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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). The chromosomes become visible as thin threadlike structures known as leptonema under a light microscope. [1]: 27 [2]: 353
The chiasmata become visible during the diplotene stage of prophase I of meiosis, but the actual "crossing-overs" of genetic material are thought to occur during the previous pachytene stage. Sister chromatids also form chiasmata between each other (also known as a chi structure), but because their genetic material is identical, it does not ...
The first stage of prophase I is the leptotene stage, also known as leptonema, from Greek words meaning "thin threads". [18]: 27 In this stage of prophase I, individual chromosomes—each consisting of two replicated sister chromatids—become "individualized" to form visible strands within the nucleus.
In S phase, the chromosomes are replicated in order for the genetic content to be maintained. [18] During G 2, the cell undergoes the final stages of growth before it enters the M phase, where spindles are synthesized. The M phase can be either mitosis or meiosis depending on the type of cell.
The formation of a bivalent occurs during the first division of meiosis (in the zygotene stage of meiotic prophase 1). In most organisms, each replicated chromosome (composed of two identical sister chromatids [1] [2]) elicits formation of DNA double-strand breaks during the leptotene phase. [3]
[3]: 98 [12] The paired homologous chromosome bound by the synaptonemal complex are referred to as bivalents or tetrads. [10] [3]: 98 Sex (X and Y) chromosomes do not fully synapse because only a small region of the chromosomes are homologous. [3]: 98 The nucleolus moves from a central to a peripheral position in the nucleus. [14]
It is visible on a chromosome during the prophase of meiosis and mitosis. Giant banded chromosomes resulting from the replication of the chromosomes and the synapsis of homologs without cell division is a process called endomitosis. These chromosomes consist of more than 1000 copies of the same chromatid that are aligned and produce alternating ...
During mitosis and meiosis, chromatin facilitates proper segregation of the chromosomes in anaphase; the characteristic shapes of chromosomes visible during this stage are the result of DNA being coiled into highly condensed chromatin. The primary protein components of chromatin are histones.