Can anybody give me full details about the PhD in US and in UK where you can directly go for PhD after clearing your qualifiers exam and why is it in UK it takes less time to complete PhD and in US it takes more time.And don't they give second chance if we fail to qualify in the qaulifiers.

I did my MSc in Analytical chemistry,can I go directly for PhD after giving GRE or do I have to again do MS and then pursue GRE in US.Please help in this matter.