Keeping chromosomes from cuddling up

If chromosomes snuggle up too closely at the wrong times, the results can be genetic disaster.

Now researchers have found the molecular machines in fruit flies that yank chromosomes, the DNA-carrying structures, apart when necessary.

The machines, proteins called condensin II, separate chromosomes by twisting them into supercoils that kink up and therefore can no longer touch.

Scientists had known of condensin II but did not know how it functioned inside cells.

Keeping specific parts of chromosomes from touching can change how the instructions carried in the DNA are read, said research team leader Giovanni Bosco of The University of Arizona in Tucson.

"It's like picking up your favorite book and, depending on what chair you chose to sit in, it turned into a different story -- even though the printed words in the book never changed," Bosco, a UA assistant professor of molecular and cellular biology, wrote in an e-mail.

"This now changes the way we think about genetic information. Taking a literal reading of it is not what actually happens," he wrote. "Instead, context matters."

The team also found that condensin II plays a key role in making sure that fruit fly sperm cells each receive the proper number of chromosomes -- not too many, not too few.

Bosco suspects that condensin II plays the same role in the formation of human sperm and eggs.

Having too many or too few chromosomes in egg or sperm cells is the source of several important genetic disorders, including Down syndrome.

Abnormalities in chromosome number is also the cause of some miscarriages of early-term fetuses in humans.