DNA is not exactly in the blood, but in the white blood cells of blood components.
Blood is composed of two parts: plasma and blood cells. The plasma is light yellow and translucent, containing a large amount of water, as well as proteins, glucose, inorganic salts, etc. The main function of plasma is to transport blood cells, nutrients, and waste.
Blood cells include red blood cells, white blood cells, and platelets: red blood cells have no nucleus and are in a concave circular cake shape on both sides; There are various types of white blood cells, including a nucleus, which is larger than red blood cells and has a smaller number; It is much smaller than red blood cells and white blood cells, with irregular shapes and no nucleus. Therefore, the DNA extracted from blood by "DNA paternity testing" comes from white blood cells, because only white blood cells contain nuclei.