how PCR can detect very low amount of dna?

 PCR detects low amount of DNA by the amplification of the small DNA fragment with the help of promoters and enzymes.

also check this out for more details: http :// www.ncbi .nlm.nih. gov/ pubmed/ 11211226

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PCR means Polymerase Chain Reaction.
When the question says that "PCR can detect very low amounts of DNA", it means the chains of nucleotides of bacteria or virus that are present inside, contaminating the host body. 
PCR can detect these by amplifying their nucleic acids. 
Personally, I'd like to think it as:
When you look for a bacteria with naked eye, you can't really see anything.
But when you look for the bacteria under a microscope, you automatically are magnifying it, and you can see this bacteria.

So, what PCR does is very simple. It takes the sample DNA, multiplies it (kind of like magnification), so that viral or bacterial DNA also multiplies along with it, so that the small concentrations of disease causing substances can be spotted easily. 
 
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The PCR helps to detect the low amount of DNA by amplifying that piece of DNA to form many copies. The amplified DNA can be then easily detected by using other testing procedures. This technique is used to detect infection at the early stage by amplifying the DNA of microbe.
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