ABSTRACT
Viroids are the smallest known pathogens and have only been found in plants. Viroids range in size from 246 to approximately 375’ bases and have strong internal nucleotide sequence homologies, leading to substantial base pairing, which gives them a rod-like shape in their native state. Short extraction methods can sometimes be devised for routine screening of one particular host species after a viroid has been characterized, but good controls are crucial. Due to their characteristic molecular properties, viroids migrate in most gel systems with a mobility less than that expected for a molecule of their molecular weight. Viroid purified in low-melting-point agarose can be recovered by melting and phenol extraction. However, partial denaturation of the RNA is likely to occur during the melting of the agarose. Electron microscopy can be used to prove circularity and estimate the size of purified molecules thought to be viroids when spread under denaturing conditions.
