Before a cell divides, it duplicates its DNA in a copiying process called replication. This process ensures that each resulting cell will have a comlete set of DNA molecules. In mitosis, This happens during interphase in the S phase.
DNA replication is carried out by a series of enzymes.
Also check out:
DNA Translation: http://www.youtube.com/watch?v=EEVRS1h6Qzg
DNA Transcription: http://www.youtube.com/watch?v=77AyKbYcxrA