The process of meiosis contains two meiotic divisions. Within the first meiotic division, the first of four stages, Prophase (I) contains five substages itself, and the first substage is Leptotene. As stated by Hartl et al (1) The substage Leptotene, is where the chromosomes first become long, thin, visible structures. Leptotene is regarded as the initial phase of chromosomal condensation, where the pair of sister chromatids become visible under an electron microscope. (2) "In this initial phase of chromosome condensation, numerous dense granules appear at irregular intervals along their length. These localised contractions called chromomeres have a characteristic number, size, and position in a given chromosome."
Hartl.D, Ruvolo.M, (2012), Genetics:,(124-125), 8th edition, Burlington, Jones & Bartlett Learning