Deoxyribonucleic acid -- DNA -- contains genetic information that determines how organisms grow, develop and function. This double-stranded molecule is found in every living cell and resembles a twisted ladder. The organism’s genetic information is expressed as proteins that have specific functions in the cells. This information is first copied from DNA to a single-stranded molecule -- messenger RNA, or mRNA -- and then from mRNA to the amino acids that make up proteins. The coding and template strands are terms that refer to the transfer of genetic information from DNA to mRNA, a process called transcription.
DNA is a double-stranded molecule made of subunits called nucleotides. These smaller units have phosphate and sugar components that form the sides -- or backbone -- of the twisted ladder. Nucleotides also include one of four nitrogeneous bases that form the letters of the genetic language. The bases, along with their shorthand designation, are adenine (A), thymine (T), cytosine (C) and guanine (G). The two DNA strands are joined in the middle by complementary base pairs from each strand -- adenine pairs with thymine, and cytosine pairs with guanine.
As with DNA, ribonucleic acid -- or RNA -- is composed of smaller units called nucleotides. The phosphate and sugar components of the nucleotides form the backbone of the single-stranded RNA molecule. Likewise, RNA has nitrogenous bases that are the same as DNA, except for thymine (T), which is replaced by uracil (U). The RNA bases can form complementary pairs with the bases of DNA -- cytosine with guanine, adenine with uracil, or thymine with adenine. There are three major types of RNA that are involved in the production of proteins -- messenger RNA (mRNA), ribosomal RNA (rRNA), and transfer RNA (tRNA).
In order for the genetic information stored in DNA to be useful, it must first be transferred to the mRNA, which is used as a template for creating proteins. Proteins in the cell called ribosomes split apart the two strands of DNA, read the code stored in the DNA bases and assemble the mRNA. This resembles unzipping a zipper and creating another half of the zipper that is complementary to the original.
During transcription, only one of the two DNA strands is copied. This is called the template strand because it acts as a template for the mRNA that is being assembled by the ribosomes. The sequence of the mRNA is complementary to the sequence of the template strand. For example, a cytosine on the DNA template strand pairs with a guanine on the mRNA strand, and an adenine pairs with a uracil.
The non-template strand is known as the coding strand. Because the coding strand and the mRNA formed from the template strand are both complementary to the template strand, they will have the same sequence. The only exception is that wherever there is a thymine in the DNA coding strand, there will be a uracil in the mRNA. Remember, RNA has a uracil base instead of the thymine base that is found in DNA.