Update 02-workflow.md

_episodes/02-workflow.md

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 - "scRNA-seq requires much pre-processing before analysis can be performed."
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+## Challenge
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+Download the latest mouse transcriptome fasta file from GENCODE. What do the entries look like? Tip: to read the file into R, consider the `readDNAStringSet()` function from the `Biostrings` package.
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 # scRNA-Seq Experimental Workflow
 
 Single cell RNA sequencing (scRNA-seq) is a novel technique for extracting more detailed information from genome.