Positive-sense

In molecular biology and genetics, sense is a concept used to compare the polarity of nucleic acid molecules, such as DNA or RNA, to other nucleic acid molecules. Depending on the context within molecular biology, sense may have slightly different meanings.

Molecular biologists call a single strand of DNA, sense (or positive (+)) if an RNA version of the same sequence is translated or translatable into protein. Its complementary strand is called antisense (or negative (-) sense). Sometimes the phrases coding strand (for sense) and template strand (for antisense) are encountered; however, protein coding and non-coding RNAs can be transcribed from the sense strand. Additionally, the terms "sense" and "antisense" are relative to the RNA transcript in question and not to the entire DNA strand as a whole. In other words, either DNA strand can serve as the sense or antisense strand for a particular RNA transcript. In some cases, RNA transcripts can be transcribed in both directions (i.e. on either strand) from a common promoter region, or be transcribed from within introns on either strand (see "ambisense" below).

The two complementary strands of double-stranded DNA (dsDNA) are usually differentiated as the "sense" strand and the "antisense" strand. The DNA sense strand looks like the messenger RNA (mRNA) and can be used to read the expected protein code; for example, ATG in the sense DNA may correspond to an AUG codon in the mRNA, encoding the amino acid methionine. However, the DNA sense strand itself is not used to make protein by the cell. It is the DNA antisense strand which serves as the source for the protein code, because, with bases complementary to the DNA sense strand, it is used as a template for the mRNA. Since transcription results in an RNA product complementary to the DNA template strand, the mRNA is complementary to the DNA antisense strand. The mRNA is what is used for translation (protein synthesis).

Hence, a base triplet 3'-TAC-5' in the DNA antisense strand can be used as a template which will result in an 5'-AUG-3' base triplet in mRNA (AUG is the codon for methionine, the start codon). The DNA sense strand will have the triplet ATG, which looks just like AUG but will not be used to make methionine because it will not be used to make mRNA. The DNA sense strand is called a "sense" strand not because it will be used to make protein (it won't be), but because it has a sequence that looks like the protein codon sequence.

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