MappingΒΆ

This module is a wrapper to Bowtie. We always allow multiple matches per read (up to 5), and document this in the bam output by adding the optional NH flag, see bbcflib.mapseq.add_nh_flag().

We use two C programs to analyze bam files:

The first generates statistics on reads mapping. In particular, the number of reads per strand, if unbalanced, can indicate a sequencing bias (see the Bowtie documentation).

The second creates genomic density profiles per strand or merged. We have implemented several optional features described in Leleu et al.:
  • Correct for PCR artifacts: only at most n reads per strand-specific genomic position are kept, where n is computed as the 95% percentile of a Poisson distribution with the same mean as the average genome coverage. This is done by parsing the position-ordered output bam and counting the successive lines with the same position, strand and library, see bbcflib.mapseq.remove_duplicate_reads().
  • Read density smoothing: the read length has no biological meaning, in principle only the first maping position should be counted in the genomic density. However extending the coverage to n bases downstream makes a smoother density which is visually more appealing. The value of this extension is arbitrary and can be less or more than the actual read length.
  • Down-weight multiple hits: using the NH flag described above, we count 1/NH read at each alignement, so that the sum of all scores in the genome is always equal to number of reads times the read extension.
  • Normalization: use total number of mapped reads times 1e-7 as a normalization factor.
  • Strand shifting and merging: reads mapping to the reverse and forward strands correspond to the ends of the DNA fragment making the sequencing library; it is generally advised to shift the mapping position downstream by half the difference between fragment size and read extension.

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