Meiosis is a specialized cell division process that unfolds in two distinct stages: Meiosis I and Meiosis II. Telophase I marks the final stage of Meiosis I, occurring before the cells proceed to the second meiotic division.
Chromosomal Transformations
During Telophase I, the homologous chromosomes, which were separated and pulled to opposite poles during the preceding Anaphase I, arrive at their respective destinations within the cell. At each pole, these chromosomes begin to decondense, meaning they uncoil and become less compact and distinct. This process reverses the condensation that occurred earlier in meiosis, making the genetic material more diffuse.
Simultaneously, a new nuclear envelope begins to form around each set of chromosomes at both poles of the cell. Each newly formed nucleus contains a haploid set of chromosomes, meaning it has one chromosome from each homologous pair that was present in the original cell. Even though the chromosome number is halved, each chromosome within these new nuclei still consists of two sister chromatids, which are identical copies joined together.
Cellular Separation
Following the chromosomal changes in Telophase I, the physical division of the cell’s cytoplasm, known as cytokinesis, typically commences. This process can happen concurrently with Telophase I or immediately afterward, physically separating the single cell into two distinct daughter cells. Cytokinesis ensures that each newly formed nucleus is enclosed within its own cellular boundary.
The mechanism of cytokinesis differs between animal and plant cells. In animal cells, a cleavage furrow forms around the middle of the cell, pinching the cell membrane inward until it eventually divides the cell into two. This furrow is created by a contractile ring made of actin filaments that contracts. In contrast, plant cells, with their rigid cell walls, form a cell plate in the center of the cell. This cell plate originates from vesicles derived from the Golgi apparatus, which fuse together and grow outwards, eventually forming a new cell wall that separates the two daughter cells.
The Meiosis I Product
Upon the completion of Telophase I and cytokinesis, the outcome of Meiosis I is the formation of two haploid daughter cells. These cells are considered haploid because they each contain half the number of chromosomes as the original parent cell. A characteristic of these cells is that each chromosome still comprises two sister chromatids.
This reduction in chromosome number is why Meiosis I is often referred to as a “reductional division”. The genetic content has been significantly altered, preparing these cells for the subsequent stage of cell division. These two haploid cells, each with duplicated chromosomes, are now ready to enter Meiosis II, where the sister chromatids will separate.