Leptotene is a phase or stage that occurs in prophase I of meiosis - a type of cell division. Leptotene stage is the stage where the chromosomes which have been duplicated in the previous phase of the interphase of the cell division, start forming ‘thin-thread’ like structures from the previously diffused state. This stage is one of the five stages of prophase I of the meiosis cell cycle. It is the preparatory stage of the chromosomal recombination in cells undergoing meiosis, especially in gametes i.e. the sperm and the ovum.
Since, you know what is leptotene in general, let us understand it in detail.
You know that gametes usually undergo meiosis and they create four haploid daughter cells with half the number of chromosomes as compared to the single diploid parent cells. This happens in two cycles of meiosis which are known as meiosis I and meiosis II. In meiosis I the division of chromosomes from diploid to haploid takes place. The two daughter cells produced at the end of meiosis I have a haploid number of chromosomes while meiosis II is like mitosis where there is no decrease in the ploidy of chromosomes.
The four well-known phases of the cell cycle - prophase, metaphase, anaphase, and telophase, occur in both meiosis cycles. But there are significant differences in the same phases of both meiosis I and meiosis II. The prophase I of meiosis I is divided into five subphases unlike in meiosis II. During these five subphases of prophase I the homologous maternal and paternal chromosomes in the parent cell undergo pairing, synapsis, and homologous recombination for the exchange of genetic information by crossover. This entire chain of events helps for more accurate segregation of the homologous chromosomes at the end of the first meiotic cell division. These five subphases are: (i) Leptotene, (ii) Zygotene, (iii) Pachytene, (iv) Diplotene and, (v) Diakinesis.
The Leptotene stage marks the beginning of prophase I. In this stage, the diffused chromatin state no longer exists and there begins to appear long thin-thread-like structures. These structures are condensed chromosomes that become more visible in the nucleoplasm. Each of these thin-thread like structures consist of two replicated sister chromatids. These replicated sister chromatids are paired longitudinally with one another having a longitudinal axis. These longitudinally arranged chromosomes form a linear array of loops facilitated by a protein complex called cohesin as shown below in the figure.
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The elements of the synaptonemal complex assemble forming an ‘axial element’ from which the loops seem to emerge. It is along this axis that programmed DNA strand breaks are caused by an enzyme as these breaks are going to be the sites of homologous recombination of the chromosomes. The paired chromosomes are called bivalents (two chromosomes) or tetrads (four chromatids), where each chromosome comes from one parent.
Before the transition from the leptotene stage to the zygotene stage, the individual chromosomes attach themselves to the inner membrane of the nuclear envelope via their telomeric ends. Their aggregation at the nuclear envelope gives them a bouquet-like shape. Therefore, this stage - the leptotene stage is also known as the bouquet stage of meiosis I. Following the leptotene stage the zygotene stage takes place where the synapsis of homologous chromosomes occurs i.e. pairing of the chromosomes along with the central elements of the synaptonemal complex. The pachytene stage follows the zygotene where the homologous recombination including crossing over event occurs. Once the recombination process is completed the cell enters the diplotene stage in which the synaptonemal complex disassembles and during which the cell enters into a resting state as there are dramatic rearrangements within the chromatin. After the completion of these events, the cells follow through the diakinesis stage the characteristics of which is closely similar to the prometaphase of mitosis i.e. the nucleoli disappear, the nuclear membrane disintegrates and the formation of mitotic spindle begins.
After the completion of prophase I the cell then passes through the metaphase I, anaphase I, and telophase I. The recombination of chromosomes ensures the proper and precise segregation of the chromosomes in the following stages of the cell cycle. One interesting observation underlining the importance of hormones in the cell division cycle is that in the absence of testosterone produced by the pituitary gland the cell cycle stops at the leptotene stage of prophase I in turn halting the process of spermatogenesis. Thus, one can understand how important the role of hormones in the process of cell division and gametogenesis is.
Studying the mechanism of the meiosis cell division cycle will show you in detail the importance and the peculiarities of different phases. As given in the note above, you will find leptotene stage, unique to prophase I and how it is a well-oiled machinery characterized by the gathering of the chromosomes into paired structures and the formation of DNA nicks - important for further recombination of homologous chromosomes.