This baby, pictured in yellow, is a nucleus. They contain genetic information for every single eukaryotic cell (apart from mitochondrial DNA, and the DNA of chloroplasts). They are the most conspicuous organelle under the microscope, appearing as a darker, denser area. The nuclear envelope separates the nuclear contents from the cytoplasm. The double membrane (nuclear envelope) is periforated by pores 100nm in diameter. At the lip of each pore, the inner and outer membranes are continous, leaving a space between them of 20-40nm. The inside of the envelope is lined with the nuclear lamina, a netlike array of protein filaments which serve to maintain shape. There is also much evidence for a nuclear matrix within. DNA is present as chromatin or chromosomes in the nucleus, with each species having a characteristic number of chromosomes and genes. A nucleolus is present within the nucleus and is identified as densely stained granules and fibres. The nucleolus synthesises rRBA and studies suggest it also plays a part in is regulation of cellular processes (such as division).