 The nucleolus is a discrete densely stained structure found in the nucleus . It is not surrounded by a membrane , and is sometimes called a <unk> . It forms around tandem repeats of rDNA , DNA coding for ribosomal RNA ( rRNA ) . These regions are called nucleolar organizer regions ( <unk> ) . The main roles of the nucleolus are to synthesize rRNA and assemble ribosomes . The structural cohesion of the nucleolus depends on its activity , as ribosomal assembly in the nucleolus results in the transient association of nucleolar components , facilitating further ribosomal assembly , and hence further association . This model is supported by observations that inactivation of rDNA results in <unk> of nucleolar structures . 
