pick out nonmapped pairs from bwa

[louis7781x]

Hi,

I have used bwa to align paired-end read against to refseq and output sam file.

Now,I have a problem.

If I want to pick out the read like bowtie's parameter --un.

How can I do like bowtie's output?

Like bowite's maual

Paired-end reads will be written to two parallel files with _1 and _2 inserted in the filename, e.g., if <filename> is unaligned.fq, the #1 and #2 mates that fail to align will be written to unaligned_1.fq and unaligned_2.fq respectively.

[swbarnes2]

As far as I know, there's no way to get it make you the fastq of the unmapped reads. However, you can get a sam/bam file containing only unmapped reads, and from that you can make the fq with a simple script.

samtools view -F 0x0004 your.bam > unmapped.sam

the 4 flag is set when the read is unmapped. -F will make sure that only reads with the 4 flag are included in the sam.

[louis7781x]

As far as I know, there's no way to get it make you the fastq of the unmapped reads. However, you can get a sam/bam file containing only unmapped reads, and from that you can make the fq with a simple script.

samtools view -F 0x0004 your.bam > unmapped.sam

the 4 flag is set when the read is unmapped. -F will make sure that only reads with the 4 flag are included in the sam.

Thanks!!

But it has a problem, if pairs has only one mapped,and the other is nonmapped.

I can't pick out both reads.

I know bowtie --un in pairs,if one is map ,one is numapped. it will also output to unmapped_1.fa and unmapped_2.fa

[swbarnes2]

samtools view -F 0x0004 your.bam > unmapped.sam

Will get all the unmapped reads

samtools view -F 0x0008 your.bam > unmapped_mates.sam

will get all the reads whose mates are unmapped.

Or, once you have the names of the unmapped read, you could go through your fastqs and pull out both reads that have that name

[louis7781x]

samtools view -F 0x0004 your.bam > unmapped.sam

Will get all the unmapped reads

samtools view -F 0x0008 your.bam > unmapped_mates.sam

will get all the reads whose mates are unmapped.

Or, once you have the names of the unmapped read, you could go through your fastqs and pull out both reads that have that name

OK I see ,Thanks a lot!

[volks]

samtools view -Sb -F 2 your.sam > not_proper_pair.bam

will give you the reads which are not mapped in proper pairs. the output can be converted to fastq or used in BWA directly (-b option).

[louis7781x]

samtools view -Sb -F 2 your.sam > not_proper_pair.bam

will give you the reads which are not mapped in proper pairs. the output can be converted to fastq or used in BWA directly (-b option).

Hi,thanks for yorr reply

I have seen manual in bwa

-b Specify the input read sequence file is the BAM format. For paired-end data, two ends in a pair must be grouped together and options -1 or -2 are usually applied to specify which end should be mapped. Typical command lines for mapping pair-end data in the BAM format are:
bwa aln ref.fa -b1 reads.bam > 1.sai
bwa aln ref.fa -b2 reads.bam > 2.sai
bwa sampe ref.fa 1.sai 2.sai reads.bam reads.bam > aln.sam

-0 When -b is specified, only use single-end reads in mapping.
-1 When -b is specified, only use the first read in a read pair in mapping (skip single-end reads and the second reads).
-2 When -b is specified, only use the second read in a read pair in mapping.

I don't understand why -b parapeter can converted to fastq in BWA directly?

[volks]

I don't understand why -b parapeter can converted to fastq in BWA directly?

with -b you can use a BAM as input, for example if you want to align the unmapped reads to a different genome. it is not converted to fastq, but there are other tools for that.