So, the authors used a fluorescent dye that only fluoresces when it is part of double stranded DNA. The surface is a glass slide / coverslip on which you have

So, the authors used a fluorescent dye that only fluoresces when it is part of double stranded DNA. The surface is a glass slide / coverslip on which you have DNA + the DNA replication enzymes (not the entire set used inside a cell... Just the most important ones for the actual process of DNA replication). They used a technique called tirf microscopy [1] which allows them to, in effect, monitor only the molecules and dyes adhered to the surface plane they want to observe. The lines are, at a simplistic level, single stranded DNA getting converted to double stranded DNA.. as the DNA gets replicated, the dye fluoresces. As the replication proceeds and the replicated length increases, more of the dye gets incorporated into double stranded DNA and starts fluorescing, acting as an indicator of ds-DNA formation, and therefore, a proxy for DNA replication success in this experimental model. The dots ( if I am interpreting correctly) are the template DNA in circular form. It's a very cool technical achievement in itself, getting this system to work! [1] http://www.leica-microsystems.com/science-lab/total-internal-reflection-fluorescence-tirf-microscopy/ http://www.leica-microsystems.com/science-lab/total-internal...