The rungs of the ladder consist of complementary base pairings between adenine and thymine, cytosine and guanine in the DNA double helix. When DNA unwinds, either side can be used as a template to manufacture more DNA. The enzyme DNA polymerase is essential for joining the appropriate nucleotides to the bases on the "unwound" DNA strands causing two identical strands to be produced. RNA is not needed for this process. When mRNA copies the DNA code, the codons mentioned in the above answer are used to assemble a polypeptide at the ribosome. Each triplet or codon represents a particular amino acid. As they are linked to form a polypeptide and eventually detach and fold into a three dimensional shape, a protein is formed. This is known as gene expression.