If a restriction enzyme cuts a recognition sequence specifically for a particular gene for gel-electrophoresis, then the fragments would all be the same size wouldn't they? Why do they turn out to be different sizes when gel electrophoresis is performed?
For example, if a piece of DNA is found at a crime scene, PCR is performed and then we have DNA.
We'd want to take/cut out a piece of DNA from the DNA at the crime scene using restriction enzymes, and then use the same restriction enzyme to cut out a part of DNA in the suspect for comparison.
If the same restriction enzyme is used, then why do the fragment lengths turn out to be different in each individual?
For example, if a piece of DNA is found at a crime scene, PCR is performed and then we have DNA.
We'd want to take/cut out a piece of DNA from the DNA at the crime scene using restriction enzymes, and then use the same restriction enzyme to cut out a part of DNA in the suspect for comparison.
If the same restriction enzyme is used, then why do the fragment lengths turn out to be different in each individual?
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The name "restriction enzymes" was given to these "scissors" simply because where they cut is restricted. Each enzyme cuts at a specific sequence. This point you seem to know well. However, your thinking that fragments should be equal in size is not accurate. If an enzyme cuts at a sequence like this GGGAC then it will only cut when and where it finds this sequence. However, this sequence does not recur at a regular pace and distance from each other. Suppose, in a piece of DNA, the sequence comes 3 times the enzyme in question will cut it into 4 pieces. The pieces are completely different in length because the sequence targeted does not come at equal distances from each other. Because human beings always have differences in DNA from each other (in exons or, more commonly in introns) then the cut pieces from one person differ from corresponding cut pieces of the same DNA site of another person. Thus you can tell if DNA found in the crime site (as an example) belongs to the same suspect you are investigating.
So: 1. Pieces cut by restriction enzyme from DNA in one individual are not equal to each other because the sequence targeted is not repeated at equal distances.
2. Same pieces cut from the same DNA site, by same enzyme from 2 different individuals are not exactly identical to corresponding pieces, because of differences in non-encodable DNA among individuals, even identical twins. So the number of pieces produced may be the same but pieces move a little differently in electrophoresis due to minor differences in their nitrogen bases.
So: 1. Pieces cut by restriction enzyme from DNA in one individual are not equal to each other because the sequence targeted is not repeated at equal distances.
2. Same pieces cut from the same DNA site, by same enzyme from 2 different individuals are not exactly identical to corresponding pieces, because of differences in non-encodable DNA among individuals, even identical twins. So the number of pieces produced may be the same but pieces move a little differently in electrophoresis due to minor differences in their nitrogen bases.