In RNA sequencing, the RNA is fragmented, DNA is synthesized complementary to the RNA fragments, which is followed by a complementary strand synthesis. Fragmentation can be done after the cDNA synthesis too. This DNA is then amplified to form a cluster that is sequenced. Most Next-Gen sequencing approaches sequence a short segment of the DNA (it has improved with experimental and technical optimization).
A read refers to the sequence of a cluster that is obtained after the end of the sequencing process which is ultimately the sequence of a section of a unique fragment. If an RNA is expressed in high copies then there will be more reads coming from it; reads can be redundant as well.
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