Messenger RNA or mRNA is a single stranded molecule that can copy the DNA code in a process called transcription. DNA is a double-stranded molecule, known as a double helix. Either side of the DNA molecule can function as a template for transcription. In DNA, there are subunits called nucleotides. These consist of a sugar--deoxyribose, a phosphate group and one of four nitrogenous bases--adenine, thymine, cytosine and guanine. RNA is a single stranded molecule, with a different sugar called ribose and it has nitrogenous bases adenine, guanine and cytosine, but it does not have thymine as in DNA. Instead, there is a base called uracil. When DNA is transcribed, complementary bases of RNA will be in the transcription. Base pairing rules for DNA are adenine pairs with thymine and cytosine with guanine. However, RNA lacks thymine so uracil is substituted. Here is an example. If the DNA code reads-
T-C-C-G-T-A, the complementary messenger RNA code would be
A-G-G-C-A-U. Notice that since RNA lacks thymine, uracil will pair with the adenine which was in the original strand of DNA.