The most apparent function of the nucleus is to sequester the DNA from the enzymatic reactions occurring in the cytoplasm. However, many activities take place in the nucleus. It contains three separate structures:
Nucleolus: The nucleolus is a dense mass inside the nucleus. Ribosomal proteins and RNAs are assembled here.
Nuclear envelope: As stated above, organelles are bounded by membranes. The membrane surrounding the nucleus has been well studied. It consists of two lipid bilayers, each similar to the cell membrane (see Chapter 11). This double membrane creates a barrier to water-soluble substances. Pores span both membranes and allow ions and small molecules to pass through. Large molecules are selectively transported across the membrane. The pores are responsible for transport both into and out of the nucleus. Ribosomes, the site of protein synthesis, are often found along the outer membrane, while chromosomes are attached to the inner membrane.
Chromosomes: Inside the nucleus is the DNA arranged in the structures called chromosomes (see Chapter 5). Chromosomes are made up of the DNA as well as proteins associated with organization, transcription, replication and repair.
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