The nuclear membrane insures that the interior of the nucleus is isolated from a cell's cytoplasm, allowing two different environments to be maintained. The membrane has two layers that enclose a distinct space in between, and although marked by large pores, the membrane is very selective, permitting only certain substances to enter or leave the nucleus. In addition, the space contained within the double-layered nuclear membrane is continuous at points with the endoplasmic reticulum, a membrane-enclosed structure that pervades the entire cytoplasm and provides a means for nuclear materials to reach all parts of the cell.
Contained within the nuclear membrane and surrounded by a granular-like nucleoplasm are two types of structures, the chromosomes and the nucleoli. The chromosomes consist of protein and the most basic units of heredity, the genes, which are composed of DNA. The nucleotide sequences of the genes' DNA contains the information that determines the nature of the cell during differentiation and that controls the metabolic activities of the cell. The genes exert such control by directing the synthesis of the enzymes that regulate all the cell's biochemical reactions. But since DNA never leaves the nucleus and since proteins (including enzymes) are synthesized in the cytoplasm, a messenger, called messenger RNA (mRNA), is needed to go from the nucleus to the ribosomes, the sites of protein synthesis in the cytoplasm.
The mRNA transcribes the DNA's nucleotide sequences, leaves the nucleus, travels to the ribosomes, and acts as the model for the ordering of amino acids during protein
