It's hard to imagine that the deoxyribonucleic acid, or DNA, that sits coiled up inside the nucleus of your cells contains the entire blueprint for your body, but it's true! One of the most fundamental processes for any organism is the central dogma of molecular biology, which describes the DNA-to-RNA-to-protein scheme that takes that genetic map and uses it to synthesize proteins.
Inside the nucleus, RNA polymerase transcribes messenger RNA using a single strand of DNA as a template. This mRNA travels into the cytoplasm where the ribosomes translate its code into strings of amino acids. These amino acids ultimately fold into proteins.
The process of transcription creates RNA using a DNA template. First, the double-stranded DNA separates into its component strands. Proteins called promoters bind to the strand of DNA and function at strategic locations known as promoter sequences. The enzyme RNA polymerase attaches 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 substitutes uracil in place of thymine. A single strand of DNA can serve as a template for countless identical molecules of RNA, using multiple copies of DNA polymerase. A termination sequence on the DNA strand indicates the stopping point for transcription.
Recall that DNA is a double helix, meaning that it resembles a ladder with the ends twisted in opposite directions. In contrast, mRNA 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. Molecules of mRNA, also called transcripts, are usually about 300 to 50,000 nucleotides long. In theory, this means 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 never become mRNA and instead serve other functions in the cell. Those destined to code for proteins are precursor mRNA. Significant portions of precursor mRNA consist 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. To become a mature mRNA transcript, these introns splice out and the remaining mRNA segments, called exons, come back together.
The business of manufacturing proteins occurs on cell components, or organelles, called ribosomes, which consist largely of a type of RNA known as rRNA, or ribosomal RNA. The ribosomes "scan" the mature mRNA strands, searching for an amino-acid start sequence. Each of the 20 amino acids in your body correlates with 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 this synthesis portion of the ribosome to the end of the growing amino-acid strand, called a polypeptide.
