Kalmar "mai tsara nucleolar" tana nufin wani yanki na DNA a cikin tsakiya na kwayoyin eukaryotic wanda ke da alhakin samuwar nucleolus, tsarin da ke cikin samar da ribosomes. Mai tsara nucleolar yana ƙunshe da kwayoyin halitta waɗanda ke ɓoye ga RNA ribosomal (rRNA), wanda shine muhimmin sashi na ribosome. Ana rubuta waɗannan kwayoyin halitta kuma ana sarrafa su a cikin nucleolus, kuma sakamakon rRNA yana haɗuwa da sunadaran ribosomal don samar da ribosomes. Mai tsara nucleolar yawanci yana kan ɗaya ko fiye da chromosomes, kuma girmansa da wurinsa na iya bambanta tsakanin nau'ikan nau'ikan daban-daban.