Origin Of Different Libraries In Sam File

Question

[Pascal]

A very simple question I guess: In a sam/bam file I notice there may be several "libraries" of inserts (LB tag). Could you explain to me where do these different libraries come from in the sequencing process? And/or give me some examples that lead to production of several libraries. Thanks.

Answer 1

[Chris Miller]

A grossly oversimplified version is as follows:

You take genomic DNA as input. Some of this DNA is cleaned up, sheared, size selected, has adaptors added, and is often amplified, which results in a library. This library DNA is then sequenced. If you still don't have adequate coverage of your genome, you may wish to prepare another library, starting again from the genomic DNA.

Since these libraries are from the same sample, the reads are often just concatenated together into one big bam. The readgroup and library tags let you trace each read back to it's origin.

[Pascal]

Thanks Chris! I really did my homework googling and reading other sequencing related documents before asking this question but I could not find easily the explanation of this. Thank you for the synthesis, this is very clear.