Leptotene

Sexual reproduction thus involves the fusion of two gametes, each one containing a complete haploid set of chromosomes. Gametes are the products of specialized diploid cells. The type of cell division, which consists in the halving of the number of the chromosome during the division, results in the production of haploid daughter cells. This kind of division is known as meiosis. 

1.0Meiosis

Meiosis ensures the production of the haploid phase in the life cycle of sexually reproducing organisms whereas fertilisation restores the diploid phase. We encounter meiosis during gametogenesis in plants(Sporic meiosis) and animals(gametic meiosis). This results in the creation of haploid gametes. The major characteristics meiosis includes the following:

Meiosis consists of two consecutive cycles of nuclear and cell division called meiosis I and meiosis II but contains only one cycle of DNA replication.

Meiosis I begins after the parental chromosomes have replicated to form identical sister chromatids at the S phase.

Meiosis includes the pairing of homologous chromosomes and recombination between non-sister chromatids of homologous chromosomes. After meiosis II, four haploid cells are obtained. Meiosis is a special type of cell division which produces haploid cells by reduction in the number of chromosomes.

Meiosis I is divided into- Prophase-I, Metaphase-I Anaphase-I and Telophase-I .

Meiosis II is divided into-Prophase-II • Metaphase-II • Anaphase-II • Telophase-II.

2.0Introduction of Leptotene

The leptotene stage, also known as leptonema, marks the initial phase of meiosis prophase I, the first among the five sub stages ( Leptotene,Zygotene, Pachytene, Diplotene and Diakinesis ). The term leptonema is rooted in the Greek language, translating to "thin threads," reflecting the characteristic appearance of chromosomal threads during this stage.

The chromatin fibres undergo progressive condensation, coiling , shortening and thickening. As chromatin condenses, it forms fine, thread-like structures. These threads represent the early stage of chromosome formation and are often referred to as "chromosomal threads" or "chromosomal fibres."

These possess darkey stained bead like structures called chromomeres along their entire length. The chromosomes are replicated but chromatids are distinguishable due to the presence of nuclear protein between them. 

The chromosomes form loops whose ends are attached to the nuclear membrane at the attachment plate. This specific arrangement of chromosomes is often called the bouquet stage. The duplicate centrioles start moving apart.

3.0Meiosis I (Prophase I)

Prophase of the first meiotic division is typically longer and more complex when compared to prophase of mitosis. 

It has been further subdivided into the following five phases based on chromosomal behavior, i.e., Leptotene, Zygotene, Pachytene, Diplotene and Diakinesis. 

During leptotene stage the chromosomes become gradually visible under the light microscope. The compaction of chromosomes continues throughout leptotene.

Chromosomes attach to the nuclear envelope at their telomeres during the leptotene stage; these then make revolutions on the envelope, tumbling total chromosomes inside the nucleus, which allows them to carry out their searches for homology to pair.