Leptotene
In this first substage of prophase I, the chromosomes have appeared within the nuclear envelope (shown in the diagram at right as a tan circle with a brown border), but are not yet fully condensed. In the diagram the two chromosomes of paternal origin are indicated in red, those of maternal origin, in blue. Each is a thin thread of DNA (lepto- is Greek for thin and -tene is Greek for ribbon or band) along which clearly defined beads of local coiling (chromomeres) can be seen. The chromosomes, while they have this threadlike form, are called chromatonemata (sing. chromonema; -nema is Greek for thread). The chromosomes appear single because the sister chromatids are still so tightly bound to each other that they cannot be separately seen. During this stage both telomeres of each chromosome are turned toward, and probably attached to, the same region of the nuclear envelope. Leptotene is also known as