Nucleus
The nucleus is the cellular control center and exists only in eukaryotes. The nucleus contains the genetic information for the cell, in the form of DNA and RNA. The genetic information is surrounded by a two-layer nuclear envelope and it is generally found at the center of the cell. The nucleus is responsible for communicating with other organelles in the cytoplasm (the gel-like space surrounding the nucleus). Messages from inside the nucleus travel through pores on the nuclear envelope to enter the cytoplasm.
Inside the nucleus, called the nucleoplasm, DNA is bound by histone proteins and organized into chromatin. During replication, the chromatins are condensed to form highly organized structures called chromosomes. Besides the genetic information, most nuclei also contain one or more spherical-shaped organelles called the nucleoli. The nucleolus is where ribosomes are assembled.
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