A gene is a segment of DNA (I hope double stranded) and each strand when transcribed form a mRNA and after being translated a protein. As from each gene there are two proteins that are being formed. Now how do these two proteins determine the expression of the gene?

I may be wrong. Please explain if so.


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  • $\begingroup$ Usually (I don't know about any counter examples), only one of the DNA strands encodes a mRNA. $\endgroup$ – Maljam Apr 18 '16 at 19:22
  • $\begingroup$ @Maljam What happens to the other strand ? Why is it not transcribed? How does a cell know which strand to transcribe and which strand not ? $\endgroup$ – Tyto alba Apr 18 '16 at 20:02
  • $\begingroup$ Genes have promoters and enhancers upstream of the start site, all on the same strand. It is not unusual to have two (sometimes completely different) genes overlapping in a certain segment of DNA. $\endgroup$ – MattDMo Apr 18 '16 at 20:55
  • $\begingroup$ All enzymes related to DNA require a 3'-> 5' strand as they can synthesize product only in 5'-> 3' direction. Same happens with RNA polymerase too. It synthesizes mRNA in 5'-> 3' direction and thus requires only 3'-> 5' strand of DNA. $\endgroup$ – another 'Homo sapien' Apr 19 '16 at 0:55

If a piece of DNA were to be transcribed on both strands, at the same time, the transcription complexes would collide and inhibit each other, like steric hindrance.

If you could overcome this interference and synthesize primary transcripts from both strands, it follows that the two RNAs would hybridize to each other and form complementary RNA-RNA hybrids. These dsRNAs are very poor substrates for translation by the ribosomes. This was the initial mechanism proposed for anti-sense RNA to down regulate gene expression.

In many, if not most, EUKARYOTIC cells, a dsRNA will trigger the siRNA/shRNA/miRNA response mediated by Dicer et al. leading to degradation of both RNA strands.

So in virtually all instances, only one strand will be transcribed, and encode a mRNA with a significant ORF.

  • $\begingroup$ But this only applies to eukaryotes. Prokaryotes do have overlapping ORFs and don't have the dicer system. $\endgroup$ – David Apr 19 '16 at 16:32

Normally only one strand of DNA is transcribed. This is possible because for the RNA polymerase to bind and start transcribing the DNA into mRNA (or pre-mRNA) a special sequences on the DNA called a promoter is needed. Promoters will normally only be positioned to allow transcription in one direction, i.e. of one DNA strand.

Bacteria and their viruses sometimes have slightly overlapping genes so that, depending on their direction, the end of one mRNA may be from the same stretch of DNA as the start of another. This may not matter if one or both is not translated in this region.

There are even some cases where the end of one protein is from the same stretch of DNA as the start of another, but the amino acid sequences are different because they come from opposite strands. In such cases one assumes that the sequence of the protein is unimportant in that particular region. One does generally find most variation in the N- and C-terminal regions of homologous proteins in different species.


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