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Condensin. (A) Condensin is a five-subunit protein complex that resembles cohesin 
(see Figure 17–23). The ATPase head domains of its two major subunits, Smc2 and Smc4, are 
held together by a linker protein called Brn1, which is associated with two additional proteins called 
Ycs4 and Ycg1. (B) These diagrams illustrate a mechanism by which condensin might generate 
DNA loops, thereby promoting the compaction of a chromosome. For an alternative model, see 
Figure 4–56. In the model shown here, the process begins when the Ycg1 subunit interacts tightly 
with a strand of chromosome DNA, anchoring the condensin firmly in place on the DNA. The 
nearby DNA curls around to interact with the hinge domain. By mechanisms that remain unclear, 
the hinge and ATPase domains work together, using energy from ATP hydrolysis, to move leftward 
along the top DNA strand (red arrow), thereby generating a chromosome loop. Such loops are 
common structural elements in chromosome packaging (discussed in Chapter 4).