Expression Vector vs Cloning Vector

[testsubjectzz]

I have this question how come with cloning why can we not clone directly into the expression vector why is it done in 2 stages?

[JackBean]

Sure you can and sometimes you even do, like e.g. if you're expressing in bacteria or have vector working both in bacteria and yeast (like pPICZ and pGAPZ).

However, the point of using two vectors is such, that you can use your protein for plenty of applications (e.g. expression, GFP fusion, crystalization etc.) and if you had to get it always from either gDNA or mRNA would be quite hard so you better clone it into one (clone) vector, which is high copy, but doesn't bother the bacteria with expression of your protein and thus you can always obtain high amount of your gene.