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[ Origins of the Nucleus ]

1. Nucleus or no nucleus? The defining feature of cells is whether they have a nucleus or not. We call simpler single-celled organisms lacking a nucleus, prokaryotes. Prokaryotes fall into two very broad classes of organisms, the bacteria, and the distantly related but similar-looking archaea. The more complex cells with a nucleus we call eukaryotes. Eukaryotes encompass an incredible diversity of organisms from single-celled yeast to humans. The nucleus contains most of the genetic material that defines the eukaryotic organism. Many of the genes in the nuclear DNA encode the instructions to build proteins, the workhorse of the cell. The first step in the central dogma of biology — the transfer of genetic information from DNA to RNA, called transcription — occurs in the nucleus. To complete the protein synthesis process, RNA must travel out of the nucleus. The second step of the central dogma — the transfer of information from RNA to build proteins, called translation — happens in the cytoplasm. Cytoplasm is all the cellular material outside the nucleus, but within the skin of the cell called the cell membrane. The nucleus has a number of fascinating features. For example, the nucleus has a double membrane, the inner and outer nuclear membrane, each of which is composed of a lipid bilayer similar to the cell membrane. Another important feature is the nuclear pores which make the nucleus look somewhat like a whiffle ball. But the pores are not passive holes allowing materials to drift in and out. Each pore has a complex machine built of many different proteins which together act as active transporters and gatekeepers to and from the nucleus. The most important material that the nuclear pores manage are the messenger RNAs (mRNAs) which carry the protein-building code from the nucleus to the cytoplasm where proteins are made. These and other features of the nucleus challenged biologist to understand its origins.