Meiotic Prophase Prophase

1 meiotic prophase

1.1 prophase i

1.1.1 leptotene
1.1.2 zygotene
1.1.3 pachytene
1.1.4 diplotene
1.1.5 diakinesis


1.2 prophase ii

meiotic prophase

meiosis involves 2 rounds of chromosome segregation and undergoes prophase twice, resulting in prophase , prophase ii. prophase complex phase in of meiosis because homologous chromosomes must pair , exchange genetic information. prophase ii similar to mitotic prophase.

prophase i

prophase divided 5 phases: leptotene, zygotene, pachytene, diplotene, , diakinesis. in addition events occur in mitotic prophase, several crucial events occur within these phases such pairing of homologous chromosomes and reciprocal exchange of genetic material between these homologous chromosomes. prophase occurs @ different speeds dependent on species , sex. many species arrest meiosis in diplotene of prophase until ovulation. in humans, decades can pass as oocytes remain arrested in prophase complete meiosis prior to ovulation.

leptotene

in first stage of prophase i, lepotene (from greek “delicate”), chromosomes begin condense. each chromosome in a haploid state , consists of 2 sister chromatids; however, the chromatin of the sister chromatids is not yet condensed enough resolvable in microscopy. homologous regions within homologous chromosome pairs begin associate each other. 

zygotene

in second phase of prophase i, zygotene (from greek “conjugation”), maternally , paternally derived chromosomes have found their homologous partner. the homologous pairs undergo synapsis, process the synaptonemal complex (a proteinaceous structure) aligns corresponding regions of genetic information on maternally , paternally derived non-sister chromatids of homologous chromosome pairs.  the paired homologous chromosome bound the synaptonemal complex are referred as bivalents or tetrads. sex (x , y) chromosomes do not synapse because small region of chromosomes homologous. 

the nucleolus moves central peripheral position in nucleus.

pachytene

the third phase of prophase i, pachytene (from greek “thick”), begins @ completion of synapsis.  chromatin has condensed enough chromosomes can resolved in microscopy. structures called recombination nodules form on the synaptonemal complex of bivalents. these recombination nodules facilitate genetic exchange between non-sister chromatids of the synaptonemal complex in event known as crossing-over or genetic recombination.  multiple recombination events can occur on each bivalent. in humans, average of 2-3 events occur on each chromosome.

diplotene

in fourth phase of prophase i, diplotene (from greek “twofold”), crossing-over is completed. homologous chromosomes retain full set of genetic information; however, the homologous chromosomes are of mixed maternal , paternal descent. visible junctions called chaismata hold the homologous chromosomes together @ locations recombination occurred the synaptonemal complex dissolves. it @ stage meiotic arrest occurs in many species.

diakinesis

in fifth , final phase of prophase i, diakinesis (from greek “double movement”), full chromatin condensation has occurred , four sister chromatids can seen in bivalents with microscopy. in mitotic prophase, meiotic prophase ends the spindle apparatus beginning form, , the nuclear membrane beginning break down.

prophase ii

prophase ii of meiosis is similar prophase of mitosis. noticeable difference prophase ii occurs a haploid number of chromosomes opposed the diploid number in mitotic prophase.  in both animal , plant cells chromosomes may de-condense during telophase requiring them re-condense in prophase ii.  if chromosomes not need re-condense, prophase ii proceeds seen in the model organism arabidopsis.