three structures containing messenger rna can be demonstrated in the cytoplasm of early sea-urchin embryos : ( i ) particles that sediment more slowly than ribosomes and contain newly synthesized dna-like rna , ( ii ) light polyribosomes , which also contain this newly synthesized rna , and ( iii ) heavy polyribosomes , which seemingly contain only already existing or `` '' maternal '' '' messenger rna and account for the bulk of the synthesis of protein .
C0035696
messenger rna
C0035668
rna