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  • d1antho
    Member
    • Mar 2012
    • 15

    #1

    Aligning paired end read files that have different numbers of reads after trimming

    Hi all,

    Basically, I'm wondering if tophat2 (calling bowtie2) can align paired reads from two different files that don't have the same number of reads? Right, that sounds a bit vague, but in a nutshell;

    I have gotten raw read files in FASTQ format back from an RNA seq project. This is from paired end sequencing. So for a single lane there are 2 files (one for each pair) and both have 29181932 reads. I then trimmed each read for a quality score, but when I was doing this I included an option to remove reads that, after trimming, contained less than 25 bps (These are 40bp reads). More reads were removed, because of this, from the second mate of the pair. Now I have different numbers of reads for each pair (pair 1 29081810; pair 2 26989485). A difference of about 2.1 million reads, but this is expected as quality is always lower on the second mate.

    The problem now is that I don't know if bowtie2 will only align reads for which the 2 corresponding mates exist or will it align each read independently? Preferably the former.

    I know I could create files with only the IDs that are in both, but I don't want to do this if bowtie2 will take care of it for me.

    Any advice would be greatly appreciated.
    Last edited by d1antho; 01-19-2013, 12:50 PM. Reason: The initial title wasnt specific enough to the problem
  • kmcarr
    Senior Member
    • May 2008
    • 1181

    #2
    No, it will not work properly with the input files you have described. Nearly every software tool for NGS insists that all pairs are matched. When the read pairs are provided in separate files (e.g. R1 and R2) the order of the reads must match exactly in the two files.

    This is why trimming and filtering tools which don't maintian pair information create big headaches. For this reason I started using Trimmomatic. It works with both of the paired reads simultaneously. If only one of the reads survives QC it is written to a separate singletons file, while surviving pairs are written to their respective paired files.

    Comment

    • d1antho
      Member
      • Mar 2012
      • 15

      #3
      Great. Thanks

      Just re-doing the trimming now

      Comment

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