Before mitosis begins, a cell doubles its amount of chromosomes by replicating, or copying all of its DNA. During replication, double-stranded DNA unwinds and each strand acts as a template for a new strand. DNA transcription is a similar process except only a part of the DNA sequence is copied to form a messenger RNA (mRNA) strand. In this investigation you will build new strands of DNA and RNA using nitrogen bases, sugars and phospates to create the nucleotide sequence. Remember, during replication when you are copying DNA, you must build two new strands of DNA, but during transcription only a single strand of RNA is built. Choose Build DNA or Build RNA to begin the investigation.