Meiosis is form of cell division that gives rise to genetically diverse sex cells or gametes. It is comprised to two successive nuclear divisions namely meiosis I and meiosis II. Meiosis I is comprised of four stages: prophase I, metaphase I, anaphase I, and telophase I. Prophase I is the first stage and consists of the following sub-stages: leptotene, zygotene, pachytene, diplotene and diakinesis.
Leptotene is the first sub-stage and it is when the replicated chromosomes start to condense into long strands inside the nucleus. The chromosomes at this stage are threadlike (thus, the term leptos, meaning thin threads). And in each chromosome, chromomeres (i.e. serially aligned beads or granules resulting from local coiling of a continuous DNA thread) can be seen. The sister chromatids that make up the chromosome are so closely associated together at this point. Because of this, the sister chromatids appear to be a single strand since they are visually indistinguishable from each other.
Word origin: Greek leptós (“thin”) + -tene ("thread")