why is it unlikely that a DNA molecule replicate incorrectly?

Answer 1

There are two parts to this. If the old strand currently has a G at the point of replication, then in the initial step the DNA polymerase will usually add the correct base (C). This is because the shape of cytosine will allow for the formation of hydrogen bonds with guanine and it is this 'fit' that triggers the polymerase to join the cytosine to the previous base. If the fit is not good, then the base will be released,allowing a different base to enter the active site.
Rarely (about 1 in 1000) the polymerase links the new base even though it is not the correct one. At this point the pairing is not good, and the DNA is distorted. When the polymerase moves on to the next base the distortion is noticed and the polymerase uses its exonuclease function to remove the last base and try again. This 'proofreading' step cuts the error rate down to closer to 1 in 1,000,000

Answer 2

DNA polymerase III checks the work as it creates the the DNA

Answer 3

there are numerous editing processes involved along the way that make it very unlikely for errors to occur, although they still happen

Answer 4

There are numerous error checking mechanisms.