Generating Read Length?
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5.8 years ago

Hello! I am new to RNA seq. I quality trimmed my fastq sequences via fastX toolkit using a phred score of 30. I would like to figure out the read length after the quality trim (phred score < 30 was removed). Any help would be much appreciated.  

 

 

-Nikelle 

RNA-Seq rna-seq read length quality phred • 2.1k views
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This kind of trimming is very severe and likely unnecessary. I assume you are aligning to a reference genome. You are probably throwing away lot of good data if you did lose a lot if bases after trimming.

See this thread for a paper referenced in there about trimming using quality: Which Phred value to use in trimming

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You might want to consult www.rnaseq.wiki for a really nice description of the steps involved in processing RNAseq. IIRC, they cover read trimming in the appropriate section

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5.8 years ago

Run FastQC before and after trimming.

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Thank you very much! One other question. Before trim, I had sequence length = 100. After trimming, I had a sequence length of 3-100. Can you make any sense of this? I think I am having a hard time figuring out what length it is measuring.

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That means some reads were trimmed to a length of 3 eliminating 97 bases and you have a range of read lengths remaining that goes from 3 to 100. See my comment for your original question.

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Thanks!

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5.8 years ago

How about this:

sed -n '1~4p' filename.fastq | perl -ne 'chomp;print length($_) . "\n"' | sort -n | uniq -c >length.dist
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Hi, Thanks Chris. If I were to input this, what would this be generating?

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Let's break it down:

sed -n '1~4p' filename.fastq

Gives you every 4th line of the file (the sequence line)

perl -ne 'chomp;print length($_) . "\n"'

outputs the length of that line

sort -n | uniq -c

condenses it into a table of counts like this:

  3  98
123  99
 22  100
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Thanks very much, that was helpful. What are the two different columns in the table?

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Count and read length (see man uniq)

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Chris, the fourth line of FASTQ is the quality score, not sequence (but it should be trimmed to the same length as the sequence string, so results should be the same).

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The command gives every 4th line, starting with the 1st. Come to think of it, that should be 2~4p, right, because of the header?
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