Explain copying errors in DNA?

When replicating DNA there is always a chance that the DNA polymerase (enzyme responsible for DNA replication) will make a mistake and incorporate the wrong nucleotide. This change in nucleotide is how the mutation is created.

As for what he is asking it sounds like they want to know how a DNA error results in a protein that functions differently. A change in a single nucleotide depending on where it is could result in a different amino acid being incorporated into the protein, a truncated protein being formed (this would happen if the nucleotide change resulted in a stop codon being placed too soon in the transcript). Even a single amino acid change can greatly affect how a protein functions. It could result in an enzyme that binds too tightly too it's substrate or a protein that looses its enzymatic function.

Hope this helps

The mutated DNA may result in a protein which contains a different amino acid than the normal protein. Having a different amino acid can affect the overall charge and shape of the molecule - so it may not assume it's secondary and tertiary shape as the normal protein would. If the protein is an enzyme this may result in a change in the active site so it's function is impaired, or if it is a structural protein, for example in membranes, it could cause weaknesses. For example, mutations in red blood cell structural proteins (eg, actin) causes a change in the red cell shape and early destruction of the cell, ultimately this causes haemolytic anaemia.