Learning modern genetics is challenging and students have difficulty acquiring a coherent cognitive mental model of abstract concepts such as DNA, bacteria and enzymes. Here we investigated students' mental models of genetics through analysis and interpretation of the discourse that took place while high-school students practised hands-on molecular biology experiments in the laboratory. The lab activity focused on DNA manipulations showing the link between gene and phenotype. The activity was conducted in a unique outreach laboratory setting, entitled Teacher-Led Outreach Laboratories (TLOL). In this setting, the high-school biology teachers themselves teach their students at our institute's laboratories, in contrast to most outreach laboratories in which academic personnel teach visiting classes. Written questionnaires, aimed at probing students' (12th-graders, n = 181) conceptions, were handed to the students before and immediately following the laboratory activity. The results demonstrated that before the activity, the students have inaccurate mental models which are physical copies of the images they were exposed to during their studies. The students' mental models of DNA and bacteria improved significantly following the activity, as did their procedural understanding of DNA manipulations. Thus, the lab activity in the framework of TLOL promotes high-school students' comprehension of concepts in molecular genetics. (Contains 1 figure and 2 tables.)