Deoxyribonucleic acid, or DNA -- which you can think of as the genetic blueprint for an organism -- is transcribed inside cells to ribonucleic acid, or RNA. The sequence of nucleotides in RNA is then translated into amino acids, which are finally assembled into proteins. This DNA-to-RNA-to-protein scheme is often called the central dogma of molecular biology, and the main subtype of RNA, called messenger RNA or mRNA, is a specific intermediary in the elegant process of protein synthesis from the DNA tied up your chromosomes. Unlike DNA, mRNA is single-stranded.
RNA is formed from DNA by a process called transcription. First, the double-stranded DNA separates into its component strands. Proteins called promoters then bind to the strand of DNA to be copied at strategic locations known as promoter sequences. The enzyme RNA polymerase binds to the promoter-DNA complex and unwinds the DNA. DNA contains the four base pairs adenine, cytosine, guanine and thymine; RNA contains the first three but contains uracil in place of thymine. Countless identical molecules of RNA can be formed from the same DNA strand using multiple copies of DNA polymerase, and the stopping point of transcription is marked by a termination sequence on the DNA strand.
Structure of mRNA
Recall that DNA is double helical in formation, meaning that it resembles a ladder with the ends twisted in opposite directions. mRNA, in contrast, is single-stranded. It also contains the five-carbon sugar ribose in its background rather than deoxyribose, and includes the nucleotide base uracil instead of thymine. mRNA molecules, also called transcripts, are usually about 300 to 50,000 nucleotides long; in theory, this means that they can code for anywhere from 100 to 15,000 amino acids each.
Like a newly manufactured automobile, a newly synthesized mRNA transcript must undergo some final processing before it's ready to do its job. Some transcripts actually never become mRNA and have other functions in the cell. Those destined to code for proteins are called precursor mRNA. Significant portions of precursor mRNA consist of of introns, which are sequences that don't code for any amino acids and may be up to 10,000 nucleotides long in large transcripts. These introns are spliced out and the remaining mRNA segments, called exons, are rejoined to create a final mRNA product -- a mature mRNA transcript.
The Translation of mRNA
The business of manufacturing proteins occurs on cell components, or organelles, called ribosomes, which themselves consist largely of a type of RNA known as rRNA, or ribosomal RNA. The ribosomes "scan" the mature mRNA strands they bind to, "searching" for an amino-acid start sequence. Each of the 20 amino acids in your body is coded for by a particular three-nucleotide-long strand of mRNA -- the triplet codon. A third type of RNA, transfer RNA or tRNA, is responsible for bringing amino acids from the part of the ribosome in which they are made to the end of the growing amino-acid strand, called a polypeptide.