisher=Sinauer Associates|year=2015|isbn=978-1-60535-255-8|location=Sunderland MA|pages=35–39}}</ref> At the start of prophase there are two identical copies of each [[chromosome]] in the cell due to replication in [[interphase]]. These copies are referred to as [[sister chromatids]] and are attached by [[DNA]] element called the [[centromere]].<ref name=":5">{{cite journal| vauthors = Zeng XL, Jiao MD, Wang XG, Song ZX, Rao S |date=2001|title=Electron microscopic studies on the Silver-stained Nucleolar Cycle of Physarum Polycephalum|url=http://www.jipb.net/pubsoft/content/2/2071/X000541(PS2).pdf|journal=Acta Botanica Sinica|volume=43|issue=7|pages=680–5|access-date=24 February 2015|archive-url=https://web.archive.org/web/20181001070430/http://www.jipb.net/pubsoft/content/2/2071/X000541(PS2).pdf|archive-date=2018-10-01}}</ref> The main events of prophase are: the condensation of [[chromosome]]s, the movement of the [[centrosome]]s, the formation of the [[Spindle apparatus|mitotic spindle]], and the beginning of [[Nucleolus|nucleoli]] break down.<ref name=":2">{{Cite book|title=Genetics From Genes to Genomes| vauthors = Hartwell LH, Hood L, Goldberg ML, Reynolds AE, Silver LM, Veres RC |publisher=McGraw-Hill|year=2008|isbn=978-0-07-284846-5|location=New York|pages=[https://archive.org/details/genetics00lela_0/page/90 90–103]|url-access=registration|url=https://archive.org/details/genetics00lela_0/page/90}}</ref>

=== Condensation of chromosomes ===
[[DNA]] that was [[DNA replication|replicated]] in [[interphase]] is condensed from DNA strands with lengths reaching 0.7 μm down to 
0.2-0.3 μm.<ref name=":2" /> This process employs the [[condensin]] complex.<ref name=":5" /> Condensed chromosomes consist of two [[sister chromatids]] joined at the [[centromere]].<ref name=":3">{{Cite book|title=Thompson & Thompson Genetics in Medicine| vauthors = Nussbaum RL, McInnes RR, Willard HF |publisher=Elsevier|year=2016|isbn=978-1-4377-0696-3|location=Philadelphia|pages=12–20}}</ref>

=== Movement of centrosomes ===
During prophase in [[Cell (biology)|animal cells]], [[centrosome]]s move far enough apart to be resolved using a [[light microscope]].<ref name=":2" /> [[Microtubule]] activity in each [[centrosome]] is increased due to recruitment of [[Tubulin#γ-Tubulin|γ-tubulin]]. Replicated [[centrosome]]s from [[interphase]] move apart towards opposite poles of the cell, powered by [[Motor protein|centrosome associated motor proteins]].<ref name=":4">{{Cite book|title=Essential Cell Biology| vauthors = Alberts B, Bray D, Hopkin K, Johnson A, Lewis J, Raff M, Roberts K, Walter P |publisher=Garland Science|year=2004|isbn=978-0-8153-3481-1|location=New York NY|pages=[https://archive.org/details/essentialcellbio00albe/page/639 639–658]|url=https://archive.org/details/essentialcellbio00albe/page/639}}</ref> Interdigitated interpolar [[microtubule]]s from each [[centrosome]] interact with each other, helping to move the [[centrosome]]s to opposite poles.<ref name=":4" /><ref name=":2" />

