LS12815E

Reconsidering the Human Genome Structure

The human genome presents compositional features of many different scales and complex long-range correlations. This suggests that compositional fluctuations are scale-invariant, and therefore that there is not a typical dominant scale of genome organization in many orders of magnitude, typically up to orders of 100 kb. At this
scale, it is well-known the existence of isochores, i.e. long DNA tracks of relatively homogeneous G+C content with a typical size of 100 kb. To date, human genome is viewed as a mosaic of isochores harboring the rest of genomic elements. However, first we present evidences showing that isochores are actually organized at much larger scales into gigantic compositional segments or superstructures, with typical sizes of the order of 10 Mb, thus challenging the current view of the human
genome. Second, we introduce a new segmentation algorithm based on rigorous statistical criteria which takes into account the long-range correlations present in
human DNA, and which is able to detect automatically these superstructures: each human chromosome is divided into a few huge segments (15-20 Mb) with homogeneous G+C content. And third, we show that gene pairs embedded in each superstructure, despite that on average they can be very distant , share many functional properties in common (even more than genes contained in the same isochore, one hundred times closer on average). This suggests the existence of a previously unreported very large scale functional organization of the human genome.

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LW12551

Attosecond control of photoelectron emission

Since the early days of atomic and molecular physics, it is known that
the natural time scale of electron motion in atoms and molecules is
the attosecond -- that is a billionth of a billionth of a second. In
the last ten years, with the advent of controllable sources of
ultrashort light pulses, the attosecond scale has become the finest
time resolution reachable in experiments. It is now possible to
observe electrons as they move that fast, in real time. In this
Letter, we demonstrate how the dynamics of an electron ejected from a
molecule, upon absorption of two photons, can be traced with
attosecond resolution using a technique designed so far to
characterize attosecond light pulses. Furthermore, we show how
electron ejection can be controlled and delayed by several
femtoseconds, marking a new step toward attosecond control of chemical
reactions at the electronic level.