Cellular DNA contains instructions for building the various proteins the cell needs to survive. In order for a cell to manufacture these proteins, specific genes within its DNA must first be transcribed into molecules of mRNA, then, these transcripts must be translated into chains of amino acids, which later fold into fully functional proteins.
This process is called protein synthesis. The first major step in protein synthesis is transcription. During transcription, enzymes called RNA polymerases build RNA molecules that are complementary to a portion of one strand of the DNA double helix. This is then further processed to form mature mRNA which is a single-stranded copy of the gene, which next must be translated into a protein molecule. During translation, which is the second major step in gene expression, the mRNA is “read” according to the genetic code, which relates the DNA sequence to the amino acid sequence in proteins.
Each group of three bases in mRNA constitutes a codon, and most codons specify a particular amino acid. The mRNA sequence is thus used as a template to assemble the chain of amino acids that form a protein. This happens in the ribosome. Interestingly, not all regions of an mRNA molecule correspond to particular amino acids. In particular, there is an area near the 5′ end of the molecule that is known as the untranslated region (UTR). This portion of mRNA is located between the first nucleotide that is transcribed and the start codon (AUG) of the coding region, and it does not affect the sequence of amino acids in a protein.
A change in code in the coding region, however, can change the protein which is formed.