=== Formation of the mitotic spindle ===
[[Microtubule]]s involved in the [[interphase]] scaffolding break down as the replicated [[centrosome]]s separate.<ref name=":2" /> The movement of [[centrosome]]s to opposite poles is accompanied in [[Cell (biology)|animal cells]] by the organization of individual radial [[microtubule]] arrays (asters) by each centriole.<ref name=":4" /> Interpolar [[microtubule]]s from both [[centrosome]]s interact, joining the sets of [[microtubule]]s and forming the basic structure of the [[Spindle apparatus|mitotic spindle]].<ref name=":4" /> Plant cells do not have centrosomes and the [[chromosome]]s can [[Nucleation|nucleate]] [[microtubule]] assembly into the [[Spindle apparatus|mitotic apparatus]].<ref name=":4" /> In [[plant cell]]s, [[microtubule]]s gather at opposite poles and begin to form the [[spindle apparatus]] at locations called foci.<ref name=":1" /> The [[Spindle apparatus|mitotic spindle]] is of great importance in the process of [[mitosis]] and will eventually segregate the [[sister chromatids]] in [[metaphase]].<ref name=":2" />

=== Beginning of nucleoli breakdown ===
The [[Nucleolus|nucleoli]] begin to break down in prophase, resulting in the discontinuation of ribosome production.<ref name=":2" /> This indicates a redirection of cellular energy from general cellular metabolism to [[Cell division|cellular division]].<ref name=":2" /> The [[Nuclear membrane|nuclear envelope]] stays intact during this process.<ref name=":1" />

== Meiotic prophase ==
[[Meiosis]] involves two rounds of [[chromosome segregation]] and thus undergoes prophase twice, resulting in prophase I and prophase II.<ref name=":3" /> Prophase I is the most complex phase in all of meiosis because [[homologous chromosome]]s must pair and exchange [[Nucleic acid sequence|genetic information]].<ref name=":2" />{{rp|98}} Prophase II is very similar to [[Mitosis|mitotic]] prophase.<ref name=":3" />

=== Prophase I ===
Prophase I is divided into five phases: leptotene, zygotene, pachytene, diplotene, and diakinesis. In addition to the events that occur in [[Mitosis|mitotic]] prophase, several crucial events occur within these phases such as pairing of [[homologous chromosome]]s and the reciprocal [[Chromosomal crossover|exchange of genetic material]] between these [[homologous chromosome]]s. Prophase I occurs at different speeds dependent on [[species]] and [[sex]]. Many species arrest [[meiosis]] in diplotene of prophase I until [[ovulation]].<ref name=":2" />{{rp|98}} In humans, decades can pass as [[oocyte]]s remain arrested in prophase I only to quickly complete meiosis I prior to [[ovulation]].<ref name=":3" />

==== Leptotene ====
{{main|Leptotene stage}}
In the first stage of prophase I, leptotene (from the Greek for "delicate"), [[chromosome]]s begin to condense. Each chromosome is in a [[Ploidy|diploid]] state and consists of two [[sister chromatids]]; however, the [[chromatin]] of the [[sister chromatids]] is not yet condensed enough to be resolvable in [[microscopy]].<ref name=":2" />{{rp|98}} [[Homology (biology)|Homologous]] regions within [[homologous chromosome]] pairs begin to associate with each other.<ref name=":0" />

==== Zygotene ====
In the second phase of prophase I, zygotene (from the Greek for "conjugation"), all maternally and paternally derived [[chromosome]]s have found their [[Homologous chromosome|homologous]] partner.<ref name=":2" />{{rp|98}} The homologous pairs then undergo synapsis,a process by which the [[synaptonemal complex]] (a proteinaceous structure) aligns corresponding regions of [[Nucleic acid sequence|genetic information]] on maternally and paternally derived non-sister [[chromatid]]s of [[homologous chromosome]] pairs.<ref name=":2" />{{rp|98}}<ref name=":3" />  The paired homologous chromosome bound by the [[synaptonemal complex]] are referred to as [[Bivalent (genetics)|bivalents]] or tetrads.<ref name=":1" /><ref name=":2" />{{rp|98}} [[Allosome|Sex (X and Y) chromosomes]] do not fully synapse because only a small region of the chromosomes are homologous.<ref name=":2" />{{rp|98}}

