The nucleoli consists of DNA in the form of chromatin because this is the form of DNA unless the cell is preparing for mitosis (in which chromosomes would be visible). It also contains rRNA (ribosomal RNA) which will drive the making of the large and small subunits of ribosomes. This is essential to have within the nucleoli so that the cell can make proteins that are needed.
Work Step by Step
See last paragraph of 6.3 "The Nucleus: Information Central" on page 102