Transcription of both sense and antisense strands in opposite directions

Question

[fabbaszadeh]

Dear All. Could someone please check if my understanding of transcription from both sense and antisense strands of DNA are correct. I am also confused about transcription of genes that exist in the opposite directions.

1- Both sense (5-3’) and antisense (3’-5’) strands of DNA can be transcribed (such as Xist and Tsix transcription; see attached file). However as transcription progresses from 5’->3’ direction, I do not know how sense strand can be transcribed as I think the transcription on this strand would be from 3’->5’? 2- Do genes that can produce both sense and antisense transcripts contain promoters on both strands in the opposite directions of each other? If this is true I think transcription on both strands do not occur simultaneously as polymerases will collide. Probably transcription on different strands will occur under different conditions in the presence of different transcription factors. 3- For a gene that is located in the opposite direction in the DNA (such as BRCA1), on which strand of DNA does the transcription occur? If it occurs on the antisense strand does that mean that the DNA denatures so mush so that promoter at the end of the gene is exposed to transcription machinery on the antisense strand?

Thank you very much for your help!

Answer 1

[Asaf]

I think that what you're missing is that the definition of the + strand and - strand is arbitrary. The DNA is symmetric and there is no "sense" strand, we defined it to make things easier. So when the sense strand is transcribed the template is the antisense and when the antisense strand is transcribed the sense strand is used as template. You're correct with point 2 - the polymerases will collide.

Answer 2

[Devon Ryan]

The antisense strand is also 5'->3', it's just flipped around in the opposite direction. There are no genes on both strands, those are two genes on opposite strands (Xist and Tsix are classic examples of that). Each gene has its own promoter, since they will necessarily be transcribed 5'->3' in opposite directions. Transcription of genes on the + strand is done by using the - strand as a template, the reverse is true for genes on the - strand (this then facilitates 5'->3' extension). Promoters are traditionally at the beginning of genes (I'm sure there are a few at the ends too...biology likes to be messy). Strands aren't denatured, a helicase separates them.

You might want to have a read through an undergrad molecular/cell biology text book.