What goes wrong with DNA?

When a cell divides, an exact copy of the DNA must be produced so that each of the daughter cells recieve a copy.This proccess of copying the DNA is called replication.

The DNA double helix unwinds from one end and the two strands split apart as the hydrogen bonds between the bases break. Free DNA nucleotides line up alongside each DNA strand and hydrogen bonds form between the complementary bases. The enzyme DNA polymerase links the adjacent nucleotides to form complementary strand. I  this way each strand of DNA acts as a template on which a new strand is buily and, overall, two complete DNA molecules are formed. These are identical to each other and to the orignal DNA molecule. 

There are 3 ways DNA could replicate - in a fragmentary way, semi-conservatively or conservatively. 

Fragmentary replication - all DNA strands are made up of a mixture of original parent DNA nucleotides and new nucleotides.

Conservative replication - one DNA molecule has 2 original parent DNA strands, the other has two 'new' strands.

Semi-conservative replication - each DNA molecule contains one original parent strand and one 'new' strand.  

Meselson and Stahl used heavy and light strands of DNA. They did this by using DNA from bacteria that had been grown in a medium containing only the heavy isotope of nitrogen, 15N. All the nucleotides in the bacteria at the start of their experiment contained heavy nitrogen, and this made the DNA denser than normal. 

Meselson and Stahl then moved the bacteria into a medium containing only normal 14N. This meant all the new nucleotides incorporated into the replicated DNA were 'light' but the original DNA nucleotides were heavy. They allowed the bacteria to divide and their DNA to replicate once. They then extracted and centrifuged the DNA.

If a test tube containing DNA dissolved in a special density-gradient solution is centrifuged the heavy DNA (containing 15N) sinks to the bottom. Light DNA (containing normal 14N) collects a band near the top, and DNA of medium density it in the middle. Medium density DNA must contain some heavy and some light nucleotides.

Meselson and Stahl's result was a single band of medium density DNA. No heavy DNA existed any longer, so they were able to reject the conservative model of DNA replication. The DNA extracted and centrifuged after 2 rounds of replication gave 2 bands - one medium and one light. The presence of both medium and light DNA bonds confirmed the semi-conservative model and ruled out the fragmentary model which would only produce one type of DNA containing a mixture of heavy and light nucleotides.

Mistakes in replication
As the 'new' strand of DNA is being built, an incorrect base may slip into place. This is an example of gene mutation. Sometimes mutations occur in the DNA of an ovary or testis cell that is dividing to form an egg or sperm. Such mutation may be passed on to future generations, present in every single cell produced from the fertilised egg. 
Some mutations have no effect on the organism. Large amounts of DNA found in a cell do not actually play a role in protein synthesis and therefore mutations that occur in these sections may have no effect. However, if a mutation occurs within a gene and a new base triplet is created that codes for a stop signal or a different amino acid, the protein formed may be faulty. This could cause a genetic disorder.