During which stage of meiosis does the nuclear envelope begin to disappear

Telophase

During telophase, the nuclear envelope reforms on the surface of the separated sister chromatids, which typically cluster in a dense mass near the spindle poles (Fig. 44.18). Some further anaphase B movement may still occur, but the most dramatic change in cellular structure at this time is the constriction of the cleavage furrow and subsequent cytokinesis.

2.3.2 In vivo reassembly

During mitosis, nucleoli disassemble during prophase and reassemble in telophase (Sirri et al., 2008). The nucleolus has been described as “an organelle formed by the act of building a ribosome” (Mélèse and Xue, 1995) and when transcription is repressed its components in part stay associated to rDNA in the NOR (Roussel et al., 1996) and in part migrate as chromosomal passengers (Hernandez-Verdun and Gautier, 1994).

At the moment of rDNA transcription restart, nucleoli are again formed via PNB formation (Dundr et al., 2000) via a progressive recruitment of proteins involved in early and late processing. PNBs, with their content of nucleolar processing proteins, pre-rRNAs and small nucleolar RNAs (snoRNA), play a role that has not yet been completely clarified. Moreover, it seems clear that proteins with a different functional role leave the PNBs at different moments. Recently, Muro et al. (2010) have demonstrated that fibrillarin passes from one incipient nucleolus to another without stopping in PNBs, while other proteins like B23 shuttle between PNBs and nucleoli. The difference in this traffic would suggest a way of regulating the assembly first of the DFC and then of the GC, and this mechanism would involve the Cajal bodies.

Several factors are probably involved in the rebirth of a nucleolus. Transcription itself is not sufficient to start the event (Section 2.3.1) but nucleolar assembly can start independently of rDNA transcription (Dousset et al., 2000). Apparently a paradox: transcription arrest means disassembly, reassembly does not mean transcription restart. Other factors, such as CDK, may intervene to regulate both transcription and processing (Sirri et al., 2008). The final assembly is rather rapid and very probably a “prenucleolar” interaction of processing proteins is required.

If one considers the incredible amount of proteins that disassemble and reassemble during mitosis, and that most of them redistribute at different locations and then are recruited to PNBs, it is not clear what could be the driving force behind. Diffusion is the easy answer for the movements, and indeed a part of nonribosomal proteins show a nucleolar localization signal (NLS), but not all of them possess this feature (Jacobson and Pederson, 1998).

Diffusion can account for a series of movements, although mediated by signal recognizing sequences, but necessity of order and time might involve other mechanisms. It is known that some proteins are recruited from PNBs in a specific, sequential order (Louvet et al., 2008). It is difficult in this case to imagine diffusion as the only mechanism. As described for other nucleolar functions such as transcription (Dundr et al., 2002) or ribosome subunit movement (Cisterna et al., 2006, 2009) there could be place for motor proteins to give directionality (impossible in diffusion mechanisms), time schedule (also possible only in active mechanisms), and releasing order, if any. The coordination found in the movements of nucleolar proteins suggests that they can maintain their interaction during mitosis; however, the mechanisms behind the interactions are still not clear. The interaction has been clearly shown by FRET analysis (Angelier et al., 2005).

Sources and Preparation of Chromosomes from Insects

Mitotic chromosomes undergoing the stages of prophase, metaphase, anaphase, and telophase can be prepared from any insect somatic tissues with dividing cells. Embryos are the best sources of mitotic divisions, but they are also seen in the midgut ceca of adults and juveniles and in the follicle cells covering very early ova in females.

Insect cytogeneticists now usually use colchicine or other mitostatic agents to arrest the chromosomes at metaphase of mitosis by inhibiting the formation of the spindle fibers required for the cells to progress to anaphase. Squashing, under a coverslip, spreads the chromosomes, and for squash preparations the cells are usually prestained. Insect cytogeneticists now often use air-drying to spread the chromosomes, since this process has the advantage of making the chromosomes immediately available for modern banding and molecular cytogenetic methods.

Male meiosis is very commonly used to study the chromosomes of insects and to analyze sex-determining mechanisms. The structure of the insect testis is very favorable to chromosomal studies because each lobe has a single apical cell that divides by a number (s) of spermatogonial divisions (Fig. 1A to yield 2S primary spermatocytes, which then undergo synchronous first and second meiotic divisions to yield 2S+1 secondary spermatocytes and 2S+2 sperm.

First meiotic prophase in insects involves the usual stages (Fig. 1). Replication of the DNA is followed by the prophase stages of leptotene (strand forming), zygotene (chromosome pairing to form bivalents), pachytene (crossing over to yield recombinants), diplotene (repulsion of the homologues), diakinesis (completion of repulsion), and premetaphase (bivalents fully condensed).

Metaphase I is followed by first anaphase, which can be a very informative stage and, in contrast to mammals, is readily available in insects. Second meiotic division is also readily observed in insects (Fig. 1) and can be useful for confirming events in earlier stages.

Meiotic chromosomes in insect females are difficult to prepare and are usually studied only in special cases, such as parthenogenesis.

What stage does the nuclear envelope disappear?

Which phase of meiosis does the nucleus disappear?

Does the nuclear membrane disappear in meiosis 2?

During which phases does the nuclear envelope start to disappear and then reappear?