[Magazine] Scientific American. Vol. 293. No 4

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In short, it appears that galaxies are not born with a given shape, barred or unbarred. They metamorphose. If three quarters of galaxies are barred, a typical galaxy must spend three quarters of its life barred. During this time, the bar prevents fresh gas from entering the central part of the galaxy. The gas accumulates in the outer region and, after the bar self-destructs, pours in and rejuvenates the galaxy. The other, better-known way to assemble mass in galaxies is cannibalism, or successive galactic mergers.

These atoms are called stable isotopes because they do not undergo radioactive decay and thus do not spontaneously one millionth the size of those then possible in my laboratory in Wisconsin. After 11 days of round-the-clock analysis and little sleep (typical conditions for this difficult procedure), we completed the measurements — and found that our predictions were wrong. 4. We were stunned. What could these high oxygen isotope ratios mean? In younger rocks the answer would be obvious, because such samples are common.

To enter the cells, HIV first adheres to a molecule called CD4 (1), an attachment that facilitates the binding of a viral protein (gp120) to CCR5 (2). This connection induces the cells to take up the attached virus (3), enabling the pathogen to release its genes and convert the infected cell into a virus-making machine. Allosteric modulators that change the shape of CCR5, making it unrecognizable to gp120 (top left panel), are already in clinical trials. Unfortunately, though, viral mutations could eventually alter gp120 so that it became able to attach to the allosterically altered CCR5 and enter T cells (top right panel).