It means the DNA that yeast contains. The DNA that makes up yeast.

YEAST = the stuff that makes bread rise/grow bigger when baked... They are talking about it's DNA.

GRAMMAR - to focus on DNA in yeast is to focus on the DNA of yeast'. It's an optional prepositional noun phrase using (of or in) here, attached to a Phrasal verb 'focus on'.

It's just the name of the organism he is studying. Yeast is a microorganism with tiny microscopic cells. Like other living organisms, it contains DNA. According to Google Translate, "yeast" is "hefe" in German and дрожжи in Russian. Its most familiar use is as leavening in bread. Traditional breadmaking uses yeast. Bread dough with yeast "rises." The microorganisms produce carbon dioxide, forming tiny bubbles. The bubbles give the bread its light, spongy texture. The wonderful smell of bread baking is the smell of yeast. Depending on conditions, yeast also produces alcohol. Beer is made with yeast, which produces both the bubbles and the alcohol. Yeast is one of many traditional "model" organisms used for biological studies, each for one kind of study for one specialized reason--like fruit flies, or guinea pigs, or white mice.

Presumably the context of this statement is the research activity of a scientist in genetics or food technology. He was working on DNA - the genetic material - of yeast, but has changed his work plan. The preposition 'in' is usually used when defining the source organism for the material [in this case DNA] used for research. e g. ’An investigation of the effects of seasonal average temperature on fertility in the rabbit’ ; ’Factors determining viral pathogenic transmission in the oak tree [Quercus spp.]’