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I am currently studying DNA extraction from various bio-originated samples. I am new to this field, and have learned how commercial DNA extraction kits work for bacteria.

I understand that the very first step is called 'lysis', and that this is supposed to break cells to pull out all the stuff inside them, including DNA.

I am studying cell-free DNA extraction procedure from human blood, and I don't understand why the lysis step is necessary in this context.

If lysis is to break cells, and cell-free DNA is already out of the cell and freely circulating, such a step seems unnecessary.

Is my understanding about lysis right? Then, why is the lysis step necessary during cfDNA isolation?

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You want to isolate the cfDNA, and leave anything else in the plasma out of your analysis — essentially, the lysis step is part of the purification.
(See here)
(I'll elaborate with edits shortly)
Edit:
Actually, it looks like lysis is not a good idea in cfDNA isolation — lysing cells in your sample will result in contamination by genomic DNA, as discussed here. It would be better to stabilize/fix cells to prevent them from lysing, to avoid this risk.
In other words, your initial instincts were correct!

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  • $\begingroup$ Thanks, you gave me a piece of confidence to solve this problem. Then is it okay to say that the lysis step for cfDNA is to clean or degrade other stuff in serum such as protein? $\endgroup$ – Kidan Feb 10 '17 at 14:27
  • $\begingroup$ Let me know if this answers your question! $\endgroup$ – Asher F. Feb 10 '17 at 16:52
  • $\begingroup$ Yes absolutely it does! I never knew there could be WBC in serum... Then lysis step might cause a mess in my sample. Thank you very much :) $\endgroup$ – Kidan Feb 10 '17 at 17:03
  • $\begingroup$ @Kidan My pleasure! Please do mark the answer as accepted if it helped you :) $\endgroup$ – Asher F. Feb 10 '17 at 17:04

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