After we separate the two DNA strands, what makes one of the two strands combine with the labelled mRNA strand instead of the old complementary strand?
1$\begingroup$ You usually add your labelled probe in a large excess to avoid this. $\endgroup$– Chris ♦Nov 22, 2016 at 16:48
1$\begingroup$ Like chris said it is only a question of Ratio, some will recombine with their complementary strand, but a lot more with the mRNA. $\endgroup$– Dart FeldNov 22, 2016 at 18:31
$\begingroup$ Then why don't you guys write an answer? Comments are NOT for answers. It says so when you open the comment box. $\endgroup$– DavidNov 22, 2016 at 20:39
$\begingroup$ @David Because I had no time to write up a proper answer. One line is not an answer. $\endgroup$– Chris ♦Nov 23, 2016 at 15:44
$\begingroup$ Yup, same here. I really dislike not having references in the biology SE. I guess for this type of question it is ok. $\endgroup$– Dart FeldNov 23, 2016 at 15:46
In this type of experiment the concentration of the mRNA will be greater than the concentration of the starting DNA.
When the DNA strands separate, the DNA and mRNA will be in competition for the complementary strand. Using really high concentrations of mRNA compared to the DNA will give it advantage to recombine with a higher frequency and give a higher ratio of the DNA-mRNA combination.
It is all a question of ratio.