DNA is organized into chromosomes through an array of protein-DNA interactions. Our group uses single-DNA micromanipulation – stretching and twisting of the double helix – as a tool to analyze a variety of enzymes acting on DNA. I will describe a few different kinds of “magnetic tweezers” experiments we are doing that are aimed at understanding enzymes that help to package DNA and to change its topology. We also use analogous but larger-scale micropipette-based micromanipulation approaches to study the large-scale structure of metaphase chromosomes; I will discuss experiments that tell us the metaphase chromosomes behave as “chromatin gels”, apparently stabilized in part by DNA entanglement. Recent experiments on effects of depletion of condensin SMC complexes will also be discussed.