This is a nucleus with a double surrounding membrane. Whether the nucleus represents any kind of endosymbiosis is still controversial. The membrane has pores at intervals to permit exchange of some very large polymers between the cytosol (outside the nucleus) and the nucleosol (inside the nucleus).

The most important nucleosol component is DNA molecules. These molecules are polymers of nucleotides. The sequence of nucleotides represents genetic information. A sequence of nucleotides ultimately used to make a polypeptide is called a gene. There can be many genes in each DNA molecule. There can be many DNA molecules in the nucleus. For example there are 46 in human cell nuclei, but some ferns have more than 200!

The DNA molecules are not naked in the nucleus. They are bound to proteins, called histones. The portions of the DNA that are tightly bound to histones cannot be used. Thus histones "silence" long stretches of DNA. This way only those genes needed for a particular cell type are actually used in the nucleus of that cell type. Each cell has enough genetic information for a whole organism, but uses only the information needed for its particular cell type. Neat! The complex of one DNA molecule and its histone proteins is called a chromosome.

The nucleosol also contains enzymes. Some of these are responsible for making copies of the information in DNA. For common use, RNA polymerase (and other enzymes) makes disposable RNA copies of genes to be expressed. This process is called transcription. The resulting RNA is sometimes called messenger RNA. This RNA copy of a gene will be modified and sent out to the cytosol for use in protein synthesis.

One area of active transcription in the nucleus is called the nucleolus. Here the RNA transcribed is not called messinger RNA; it is ribosomal RNA. This RNA is used to assemble ribosomes that, back out in the cytosol, play a central role in "reading" the messenger RNA to assemble the appropriate amino acids into a polypeptide sequence of a protein. Because of the high concentration of RNA in the nucleolus region, it stains differently than the rest of the nucleus.

Enzymes in the nucleosol are also used to make permanent copies of DNA. DNA polymerase (and other enzymes) produces copies of entire DNA molecules. This process is called replication. The copy and original DNA are held together for orderly separation in a later process called mitosis. Thus replication is a prelude to cell division.

The pores in the nuclear membrane are the ports of exchange. Through these pores large macromolecules are transported. Enzymes for transcription and replication are brought into the nucleus from their synthetic origin in the cytosol. Completed messenger RNA and ribosomal RNA are moved out to the cytosol. All borders are meant for crossing and exchange of materials and information (a hard-learned lesson for Chinese Emperors and Berlin Communists). The nucleus is no different.

Please learn to pronounce "nucleus." It is "new-clee-us" not "new-cue-luss." Yes, yes, you hear it wrong on TV all the time. Even US Presidents cannot pronounce "nuclear power" correctly! But you will show attention to detail in your field if you pronounce it correctly. You gain credibility. You will also probably spell it correctly too! Also note: "probably" has three syllables, and "library" is not a fruit!

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