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 |
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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 |
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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.
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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 |
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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.
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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 |
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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.
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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
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| Wednesday October 27, 2004 1:30pm BSB-108 |
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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.
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Ofer Peleg
DIVERSITY CENTER, INSTITUTE OF EVOLUTION, UNIVERSITY OF HAIFA
Overlapping genes on the HIV genome
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| Monday October 4, 2004 2:30pm ABB-165 |
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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. |
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