The [[nucleolus]] moves from a central to a peripheral position in the [[Cell nucleus|nucleus]].<ref>{{cite journal | vauthors = Zickler D, Kleckner N | title = The leptotene-zygotene transition of meiosis | journal = Annual Review of Genetics | volume = 32 | pages = 619–97 | date = 1998 | pmid = 9928494 | doi = 10.1146/annurev.genet.32.1.619 }}</ref>

==== Pachytene ====
The third phase of prophase I, pachytene (from the Greek for "thick"), begins at the completion of synapsis.<ref name=":2" />{{rp|98}} [[Chromatin]] has condensed enough that [[chromosome]]s can now be resolved in [[microscopy]].<ref name=":1" /> Structures called recombination nodules form on the [[synaptonemal complex]] of [[Bivalent (genetics)|bivalents]]. These recombination nodules facilitate [[Chromosomal crossover|genetic exchange]] between the non-sister chromatids of the [[synaptonemal complex]] in an event known as [[Chromosomal crossover|crossing-over]] or genetic recombination.<ref name=":2" />{{rp|98}} Multiple recombination events can occur on each bivalent. In humans, an average of 2-3 events occur on each chromosome.<ref name=":4" />{{rp|681}}

==== Diplotene ====
In the fourth phase of prophase I, diplotene (from the Greek for "twofold"), [[Chromosomal crossover|crossing-over]] is completed.<ref name=":2" />{{rp|99}}<ref name=":1" /> [[Homologous chromosome]]s retain a full set of genetic information; however, the [[homologous chromosome]]s are now of mixed maternal and paternal descent.<ref name=":2" />{{rp|99}} Visible junctions called chiasmata hold the [[homologous chromosome]]s together at locations where recombination occurred as the [[synaptonemal complex]] dissolves.<ref name=":3" /><ref name=":2" />{{rp|99}} It is at this stage where meiotic arrest occurs in many [[species]].<ref name=":2" />{{rp|99}}

==== Diakinesis ====
In the fifth and final phase of prophase I, diakinesis (from the Greek for "double movement"), full chromatin condensation has occurred and all four [[sister chromatids]] can be seen in [[Bivalent (genetics)|bivalents]] with [[microscopy]]. The rest of the phase resemble the early stages of mitotic [[prometaphase]], as the meiotic prophase ends with the [[spindle apparatus]] beginning to form, and the [[nuclear membrane]] beginning to break down.<ref name=":1" /><ref name=":2" />{{rp|99}}

=== Prophase II ===
Prophase II of [[meiosis]] is very similar to prophase of [[mitosis]]. The most noticeable difference is that prophase II occurs with a [[Ploidy|haploid]] number of [[chromosome]]s as opposed to the [[Ploidy|diploid]] number in mitotic prophase.<ref name=":3" /><ref name=":1" /> In both [[Cell (biology)|animal]] and [[plant cell]]s chromosomes may de-condense during [[telophase]] I requiring them to re-condense in prophase II.<ref name=":2" />{{rp|100}}<ref name=":1" /> If chromosomes do not need to re-condense, prophase II often proceeds very quickly as is seen in the [[model organism]] [[Arabidopsis]].<ref name=":1" />

