Viroids, as a consequence of not encoding any protein, are extremely dependent 
on their hosts. Replication of these minimal genomes, composed exclusively by a 
circular RNA of 246-401 nt, occurs in the nucleus (family Pospiviroidae) or in 
the chloroplast (family Avsunviroidae) by an RNA-based rolling-circle mechanism 
with three steps: (1) synthesis of longer-than-unit strands catalyzed by host 
DNA-dependent RNA polymerases recruited and redirected to transcribe RNA 
templates, (2) cleavage to unit-length, which in family Avsunviroidae is 
mediated by hammerhead ribozymes, and (3) circularization through an RNA ligase 
or autocatalytically. This consistent but still fragmentary picture has emerged 
from a combination of studies with in vitro systems (analysis of RNA 
preparations from infected plants, transcription assays with nuclear and 
chloroplastic fractions, characterization of enzymes and ribozymes mediating 
cleavage and ligation of viroid strands, dissection of 5' terminal groups of 
viroid strands, and in situ hybridization and microscopy of subcellular 
fractions and tissues), and in vivo systems (tissue infiltration studies, 
protoplasts, studies in planta and use of transgenic plants expressing viroid 
RNAs).