The nucleus is the major site of gene expression in eukaryotic cells, separating the nuclear genome from cytoplasmic activities. Packed inside the membrane-bound nucleus are the chromosomes, consisting of long strands of DNA bound with a variety of proteins. The nucleus is the major site of transcription, where the genes carried on the chromosomes are copied into RNA sequences. These primary RNA sequences are processed in the nucleus, modified by the enzymatic addition of caps and tails, as well as splicing together of genetic sequences. Functional RNA molecules exit the nucleus through nuclear pores, and assemble in the cytoplasm on ribosomes where they are used to translate the RNA sequence information into chains of amino acids. Gene expression is controlled in the nucleus by a variety of means, including differential packing of chromatin and the number and type of transcription factors. A special region of the nucleus called the nucleolus is the site of ribosome synthesis.
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