Final answer:
The electrophoresis chamber is filled with a buffer solution during the preparation of an agarose gel for electrophoresis. DNA samples and ethidium bromide are added later in the process and are not used to fill the chamber initially.
Step-by-step explanation:
The electrophoresis chamber is filled with a buffer solution during agarose preparation. This buffer solution maintains a constant pH and provides ions that carry the electrical current through the gel. The agarose gel itself is a matrix composed of agarose powder that is added to the buffer and heated, which, after cooling, is poured into a casting tray to solidify. Once the gel has been set, DNA samples are loaded into wells within the gel, and an electric current is applied. As the negatively charged DNA moves through the gel towards the positive electrode, it is separated by size. After the electrophoresis run, the separated DNA is typically stained with a dye such as ethidium bromide for visualization under ultraviolet light.
Ethidium bromide (EtBr) is often added to the gel after it has cooled but before it solidifies to stain the DNA during the run. It's important to note that although the DNA samples and ethidium bromide are essential components of the electrophoresis process, they are added at different stages and do not constitute the initial filling of the electrophoresis chamber.