It is often useful to distinguish the two strands of DNA -- the strand that is copied into mRNA and subsequently translated has the complementary sequence to the mRNA, while the base sequence of the opposite strand directly corresponds to the codons in the mRNA.
The terms template strand, sense strand, and coding strand are commonly used to describe one of the two strands of DNA, however the nomenclature is quite confusing because different authors have used these terms to describe both strands -- one school argues that the strand copied into mRNA should be considered the template strand, but the other school argues that the opposite strand which reflects the sequence in the mRNA should be considered the template because the corresponding codons are copied into protein. The first definition is used in the figures below, however, to avoid confusion, when using the words template, sense, or coding, it is essential to explicitly define how you are using the terms. I believe that these terms are best defined as described below.
The term template strand refers to the sequence of DNA that is copied during the synthesis of mRNA.
The opposite strand (that is, the strand with a base sequence directly corresponding to the mRNA sequence) is called the coding strand or the mRNA-like strand because the sequence corresponds to the codons that are translated into protein.
Although RNA polymerase must recognize sequences on the template strand, by convention we draw the DNA sequence and regulatory signals on the "mRNA-like" strand. (This makes it simpler to directly determine the sequence of the resulting RNA.) The following cartoon shows this concept for a hypothetical gene.
It may be useful to consider a real gene as well. The DNA sequence of the phage P22 arc gene and some important regulatory sites is shown below. The upper strand of DNA is the "mRNA-like" strand. The lower strand is the strand that is complementary to the mRNA. The -35 region (TTGACA) and -10 region (TATATT) of the promoter sequence and the transcriptional start site (the A) is indicated on the coding strand. Also note that the DNA sequence of the coding strand corresponding to the RNA codons is shown in bold (of course, the T is a U in the RNA) -- the first codon is ATG the translational start site (fMet) and the last codon is TAA (Ochre) the translational stop codon.
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Last modified July 12, 2002