The Nucleus

In a typical animal cell the nucleus has a diameter of ~5 µm and a volume of 65 µm3. Except at the time of cell division, it is densely and almost uniformly packed with DNA. The amount of DNA present is larger than that in bacteria as is indicated in Table 1-3. Yeast contains about three times as much genetic matter as E. coli and a human being or a mouse about 700 times as much. However, genes are sometimes duplicated in higher organisms and large amounts of repetitive DNA of uncertain significance are often present. Some amphibians have 25 times more DNA per cell than do humans. The fruit fly Drosophila
contains about 13,600 functioning genes and a human being perhaps 50,000. 37
Because of its acidic character, DNA is stained by basic dyes. Long before the days of modern biochemistry, the name chromatin was given to the material in the nucleus that was colored by basic dyes. At the time of cell division, the chromatin is consolidated into
distinct chromosomes which contain, in addition to 15% DNA, about 10% RNA and 75% protein. Nearly all of the RNA of the cell is synthesized (transcribed) in the nucleus, according to the instructions encoded in the DNA. Some of the RNA then moves out of the nucleus into the cytoplasm where it functions in protein synthesis and in some other ways.
Many eukaryotic genes consist of several sequences that may be separated in the DNA of a chromosome by intervening sequences of hundreds or thousands of base pairs. The long RNA transcripts made from these split genes must be cut and spliced in the nucleus to form the correct messenger RNA molecules which are then sent out to the ribosomes in the cytoplasm. Each cell nucleus contains one or more dense nucleoli, regions that are rich in RNA and may contain 10–20% of the total RNA of cells. Nucleoli are sites of synthesis and of temporary storage of ribosomal RNA, which is needed for assembly of ribosomes. The nuclear envelope is a pair of membranes, usually a few tens of nanometers apart, that surround the nucleus. The two membranes of the pair separate off a thin perinuclear space. The membranes contain “pores” ~130 nm in diameter with a complex structure. 38,39 There is a central channel ~42 nm in diameter, which provides a route for controlled passage of RNA and other large molecules from the nucleus into the cytoplasm and also from the cytoplasm to the nucleus. Smaller ~10 nm channels allow passive diffusion of ions and small molecules.