Mitosis Nucleoli

Nucleolus disintegrates during mitosis. Nucleolus divides into 2 polar masses during mitosis. 

Nucleus The nucleus is separated from the cytosol by a double membrane, the nuclear envelope. The DNA is complexed with basic proteins (histones) to form chromatin fibers, the material from which chromosomes are made. A distinct RNA-rich region, the nucleolus, is the site of ribosome assembly. The nucleus is the repository of genetic information encoded in DNA and organized into chromosomes. During mitosis, the chromosomes are replicated and transmitted to the daughter cells. The genetic information of DNA is transcribed into RNA in the nucleus and passes into the cytosol where it is translated into protein by ribosomes. 

The nucleolus is a web of threads with no surrounding membrane, and is rich in RNA. The nuclear sap contains chromatin threads which, at mitosis, become more densely organized and are seen as chromosomes (DNA+proteins). The total length of DNA in a nucleus is about 1 m, made up of about 3X10 nucleotides each with a molecular weight of about 350. F. Sanger has compared the amount of information stored in this quantity of DNA to that available in a large library. 

Go stage of the cell cycle. Monocytes and B cells are not stimulated by PHA. Not only is this model system used to study activation of the nucleolus but it is also used to study events in the cell cycle, because unstimulated resting cells start out in Go and then enter G phase of the cell cycle after 24 hr exposure to PHA in culture, S phase at 48 hr, and mitosis after approximately 72 hr in culture. 

The osteoblast is found in young bones and is associated with bone development. The origin of the osteoblast is still debated. It is assumed to be derived from the differentiation of fibroblasts. The quasitotal absence of mitosis in osteoblasts is in agreement with such a hypothesis. The osteoblast looks like a meta-bolically active cell. It contains a large nucleus with a well-developed nucleolus. Its endoplasmic reticulum is made of densely packed lamellae studded with ribosomes. Occasionally, the endoplasmic lamellae are dilated to form cisternae. The Golgi apparatus can usually be readily recognized, and the osteoblast usually contains only a few mitochondria. 

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