![]() ![]() Why does this mean that they are a match for the crime scene hair? How does having the same length for a sequence indicate identical genomes? Let's say, somehow, only one suspect has 5000 bp for that sequence. At this point, why would the different suspects have different lengths for that sequence? Doesn't a certain sequence have a fixed length? Then, you take the hair cells of a few suspects (for simplicity, you know for sure that the culprit HAS to be one of them) and do PCR for the same sequence and gel electrophoresis. (Side question: why wouldn't you know the length of your target sequence before doing PCR?) Then, let's say you did gel electrophoresis and found out the DNA for the chosen sequence is 5000 bp long. Firstly, how would you select the sequence to do PCR with? Let's say you do PCR for a specific sequence on a hair cell from a crime scene. There's much that might be wrong, so please let me know: Here's how I'm guessing it happens and the questions I have about it. I don't understand how the length of a DNA fragment can be used to identify a person. I myself once had the same question in my mind but realized that the viscous drag is the game changer of the scenario. Yes all your equations are correct and they travel at the same acceleration but it's the viscous drag through the gel that causes heavier DNA fragments to move slower than the lighter DNA fragments. ![]() The more electrons in the DNA fragment ,the stronger the intermolecular london forces of attraction between the DNA fragment and the gel molecules so the greater the viscous drag experienced by the DNA fragment. The longer the DNA fragment ,the more atoms it contains so the more electrons it contains. The mass of the DNA fragment depends on the length of the DNA fragment. ![]() This viscous drag is proportional to the mass of the DNA fragment. The DNA fragments move through the agarose gel so they are experiencing viscous drag as they move through the gel. However, there's one more factor that comes in play. When you define that a=qE/m ,you assume that the only force acting on the DNA is the electric force from the electric field so that's why your calculation takes this as the resultant force since it is the resultant force which will accelerate a body according to Newton's second law. Your equations are correct but your model has assumptions. ![]()
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