==Prophase I arrest==
Female mammals and birds are born possessing all the oocytes needed for future ovulations, and these [[oocyte]]s are arrested at the prophase I stage of [[meiosis]].<ref name = Mira1998>{{cite journal | vauthors = Mira A | title = Why is meiosis arrested? | journal = Journal of Theoretical Biology | volume = 194 | issue = 2 | pages = 275–87 | date = September 1998 | pmid = 9778439 | doi = 10.1006/jtbi.1998.0761 | bibcode = 1998JThBi.194..275M }}</ref>  In humans, as an example, oocytes are formed between three and four months of [[gestation]] within the fetus and are therefore present at birth.  During this prophase I arrested stage ([[dictyate]]), which may last for decades, four copies of the [[genome]] 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 to [[DNA repair|repair damage in the DNA]] of the [[germline]].<ref name = Mira1998/>  The repair process used appears to be [[homologous recombination]]al repair<ref name = Mira1998/><ref name = Stringer2020>{{cite journal | vauthors = Stringer JM, Winship A, Zerafa N, Wakefield M, Hutt K | title = Oocytes can efficiently repair DNA double-strand breaks to restore genetic integrity and protect offspring health | journal = Proceedings of the National Academy of Sciences of the United States of America | volume = 117 | issue = 21 | pages = 11513–11522 | date = May 2020 | pmid = 32381741 | pmc = 7260990 | doi = 10.1073/pnas.2001124117 | bibcode = 2020PNAS..11711513S | doi-access = free }}</ref> Prophase arrested oocytes have a high capability for efficient repair of [[DNA damage (naturally occurring)|DNA damages]].<ref name = Stringer2020/>  DNA repair capability appears to be a key quality control mechanism in the female germ line and a critical determinant of [[fertility]].<ref name = Stringer2020/>

== Differences in plant and animal cell prophase ==
[[File:Preprophase.jpg|thumb|''Arabidopsis thaliana'' cell in preprophase, prophase and prometaphase. Preprophase band is present along the cell wall from images 1–3, is fading in image 4, and disappears by image 5.<ref name="Nussbaum 2016 12–20"/>]]
The most notable difference between prophase in [[plant cell]]s and [[Cell (biology)|animal cells]] occurs because plant cells lack [[centriole]]s. The organization of the [[spindle apparatus]] is associated instead with foci at opposite poles of the cell or is mediated by chromosomes. Another notable difference is [[preprophase]], an additional step in plant [[mitosis]] that results in formation of the [[preprophase band]], a structure composed of [[microtubule]]s. In [[Mitosis|mitotic]] prophase I of plants, this band disappears.<ref name=":1" />

== Cell checkpoints ==
Prophase I in [[meiosis]] is the most complex iteration of prophase that occurs in both [[plant cell]]s and [[Cell (biology)|animal cells]].<ref name=":2" /> To ensure pairing of [[homologous chromosome]]s and [[Homologous recombination|recombination of genetic material]] occurs properly, there are [[Cell cycle checkpoint|cellular checkpoints]] in place. The meiotic checkpoint network is a [[DNA damage]] response system that controls [[DNA repair|double strand break]] repair, [[chromatin]] structure, and the movement and pairing of [[chromosome]]s.<ref>{{cite journal | vauthors = Hochwagen A, Amon A | title = Checking your breaks: surveillance mechanisms of meiotic recombination | journal = Current Biology | volume = 16 | issue = 6 | pages = R217-28 | date = March 2006 | pmid = 16546077 | doi = 10.1016/j.cub.2006.03.009 | doi-access = free | bibcode = 2006CBio...16.R217H }}</ref> The system consists of multiple pathways (including the [[meiotic recombination checkpoint]]) that prevent the cell from entering [[metaphase I]] with errors due to recombination.<ref>{{cite journal | vauthors = MacQueen AJ, Hochwagen A | title = Checkpoint mechanisms: the puppet masters of meiotic prophase | journal = Trends in Cell Biology | volume = 21 | issue = 7 | pages = 393–400 | date = July 2011 | pmid = 21531561 | doi = 10.1016/j.tcb.2011.03.004 }}</ref>

== See also ==
*[[Prometaphase]]
*[[Metaphase]]
*[[Anaphase]]
*[[Telophase]]
*[[Meiosis]]
*[[Mitosis]]
*[[Cytoskeleton]]
*[[Homologous chromosome]]

== References ==
<references />

== External links ==
* {{commons category-inline}}

{{Cell cycle}}

[[Category:Genetics]]
[[Category:Cell cycle]]
[[Category:Cellular processes]]

[[de:Mitose#Prophase]]