## Events - Daily

**March 26, 2013**>>

Tuesday,
March 26 |
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From molecules to development: revealing simple rules of biological clocks - Qiong Yang, Stanford University Organisms from cyanobacteria through vertebrates make use of biochemical and genetic oscillators to drive repetitive processes like cell cycle progression and vertebrate somitogenesis. Oscillators also allow organisms to anticipate natural environmental rhythms, as exemplified by the circadian clock. Despite the complexity and variety of biological oscillators, their core design is thought to be shared. Notably, they all possess an essential negative feedback loop. However, absent crucial elements negative feedback circuits often settle into a stable steady state rather than oscillating. In this talk, I first discuss computationally how several modifications of the basic activator/repressor circuit can promote oscillation. Then I ask which of these strategies are actually utilized in the complex biological oscillator circuits found in nature by dissecting a mitotic oscillator in the Xenopus laevis early embryos. I found that the core negative feedback system of Cdk1-APC/CCdc20 operates as a time-delayed, digital switch, with a time lag of ~15 min between the activation of Cdk1 and its repressor APC/CCdc20 and a tremendously high degree of ultrasensitivity. Mathematical modeling indicates that this time delay must be coupled to the ultrasensitivity to ensure robust oscillations and segregation of cell-cycle phases. Principles uncovered here may also apply to other activator-repressor oscillators and help in designing robust synthetic clocks. Joseph Henry Room · 1:30 p.m.– 6:00 p.m. |

High Energy Theory Informal Seminar - IAS - Tomasz Taylor, Northeastern U. - Superstring Amplitudes as a Mellin Transform of Supergravity At the tree level, the maximally helicity violating amplitudes of N gauge bosons in open superstring theoryand of N gravitons in supergravity are known to have simple representations in terms of tree graphs.For superstrings, the graphs encode integral representations of certain generalized hypergeometric functions of kinematic invariants while for supergravity, they represent specific kinematic expressions constructed from spinor-helicity variables. We establish asuperstring/supergravity correspondence for this class of amplitudes, by constructing a mapping between thepositions of gauge boson vertices at the disk boundary and the helicity spinors associated to gravitons. After replacing vertex positions by a larger set ofN(N-3)/2 coordinates, the superstring amplitudes become (multiple) Mellin transforms of supergravity amplitudes, from the projective space into the dual Mellin space of N(N-3)/2 kinematic invariants. We elaborate on the properties of Mellin and inverse Mellin transformsin the framework of superstring/supergravity correspondence. Bloomberg Lecture Hall - Institute for Advanced Study · 1:30 p.m.– 2:30 p.m. |