Please see the attached file.
1.Why was the gel probed with GAPDH cDNA? Briefly describe how the level of GAPDH mRNA varied between different lanes?
GAPDH is referred to as a 'housekeeping' gene, it is expressed at significant levels all the time, so it should be detectable in all samples. This is important as a positive control to show the Northern blot has worked, as no signal would indicate a technical problem. It also acts an an internal standard. As the level of this mRNA remains approximately constant in each cell, regardless of the age of the mouse, it gives a measure of how much sample has been added to each lane. By comparing the signals from the test mRNAs with the GAPDH signal for mice of each age, you can normalize expression of all the test genes relative to GAPDH across mice of different ages. For example if the GAPDH signal was markedly higher in mice of some ages, then the test mRNA signals would also be higher, but they could only be compared with the samples from the mice at other ages after determining expression relative to the GAPDH signal.
The level of GAPDH mRNA was similar ...
This solution performs one of many tasks: describe the changes in Egr-1 mRNA expression in the liver of animals from embryonic age E18 to 9 months of age.