The diplotene or diplonema is the fourth subfasede prophase I of meiotic cell division and is distinguished by the separation of chromatids of homologous chromosomes. During this subphase, you can see the places on the chromosomes where recombination occurred, these places are called chiasmas.
Recombination occurs when a strand of genetic material is cut to join another molecule with different genetic material. During diplotene, meiosis may experience a pause and this situation is unique to the human race. This state of pause or latency experienced by the ovules is called dictyotene.
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By Doc. RNDr. Josef Reischig, CSc. (Author's archive), via Wikimedia Commons
In this case, the human ovules will cease their activity, until the seventh month of embryonic development and, the activity will restart, when the individual reaches sexual maturity.
Diplotene begins when chromosomes separate and simultaneously increase in size and separate from the nuclear membrane.
Tetrads (two chromosomes) of four chromatids are formed and the sister chromatids in each tetrad are linked by the centromeres. The chromatids that have crossed will be joined by chiasmata.
Meiosis
Meiosis is a specialized class of cell division that cuts the number of chromosomes in half, producing four haploid cells.
Each haploid cell is genetically different from the mother cell that originated it and from it come the sex cells, also called gametes
This procedure occurs in all unicellular (eukaryotic) and multicellular beings of sexual reproduction: animals, plants and fungi. When errors occur in meiosis, aneuploidy is evident and is the leading known cause of miscarriage and the most common genetic cause of disabilities.
Phases
The meiotic process occurs in two stages or phases: Meiosis I and Meiosis II. Meiosis I, in turn, is made up of four stages: prophase I, metaphase I, anaphase I and telophase.
The first division is the more specialized of the two divisions: the cells that result from it are haploid cells.
At this stage there is a reductional division of the genome and its most important moment is prophase, which is a long and complex stage in which the separation of homologous chromosomes occurs.
In prophase I, homologous chromosomes pair up and there is DNA swapping (homologous recombination). Chromosome crossing occurs, which is a decisive process for the coupling of homologous chromosomes and, consequently, for the specific separation of chromosomes in the first division.
The new DNA mixtures produced in the crossing are a significant source of genetic variation that originates new combinations of alleles, which can be very favorable for the species.
Paired and replicated chromosomes are called bivalent or tetrads, which have two chromosomes and four chromatids, with one chromosome coming from each parent.
The coupling of homologous chromosomes is called a synapse. At this stage, non-sister chromatids can intersect at points called chiasmata (plural; singular chiasma).
Prophase I is the longest phase of meiosis. It is divided into five substages that are named based on the appearance of the chromosomes: leptotene, zygotene, pachytene, diplotene, and diakinesis.
Before starting the diplotene substage, a homologous recombination occurs and crosses occur between the chromosomes of the non-sister chromatids, in their chiasms. At that precise moment, the chromosomes are closely paired.
Description of diplotene
Diplotene, also called diplonema, (from the Greek diploo: double and tainia: ribbon or thread) is the sub-stage that succeeds pachytene. Prior to diplotene, homologous chromosomes have paired forming tetrads or bivalent (genetic value of both parents), they are shortened, thickened and sister chromatids differentiate.
A zipper-like structure, called a synaptonemic complex, forms between chromosomes that have paired up and then break down, in the diplotene stage, causing homologous chromosomes to separate slightly.
Chromosomes unwind, allowing DNA transcription. However, the homologous chromosomes from each pair formed remain closely linked in the chiasms, the regions where the crossover occurred. Chiasms remain on the chromosomes until they separate in the transition to anaphase I.
In diplotene the synaptonemic complexes separate, the central space enlarges and the components disappear, remaining only in the regions where there were chiasmata. The lateral elements are also present, which are thin and separate from each other.
In advanced diplotene, the axes are interrupted and disappear, only remains in the centromeric and chiasmatic regions.
After recombination, the synaptonemic complex disappears and the members of each bivalent pair begin to separate. In the end, the two homologues of each bivalent only remain united at the points of crossover (chiasmata).
The mean number of chiasmas in human spermatocytes is 5, that is, several per bivalent. In contrast, the proportion of oocytes in pachytene and diplotene increase in fetal development.
As they get closer to diplotene, the oocytes enter the so-called meiotic arrest or dictyotene. At approximately six months of gestation, all germ cells will be found in this substage.
Importance of the diplotene substation
Around the eighth month of embryonic development, the oocytes are more or less synchronized in the diplotene stage of prophase I.
The cells will remain in this sub-phase from birth to puberty, when the ovarian follicles begin to mature one by one and the oocyte restarts the final phase of diplotene.
During the process of oogenesis (creation of ovules), human oocytes stop their maturation process at the diplotene stage, before birth. Upon reaching the puberty phase, the process is restarted, this suspended state of the meiotic division is known as dictyotene or dictyate.
When ovulation begins, the oocyte is between the first and second meiotic divisions. The second division is suspended until fertilization, which is when the anaphase of the second division occurs and the female pronucleus is ready to unite with the male.
This resumption of maturation of the oocytes occurs in order to prepare them for ovulation.
References
- Biology online, 10/26/2011, «Diplotene», Available at: biology-online.org/dictionary/Diplotene
- Cabero, L., Saldívar, D. and Cabrillo, E. (2007). Obstetrics and maternal-fetal medicine. Madrid: Editorial Médica Panamericana.
- Hartl, D. and Ruvolo, M. (2012). Genetics: analysis on genes and genomes. United States: Jones & Bartlett Learning.
- Nussbaum, RL and McInnes, RR (2008). Thompson & Thompson: Genetics in Medicine. Barcelona: Elsevier Masson.
- Solari, A. (2004). Human genetics: fundamentals and applications in Medicine. Buenos Aires: Editorial Médica Panamericana.