The leptotene It is the first and longest of the five phases into which prophase I divides. It occurs during the process of cell division known as meiosis. It is also known by the names of leptotene (in English) and leptonema.
The term means "thin band", its origin comes from two Greek voices: leptos which means thin or fine, and tainìa which means band. The word was proposed by the renowned Belgian gynecologist and cytologist Hans von Winiwater, at the beginning of the 20th century.
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Meiosis is a process of cell division where a diploid cell (with two sets of chromosomes) goes through two divisions of the nucleus to generate four haploid cells (with one set of chromosomes or only half the normal charge).
This process is carried out by organisms to obtain sex cells or gametes, either sperm or eggs. The process consists of two stages and several phases (as has already been mentioned) that are known as:
This process consists of the phases: prophase I, metaphase I, anaphase I and telophase I. In this stage, homologous chromosomes are separated and two daughter cells are obtained with half the genetic load of the progenitor cells..
This second part of the process consists of the phases: prophase II, metaphase II, anaphase II and telophase II. Scientists consider it to be very similar to mitosis (non-reductive cell division).
In this stage a new division occurs, where chromatids are separated from each chromosome and distributed among the nuclei of the daughter cells to obtain four haploid cells..
Prophase is the first phase of cell division of both mitosis and meiosis:
This stage represents, in mitosis, approximately 40% of the entire cell division process. During this phase the chromosomes become evident, a region of the nucleus called the nucleolus disappears and the nuclear membrane also disappears, among other characteristics.
In this process there are two stages called prophase and they are the following:
It is the first stage of meiosis, the longest and also the most complex of all the meiotic phases. In this the replication of chromosomes occurs. Being the longest, it is the only stage that is subdivided into five phases called: leptotene, zygotene, pachytene, diplotene and diakinesis.
This second prophase has a very short duration. The nuclear envelope is broken. There is no replication of the chromosomal material, and it is in this phase where the mitotic use is formed.
Leptotene is the first of the five subphases belonging to prophase I of the meiotic division. It occurs in both plants and animals that exhibit sexual reproduction.
In this subphase, the chromosomal material appears within a nuclear sac or envelope. The distribution of said chromosomal material in the nucleus is not random, in some plants the chromosomes are grouped on one side of the nucleus, leaving the other side clear of this.
In animals, on the other hand, the distribution of chromosomes in the nucleus is polarized, where the distal regions of the chromosomes unite towards the region of the nuclear membrane that is close to the centriole, occasionally showing a clear union to the nucleus. nuclear envelope.
Although the union between chromosomes and the nuclear envelope is sometimes not obvious, they are always linked (in both animals and plants) by means of a structure called the junction plate..
In leptotene the nucleus increases in volume. The chromosomal material is paired and counted, being the first step for the spirilization of chromosomes, but they do so before what scientists call a brief synapse (union) occurs..
The condensing chromosomes are visible under the microscope as thin threads, which gives rise to the name of the subphase.
Along the threads (chromosomes), the centromeres can be visible as a small necklace of coiled chromatids. Each chromosome in this phase has replicated (prior to this phase) and is made up of a pair of chromatids that are very close or closely linked (sisters).
Due to the proximity between the sister chromatids of each chromosome, under the microscope they appear as one. Only at the end of prophase I is it possible to see both sister chromatids per chromosome.
Stage in which homologous chromosomes pair up to recombine and form tetrads. The synaptonemic complex is also formed and DNA replication ends..
Crossover occurs, that is, exchange of genetic material between paired chromosomes.
The two chromatids of each chromosome are evident, as well as the chiasmas or sites where the crossover occurred.
There is greater condensation of the chromosomes and the crossover sites are more evident, at the end of this stage RNA stops being synthesized, the nuclear envelope is broken and the nucleolus disappears.
To talk about the importance of leptotene, it is necessary to talk in general about the importance of meiosis and therefore of prophase I in meiotic cell division.
Meiosis is a characteristic process of organisms that have sexual reproduction, a vital process for populations, since it leads to gene segregation and recombination.
Genetic recombinations are a fundamental part of the survival, adaptation and diversification of populations of organisms. And this is possible thanks to meiotic cell division.
Prophase I and its five subphases, could be said to be the most important stage of all meiosis, even though without the other phases there would be no meiosis. At this stage is when homologous chromosomes pair up and exchange genetic information.
The case of leptotene is as fundamental as the rest of the four sub-phases that follow. This phase is particularly important because in it the condensation and pairing of homologous chromosomes occurs; In addition, the first step of espirillation occurs..
Some scientists join the leptotene and zygotene phases or study the transition between both, as one gives way to another, and between both threads (more in the zygotene) the synaptonemic complex is formed, which is where the pairing and recombination of chromosomes occurs..
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