The leptotene stage, also known as leptonema, is the first of five substages of prophase I during meiosis, the specialized cell division that reduces the chromosome number by half to produce haploid gametes in sexually reproducing organisms.
During the leptotene stage, the duplicated chromosomes - each consisting of two sister chromatids - condense from diffuse chromatin into long, thin strands that are more visible within the nucleoplasm (nucleus contents).
[1]: 27 [2]: 353 Each chromosome consists of two identical sister chromatids held together by cohesin proteins along the entire length, connected at the centromere region.
At the transition to the zygotene stage, the telomeres usually aggregate at a sector of the nuclear envelope, thereby forming a "meiotic bouquet" arrangement.
This involves programmed DNA double-strand breaks and their repair to generate crossovers between non-sister chromatids of homologous pairs.