High resolution ray-traced model of a nucleosome, isolated on black.
A nucleosome is the basic unit of DNA packaging in eukaryotes, consisting of a segment of DNA wound in sequence around four histone protein cores. This structure is often compared to thread wrapped around a spool.
Nucleosomes form the fundamental repeating units of eukaryotic chromatin, which is used to pack the large eukaryotic genomes into the nucleus while still ensuring appropriate access to it. In mammalian cells approximately 2 m of linear DNA have to be packed into a nucleus of roughly 10 µm diameter.
Nucleosomes are folded through a series of successively higher order structures to eventually form a chromosome; this both compacts DNA and creates an added layer of regulatory control, which ensures correct gene expression.
A collection of planetary nebulae; clouds of ionized gas that form as red giant stars die and cast off their outer layers. This process enriches interstellar space with heavy elements that go on to form new stars and planets.
What’s that behind Titan? It’s another of Saturn’s moons: Tethys. The robotic Cassini spacecraft orbiting Saturn captured the heavily cratered Tethys slipping behind Saturn’s atmosphere-shrouded Titan. The largest crater on Tethys, Odysseus, is easily visible on the distant moon. Titan shows not only its thick and opaque orange lower atmosphere, but also an unusual upper layer of blue-tinted haze. Tethys, at about 2 million kilometers distant, was twice as far from Cassini as was Titan when the above image was taken. In 2004, Cassini released the Hyugens probe which landed on Titan and provided humanity’s firstviews of the surface of the Solar System’s only known lake-bearing moon.