|MoBio||The Nucleus||Chapter 1|
The cell nucleus is the largest organelle in a cell. It contains DNA which, together with associated proteins, form a complex called chromatin.
Figure 1-C-1. The nucleus.
The nuclear envelope is a double-membrane structure that constitutes the outermost portion of the nucleus. Both the inner and outer membranes of the nuclear envelope are phospholipid bilayers. The outer membrane is connected to the endoplasmic reticulum.
There are many pores in the nuclear envelope, allowing substances to enter and exit the nucleus. The viscous fluid inside the nucleus is called nucleoplasm, where the chromatin is located.
Chromosomes are the condensed structures of chromatin when the cell is getting ready to divide. In humans, the germ cell (sperm or egg) contains 23 chromosomes, labeled from 1 to 22 and either X or Y. The somatic cells (cells other than germ cells) of a normal person has 46 chromosomes. For other species, the chromosome number varies from 1 to 1260.
The nucleolus is the darkly staining area within the nucleus. It contains some sections of DNA encoding ribosomal RNA (rRNA). The nucleolus aggregates the produced rRNA with associated proteins to assemble the ribosomal subunits that are then transported out through the pores in the nuclear envelope to the cytoplasm.