This Diploma thesis consists of two parts: the research part and the methodological part. The research part presents the study of conditions at which DNA forms complexes with poly(diallyldimethylammonium chloride), PDADMAC. The following methods were used: electrophoresis for mobility (including ζ-potential) determination and quasi-elastic light scattering for size determination. The effect of cation type on electrophoretic mobility and ζ-potential of PDADMAC-DNA system and size of formed complexes was investigated and confirmed. It was also shown that the direction of titration influences the complexation of polycations with DNA. In the methodological part of the Thesis entitled „The motivation in chemistry teaching - a case study” a case study method as a tool for teaching chemistry with positive results on student’s motivation is described. The case study is suggested as a two hour lesson in which targeted educational achievements of the third year high school students are related to observations from the experiment.