## Gel electrophoresis enables visualization of DNA.

However, after performing the PCR reaction, we see nothing in the test tube that we take out of the instrument. The DNA is dissolved in the solution, and we need to find out if the desired section has been amplified and if it is a section with the correct length. To visualize and separate DNA molecules according to their size or molecular weight, we mostly use gel electrophoresis. We can use it to detect the presence of any DNA or RNA, it does not have to be just a PCR result. The main component of gel electrophoresis is a gel made from a polymer, usually agarose, which forms a network of pores after boiling and solidification. The PCR sample is mixed with reagents that allow the DNA to be visualized after analysis and then the DNA sample is applied to one end of the solidified gel using a pipette. As DNA is negatively charged it will therefore move towards the positive electrode (anode) in the electric field. Consequently, after filling the electrophoresis apparatus with a current-conducting solution, the movement of the DNA molecules occurs in the direction to the positive pole, with shorter molecules (of lower molecular weight) moving faster through the pores of the agarose gel and longer fragments moving more slowly (Figure 4.2). If we turn off the apparatus after a certain time, we can track where the DNA has gone. We can see this thanks to the fluorescent dye incorporated into the double-stranded DNA, and when the gel is illuminated with light of the appropriate wavelength, we can see bands corresponding to the individual DNA fragments. To determine the actual length of these fragments, we use a DNA molecular weight marker, which is a mixture of fragments of known length. By comparing the position of band in the lane with the bands in the DNA marker (as with a ruler), we can then determine the length of the fragments we analysed.