This is a very very basic question. I've looked at methods such as chromosome sequencing and shotgun sequencing. Wikipedia says that:
DNA sequencing is the process of determining the precise order of nucleotides within a DNA molecule
For instance, with methods such as shotgun, we already have the original DNA. If we have access to the original DNA itself, don't we ALREADY know the sequence of nucleotides?
Why exactly do we need to 're-build' the DNA with methods such as shotgun and chromosome walking, when we already have the original DNA available? If we have the original DNA, don't we already have the original sequence of nucleotides?
It makes me think that these techniques for 'DNA sequencing' is sort-of pointless. But, obviously I am misunderstanding something.
Thanks in advance, any simple pointers would be great.