Small RNA library preparation and quality control. (A) Schematic of library preparation. Small RNAs of 18–30 nt were purified from X. tropicalis embryos, and Illumina RNA adapters were ligated by an RNA ligation reaction. The adapter sequence is needed to attach the template DNA to the flow cell in the sequencing machine. The ligated RNAs were reverse-transcribed and the cDNA used as the template for a large-scale PCR. The PCR library was then subjected to single-end deep sequencing. (B) Length distributions of small RNA sequencing reads and unique sequence tags. Small RNA read and tag numbers are plotted against length for the five small RNA libraries. The sequencing reads are trimmed to remove sequencing adapters and mapped to the genome with zero mismatches. The arrow indicates the miRNA population that collapses when reads are converted to tags, and the arrowhead indicates the population of 28-nt RNAs.
