 The nucleolus is a discrete densely stained structure found in the nucleus . It is not surrounded by a membrane , and is sometimes called a suborganelle . It forms around tandem repeats of rDNA , DNA coding for ribosomal RNA ( rRNA ) . These regions are called nucleolar organizer regions ( NOR ) . 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 intermingling of nucleolar structures . 
 The nucleolus <a href='drop_aux' title='1'><mark>is</mark></a>a discrete densely stained structure found in the nucleus . It is not surrounded by a membrane , and is sometimes called a suborganelle . It <a href='uninflect' title='2'><mark>forms</mark></a> around tandem repeats of rDNA , DNA coding for ribosomal RNA ( rRNA ) . These regions are called nucleolar organizer regions ( NOR ) . The main roles of the nucleolus are to synthesize rRNA and assemble ribosomes . The structural cohesion of the nucleolus <a href='uninflect' title='3'><mark>depends</mark></a> 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 intermingling of nucleolar structures . 
The nucleolus a discrete densely stained structure found in da nucleus . It ain't surrounded by a membrane , and is sometimes called a suborganelle . It form around tandem repeats of rDNA , DNA coding for ribosomal RNA ( rRNA ) . These regions are called nucleolar organizer regions ( NOR ) . The main roles of tdanucleolus are to synthesize rRNA and assemble ribosomes . Thdatructural cohesion of the nucleolus depend 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 intermingling of nucleolar structures .