Circularity and self-cleavage as a strategy for the primary chromosome in the RNA world. Thick lines represent the sense chain, and thin lines represent the antisense chain. The linear RNAs, which arise from the spontaneous break of the sense chain or partial copying of the antisense chain, may be readily broken at sites (labeled by virtual bands) between the linked genes. This feature may be implemented by the participation of short self-cleaving ribozymes residing at these sites. The products of the cleavage are the RNAs that may fold into various ribozymes (crescent-shapes). Note: the self-cleaving effect would not occur in the circular chromosome because steric constraints would inhibit the folding and function of the embedded self-cleaving ribozymes, consistent with the situations in viroids [11, 12].