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Dr. Warwick F. Vincent
LAVAL UNIVERSITY (Dept. of Biology)
Life on Snowball Earth: Insights from Polar Microbial Ecosystems
April 4, 2005    2:45pm    BSB-B136
The genetic code is the set of assignments between the 64 possible codons in DNA and the 20 possible amino acids in proteins. The code is fundamental to the way all organisms work, since it controls the process of translation (i.e. protein synthesis). The Canonical Code evolved prior to the Last Universal Common Ancestor of all current life forms. However, many modified genetic codes are found in specific genomes in which one or more codons have been reassigned to a different amino acid from that in the canonical code. Codon reassignment should be a difficult and disruptive process for an organism to go through because of a variety of negative selective effects which will occur during the change-over period.
Earth?s Precambrian (the vast stretch of geological time from the formation of the planet to the onset of the Cambrian, 570 million years ago) has traditionally been thought of as a period of continuously high temperatures in which extreme, heat-loving microbes emerged and established prolonged dominance of the biosphere. However, there is evidence that the Earth also experienced extreme low temperature conditions at several intervals during the Precambrian, and perhaps even during the earliest steps in the emergence of life. Geological evidence of widespread glaciation during the Paleoproterozoic about 2.4 billion years (Ga) before the present and during the Neoproterozoic (0.6 Ga before present) has led to the controversial ?snowball Earth hypothesis?, that freeze-up occurred during these times at a planetary scale, and that ice extended from the poles to the tropics. Extreme cooling has largely been viewed as a negative factor that would severely inhibit or even extinguish life in the biosphere. However, little attention has been given to the success and surprising biodiversity of microscopic communities that thrive today in the Polar Regions. In this talk I summarize the current ?coldness debate? about Precambrian climates, and describe new insights into microbial community structure and cold-survival strategies from our research on coastal ecosystems in the Arctic and Antarctica.


Supratim Sengupta
MCMASTER UNIVERSITY (Dept. of Physics & Astronomy)
Evolution of the Genetic Code
November 29, 2004    2:30pm    MDCL-1309
The genetic code is the set of assignments between the 64 possible codons in DNA and the 20 possible amino acids in proteins. The code is fundamental to the way all organisms work, since it controls the process of translation (i.e. protein synthesis). The Canonical Code evolved prior to the Last Universal Common Ancestor of all current life forms. However, many modified genetic codes are found in specific genomes in which one or more codons have been reassigned to a different amino acid from that in the canonical code. Codon reassignment should be a difficult and disruptive process for an organism to go through because of a variety of negative selective effects which will occur during the change-over period.
We present a new theory for codon reassignment that incorporates two previously proposed mechanisms (codon disappearance and ambiguous intermediate) and introduces two further mechanisms (unassigned codon and compensatory change). We present simulations showing that all four mechanisms can occur within our framework, depending on the parameters. We investigate the way the frequencies of the mechanisms are influenced by selection strengths, the number of codons undergoing reassignment, directional mutation pressure, and selection for reduced genome size.


Cliff Burgess
MCMASTER UNIVERSITY / PERIMETER INSTITUTE
What is the Universe Made of? The Case for Dark Energy and What it Might Be
November 22, 2004    2:30pm    MDCL-1309
For the first time in human thought it is now possible to observationally determine how much matter is in the Universe as a whole. These observations strongly support the "Concordance Model" of Hot Big Bang Cosmology, and reinforce earlier indications that ordinary matter (atoms, nuclei and electrons) make up at present at most 4% of the total of the Universal energy density. The big surprise was that the rest consists of *two* kinds of unknown forms of matter: the so-called Dark Matter and Dark Energy. This talk summarizes the various lines of evidence for their existence, why it is particularly hard to understand what the Dark Energy is. I will also briefly describe the current state of what I regard to be the best theoretical proposal for explaining the Dark Energy.


Dr. Ed Thommes
CANADIAN INSTITUTE FOR THEORETICAL ASTROPHYSICS, UNIVERSITY OF TORONTO
Chaos to order: Dynamical Shakeup During the Final Assemblage of the Terrestrial Planets
Monday November 8, 2004    2:45pm    MDCL-1309
One of the most persistent puzzles in the theory of planet formation also happens to be the one closest to home: The present-day orbits of the terrestrial planets are widely separated, and the orbits of Earth and Venus are nearly circular, features which current theories of late-stage planet formation have difficulty reproducing. We present a new model in which the final stage of terrestrial planet formation is induced by a dynamical shakeup during the dispersal of the proto stellar gas disk. In addition to being able to reproduce the orderly and stable architecture of the inner Solar System, this model provides an explanation for the mass depletion of the asteroid belt, the rapid formation of the Earth's core (as implied by recent cosmo chemical evidence), and the delivery of water-rich material to the Earth.


Tamara Davis
RESEARCH SCHOOL OF ASTRONOMY AND ASTROPHYSICS, AUSTRALIAN NATIONAL UNIVERSITY
Are We Alone? What life on our planet can tell us about life in the Universe
Wednesday October 27, 2004    1:30pm    BSB-108
Evidence shows that life on Earth began very quickly after Earth formed. We test the hypothesis that the rapid appearance of life on earth suggests that life in the universe is common.


Ofer Peleg
DIVERSITY CENTER, INSTITUTE OF EVOLUTION, UNIVERSITY OF HAIFA
Overlapping genes on the HIV genome
Monday October 4, 2004    2:30pm    ABB-165
DNA sequences are read in triplets of bases (codons). For this reason it is occasionally possible for the same piece of DNA to code for overlapping genes that are read in different reading frames. One of the most well known organisms possessing such a pattern of genome organization is the Human Immunodeficiency Virus type 1 (HIV-1). Our analysis of the genome of HIV-1 revealed novel functional RNA secondary structures in the overlapping region that are likely to have a biological function. It is usually thought that the overlapping of messages is a mechanism to increase the amount of information per unit length of DNA. I will present a model that suggests a direct evolutionary advantage of message overlapping in a genome and predicts under what conditions overlapping genes are likely to evolve.