{{Short description|Second stage of prophase I in Meiosis}}
'''Zygotene''' (from Greek for "paired threads")<ref name=":0">{{cite journal |last1=O'Connor |first1=Clare |title=Stages of Meiosis and Sexual Reproduction |journal=Nature Education |date=2008 |volume=1 |issue=1 |page=174 |url=https://www.nature.com/scitable/topicpage/meiosis-genetic-recombination-and-sexual-reproduction-210/ }}</ref> is the second stage of prophase I during meiosis, the specialized cell division that reduces the chromosome number by half to produce haploid gametes. It follows the Leptotene stage and is followed by Pachytene stage.
== Synapsis completion == The key event during zygotene is the completion of synapsis between homologous chromosomes. Synapsis began during the previous leptotene stage, with the homologous chromosomes starting to pair together and associate lengthwise, facilitated by the synaptonemal complex protein structure.<ref>{{cite book |last1=Wang |first1=Yingxiang |last2=Cheng |first2=Zhukuan |last3=Ma |first3=Hong |title=Cell Biology |chapter=Meiosis: Interactions Between Homologous Chromosomes |date=2014 |pages=1–34 |doi=10.1007/978-1-4614-7881-2_18-1 |isbn=978-1-4614-7881-2 }}</ref>
In zygotene, the synaptonemal complex forms more extensively between the paired chromosomes. It zips the homologs together along their entire length, with the lateral elements of the complex associated with each chromosome and the central region holding them together. This allows intimate pairing and genetic recombination events.<ref>{{cite journal |last1=Qiao |first1=Huanyu |last2=Chen |first2=Jefferson K. |last3=Reynolds |first3=April |last4=Höög |first4=Christer |last5=Paddy |first5=Michael |last6=Hunter |first6=Neil |title=Interplay between Synaptonemal Complex, Homologous Recombination, and Centromeres during Mammalian Meiosis |journal=PLOS Genetics |date=28 June 2012 |volume=8 |issue=6 |article-number=e1002790 |doi=10.1371/journal.pgen.1002790 |doi-access=free |pmc=3386176 |pmid=22761591 }}</ref><ref>{{cite journal |last1=Zhang |first1=Feng-Guo |last2=Zhang |first2=Rui-Rui |last3=Gao |first3=Jin-Min |title=The organization, regulation, and biological functions of the synaptonemal complex |journal=Asian Journal of Andrology |date=November 2021 |volume=23 |issue=6 |pages=580–589 |doi=10.4103/aja202153 |doi-access=free |pmc=8577265 |pmid=34528517 }}</ref>
== Chromosome condensation == The chromosomes continue condensing during zygotene into distinct threadlike structures. Each chromosome now appears thicker as the sister chromatids are closely aligned.<ref>{{cite journal |last1=Zickler |first1=D. |last2=Kleckner |first2=N. |title=The leptotene-zygotene transition of meiosis |journal=Annual Review of Genetics |date=December 1998 |volume=32 |issue=1 |pages=619–697 |id={{Gale|A54257693}} |doi=10.1146/annurev.genet.32.1.619 |pmid=9928494 }}</ref>
== Recombination nodules == As synapsis completes, proteinaceous recombination nodules begin to appear along the synaptonemal complex between the homologous chromosomes. These represent sites of genetic crossover events, where exchange of chromosomal segments occurs between the non-sister chromatids.<ref>{{cite journal |last1=Zhang |first1=Fengguo |last2=Liu |first2=Mengfei |last3=Gao |first3=Jinmin |title=Alterations in synaptonemal complex coding genes and human infertility |journal=International Journal of Biological Sciences |date=2022 |volume=18 |issue=5 |pages=1933–1943 |doi=10.7150/ijbs.67843 |pmc=8935243 |pmid=35342360 }}</ref><ref>{{cite journal |last=Naranjo |first=Tomás |date=2012 |title=Finding the Correct Partner: The Meiotic Courtship |journal=Scientifica |volume=2012 |article-number=509073 |doi=10.6064/2012/509073 |doi-access=free |pmc=3820632 |pmid=24278707 }}</ref>
Key recombination proteins like MLH1/3 and MSH4/5 mark the sites of crossover formation. The number and positioning of these crossovers is regulated to ensure at least one crossover per chromosome arm for proper segregation in later meiotic stages.<ref>{{cite journal |last1=Zickler |first1=Denise |last2=Kleckner |first2=Nancy |title=Recombination, Pairing, and Synapsis of Homologs during Meiosis |journal=Cold Spring Harbor Perspectives in Biology |date=June 2015 |volume=7 |issue=6 |article-number=a016626 |doi=10.1101/cshperspect.a016626 |pmc=4448610 |pmid=25986558 }}</ref>
== Transition to pachytene == Once synapsis and crossing over are complete, the cell transitions to the pachytene stage of prophase I. Pachytene features fully condensed and paired chromosomes along their length, with distinctly visible recombination nodules.<ref>{{cite journal |last1=Azumi |first1=Y. |title=Homolog interaction during meiotic prophase I in Arabidopsis requires the SOLO DANCERS gene encoding a novel cyclin-like protein |journal=The EMBO Journal |date=17 June 2002 |volume=21 |issue=12 |pages=3081–3095 |doi=10.1093/emboj/cdf285 |pmc=126045 |pmid=12065421 }}</ref><ref>{{cite journal |last1=Grant |first1=W |last2=Owens |first2=E |title=Zea mays assays of chemical/radiation genotoxicity for the study of environmental mutagens |journal=Mutation Research/Reviews in Mutation Research |date=September 2006 |volume=613 |issue=1 |pages=17–64 |doi=10.1016/j.mrrev.2006.04.002 |pmid=16828334 |bibcode=2006MRRMR.613...17G }}</ref>
== Importance == The zygotene stage is crucial for genetic recombination and proper chromosome segregation in meiosis.<ref name=":0" /> Defects in synapsis, recombination, or crossover regulation can lead to aneuploidy and chromosomal abnormalities in gametes.<ref>{{cite journal |last1=Saito |first1=Takamune T. |last2=Colaiácovo |first2=Monica P. |title=Regulation of Crossover Frequency and Distribution during Meiotic Recombination |journal=Cold Spring Harbor Symposia on Quantitative Biology |date=2017 |volume=82 |pages=223–234 |doi=10.1101/sqb.2017.82.034132 |pmc=6542265 |pmid=29222342 }}</ref>
== References == {{Reflist}}
Category:Cellular processes Category:Meiosis