The nucleus houses most of the genetic material of the cell (Fig. 2.1). [Mitochondria have their own genetic material, which is inherited from the mother in humans and remains distinct from the so-called nuclear DNA (Gilbert, et al. 2003)]. The DNA, or genetic material, guides the cellular machinery. Through the processes of transcription and translation described in Secs. 7.4 and 7.5, genetic material guides the formation of RNA and subsequently proteins, which turn on and off a multitude of functions in the cell. The appearance of the nucleus varies. In metabolically active cells producing many proteins, the chromatin is dispersed for easy transcription and gives the nucleus a pale appearance. The dispersed chromatin is called euchromatin. Tightly packed chromatin that is more visible with typical stains is called heterochro-matin. Cells with heterochromatic nuclei are less actively making proteins.

2.4.1 Nucleolus

Inside the nucleus is found the nucleolus, or little nucleus. This spherical mass is composed of RNA and proteins. The nucleolus is the source of the ribosomes (see Sec. 2.3.1). Ribosomal subunits are made and released from the nucleolus and then transported into the cytoplasm from the nucleoplasm (fluid matrix of the nucleus). Passageways, nuclear pores, in the nuclear envelope allow ribosomal subunits and mRNA to leave the nucleus and enter the cytoplasm or lead directly into the endoplasmic reticulum.

Also entering and leaving the nucleus are nucleic acids used to make DNA and RNA. A central dogma of biology states that DNA leads to RNA, which leads to the production of protein (Raven, et al., 2005).

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