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enter image description here

Recently, I came across this picture which represents the DNA sense strand of a gene.

The ‘Altered’ picture shows a (frameshift) mutation in the nucleotide sequence in which a T base is deleted.

The sense strand is not transcribed — the anti-sense strand is transcribed. So why is there a change in the amino acids of the translation product when a mutation occurs in the sense strand, if the sense strand does not produce amino acids? Does a mutation in the sense strand change the nucleotide sequence of the transcribed sense strand?

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  • $\begingroup$ I have edited your question so that it uses only unambiguous strand designations as discussed in my answer to your recent related question. $\endgroup$ – David Dec 18 '18 at 17:06
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We’re doing it for the children!

The basic misconception in this question is that the important effects of mutations are seen in the cell that undergoes mutation. This is not the case. So focusing on the whether the strand that receives a mutation is the non-transcribed, sense, strand or the transcribed (anti-sense) strand is of little relevance.

What is important is whether or not the mutation is passed on to the progeny.

Don’t we have a life too?

Why isn’t it important? Consider a mutation occurring in a single E. coli cell in a population of millions of E. coli, or a single muscle cell in a human muscle. Even if there is a mutation in the strand that is transcribed into mRNA and translated into protein, the change in that one cell will not have a significant effect on the overall population.

The birds and the bees

The way in which a mutation — on sense or anti-sense strand — can affect a population of bacterial cells or whole tissue in an organism is:

  • (For bacteria or in tumour cells) if the mutation is passed on to some of progeny, which then have a selective advantage so that they can outgrow the wild-type population
  • (For normal tissues in animals) if the mutation is passed on to the progeny through the germ line so that it is present in all (e.g. muscle) cells

Still harking on about sex (Yawn)

The key point is easier to explain with bacteria. Consider a simplistic situation in which a mutation occurs on one strand (and is not corrected by the DNA-repair mechanisms). Before the cells divide into two daughter cells the double-stranded DNA replicates — each strand is the template for a complementary strand in a daughter double-stranded DNA. One cell will have the daughter dsDNA (double-stranded DNA) copied from the unmutated strand and the other will have the daughter dsDNA copied from the mutated strand.

In the case of the dsDNA copied from the mutated strand, both strands will clearly carry the mutation, so it is irrelevant whether the mutated strand in the parent was sense or anti-sense.

If the mutation is advantageous these cells will grow and divide and out-compete the other bacteria in the population.

A similar explanation holds for tumour cells in a differentiated animal tissue.

In the case of germ-line cells in animal tissues the situation is slightly different — the mutation either will be present or absent in both strands of the dsDNA in the fertilized egg, and so will affect all of the cells in the tissues of the progeny in which it is expressed.

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  • $\begingroup$ Thank a lot. The mutation doesn't always occur in the original DNA strands, right? Can there be mistakes in the replicating strand? For example, if there is an A nucleotide on one original DNA strand, can the replicating strand accidentally bring a C? Not sure if the term replicating strand is correct. $\endgroup$ – Christopher U'Ren Dec 19 '18 at 0:29
  • $\begingroup$ I'm not quite sure what you mean. Both strands are replicated in ds genomes. There are several different types of mutation with different mechanisms and consequences. I suggest you look at the Nature Scitable article which covers them quite well. $\endgroup$ – David Dec 19 '18 at 10:35
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Does a mutation in the coding strand change the base sequence of the template strand?

The DNA molecule is composed of two paired, complementary strands of DNA which--by definition--must be paired and complementary. A deletion of a base on one strand would be replicated such that the new DNA molecule would be deficient at that position on both strands. Here's a nice summary that also includes animations.

And, if you are instead asking whether there are ever exceptions to this very basic rule of molecular biology, then yes, so-called "bulged bases" have been reported (e.g.here) but they are still considered anomalous.

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  • $\begingroup$ Sorry, but what does deficient mean in a biological sense? $\endgroup$ – Christopher U'Ren Dec 18 '18 at 4:54
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    $\begingroup$ @ChristopherU'Ren — It has no special biological meaning. DavidR clearly meant "deficient in a base" in the sense of lacking or missing a base — the base that was deleted in the first strand. $\endgroup$ – David Dec 18 '18 at 16:24

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