Re: How scientist determine the chemicals required for the isolation of DNA.

The selection of specific reagents for any protocol depends on two main criteria:

1. Biological activity and properties

2. Cost

Without seeing your protocol, I would guess the Tris is used at the end as part of a solution to dissolve the isolated DNA. It is being used to buffer the pH, that is, to ensure that small additions of acids or bases do not significantly affect the pH of the DNA solution. Tris is more expensive (cirterion 2) than other possible buffering agents, such as phosphate, but the "complex structured chemical" you saw makes it much less likely to interfere with other biological processes where the phosphate ion might intrude, such as inhibiting enzymatic reactions (criterion 1).

There are other buffering agents that should be equally effective (one frequently sees HEPES, MOPS, PIPES and MES in biochemical and molecular biology protocols), but these are typically more costly than Tris.

Incidentally, in other application, such as those involving living cells rather than isolated macromolecules, solutions buffered with phosphate are commonly used, largely due to their low cost and ease of preparation.

Looking at other reagents in DNA isolation, there is a long-running debate about the use of ethanol vs. isopropanol to precipitate DNA after its isolation. There's a very nice article available explaining the relative merits of each.