Construction of this network occurs at a later phase. During mitosis and in male gametes, centrosomes are responsible for constructing a network of microtubules which extend from within the centrosome towards the cell’s center. Two centrosomes, each containing a centriole pair, migrate to either end of the cell – the poles. From this point on they are referred to as centrosomes.
During the normal cell cycle, centrioles replicate to form pairs that are later enveloped in pericentriolar material (PCM). Centrioles are constructed from microtubules and, certainly in male gametes and during mitotic cell division, play an important role in spindle apparatus construction. Building a Spindle Apparatus Without CentrosomesĬurrent research is looking at the role of centrioles in human spindle apparatus formation, as female gametes – oocytes – do not have them. These four steps characterize prophase II. Even in the absence of centrosomes, the reconstruction of the spindle apparatus used during meiosis I can be initiated. Simultaneously, the nuclear membrane dissolves, leaving an open area of cytoplasm in which a network of proteins (microtubules) has enough space to create pathways reaching from one side of the cell to the other – the spindle apparatus.Īfter the condensing of chromatin into chromosomes, and after the disintegration of the nuclear envelope containing these chromosomes, the centrosomes migrate to either pole. In order to prepare for the next division, prophase II condenses chromatin first into chromatids, and then more tightly into chromosomes. Prophase II begins immediately after cytokinesis – the splitting of the diploid gamete into two haploid daughter cells. A much less complex phase than prophase I, prophase II does not include the steps of leptotene, zygotene, pachytene, diplotene, and diakinesis but more resembles the simpler process of mitotic prophase in non- gamete (somatic) cell division. Prophase II prepares the cell for secondary meiotic division where two haploid cells eventually form four haploid cells, each containing half of the genetic information previously contained in the original, replicated diploid cell. As no DNA replication takes place in this second step of meiosis, the cell division process immediately begins. Meiosis II occurs in both daughter cells that were formed during meiosis I. However, centrosomes are not present in all cells. These are the condensing of chromatin into chromosomes, disintegration of the nuclear envelope, migration of centrosomes to either pole, and the reconstruction of the spindle apparatus. During prophase II of meiosis II, four important steps occur.
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