A sonic boom heard in California last week had an out-of-this world origin as ”a large meteoric event” according to NASA’s Meteoroid Environment Office. Scientists now estimate the blast measured in near 5 kilotons or roughly 1/3 the power of the atomic bomb dropped on Hiroshima, Japan during World War II.Bill Cooke of the Meteoroid Environment Office at NASA’s Marshall Space Flight Center, estimates the object was about the size of a minivan, weighed in at around 154,300 pounds.
“Most meteors you see in the night’s sky are the size of tiny stones or even grains of sand and their trail lasts all of a second or two,” said Don Yeomans of NASA’s Near-Earth Object Program Office at the Jet Propulsion Laboratory in Pasadena, Calif. “Fireballs you can see relatively easily in the daytime and are many times that size – anywhere from a baseball-sized object to something as big as a minivan.”
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Strange Cousins: Molecular Alternatives to DNA, RNA Offer New Insight Into Life’s Origins
ScienceDaily (Apr. 19, 2012) — Living systems owe their existence to a pair of information-carrying molecules: DNA and RNA. These fundamental chemical forms possess two features essential for life: they display heredity — meaning they can encode and pass on genetic information, and they can adapt over time, through processes of Darwinian evolution.
A long-debated question is whether heredity and evolution could be performed by molecules other than DNA and RNA.
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First Glimpse at the Viral Birth of DNA
EARLY life underwent a massive system upgrade around 4 billion years ago. DNA’s simple code can encrypt a huge amount of information and its trademark double helix makes it remarkably stable. But most biologists agree that life began with a soup of RNA, a less stable genetic molecule. So at some point the vast majority of life must have switched its code.
For the first time, biologists have had a glimpse at how this may have happened. The rare insight points to archaic viruses as the inventors of DNA. Better yet, the process that enabled the ancient upgrade occasionally still happens today.
According to the prevailing dogma, the earliest life forms arose from a loose mix of proteins and nucleic acids that used RNA as their genetic material. At some point, most of life began storing genetic information in DNA; all the cellular life we know today, and most modern viruses as well, are DNA-based. The switch created a problem familiar to anyone who has upgraded their laptop to a new operating system: how do you port over your old software to the new platform? The genes of RNA life contained solutions to many of the challenges of existence, but because RNA cannot combine with DNA there was no obvious way for the new DNA life to use this information.
The discovery of an unusual hybrid virus living in one of the harshest environments on the planet suggests a solution. Ken Stedman, of Portland State University in Oregon, stumbled on it by accident while studying the microbes that live in a hot, acidic lake in California’s Lassen Volcanic National Park. He filtered all the virus-sized particles from 40 litres of lake water, and randomly sequenced some 400,000 pieces of viral DNA to see what was there.
He found something odd: a gene, made of DNA, that looked like the gene for a protein coat from an RNA virus. Some viruses, called retroviruses, have a reverse transcriptase enzyme to translate RNA into DNA, but this gene did not come from a retrovirus. So how had the gene leapt from RNA to DNA?
Intrigued, Stedman’s student, Geoff Diemer, produced a full sequence of the strange virus’s genome. He found that alongside the RNA-derived gene it contained a gene for DNA replication typical of a DNA virus.
Finding these two genes in one organism was a bit like finding a sunflower gene in a chimpanzee, except that plants and animals probably share a much more recent common ancestor than DNA and RNA viruses, which are thought to have diverged billions of years ago. “Our first thought was that we messed up somehow,” says Stedman.
They re-sequenced the entire viral genome but the two genes were still there, Diemer reported this week at NASA’s Astrobiology Science conference in Atlanta, Georgia. The work will appear in Biology Direct.
To see whether this motley virus was just a one-off, Stedman and Diemer scanned databases of viral DNA sequences. They found that something very similar had turned up in samples of ocean water sequenced by a team led by Craig Venter, of the J Craig Venter Institute. “These hybrid viruses are present not just in this acidic hot lake, but in at least a couple of oceanic samples, and probably other places as well,” says Stedman.
The find proves that modern viruses can combine information coded in the two normally separate genetic molecules. And it lends support to the idea that it was viruses that performed the upgrade from RNA and effectively gave rise to DNA.
Stedman and Diemer’s hybrid virus is not a living fossil - a left-over that has stuck around since the dawn of life. Its genes are similar to their parent genes in RNA and DNA viruses, and the team estimates that it hybridised within the last 10 million years.
Stedman suggests that it may have formed when an RNA virus, DNA virus and retrovirus all infected a cell at the same time. This perfect viral storm could have triggered a three-way genetic mash-up (see diagram). The retrovirus used its reverse transcriptase enzymes to mistakenly make a DNA copy of an RNA virus gene, which combined with the DNA virus’s genome to yield the unlikely hybrid. A few earlier studies had hinted that such viral super-hybrids could exist, but Stedman’s study is the first to show it directly.
“These are two lineages that we never think of as overlapping,” says virologist Luis Villarreal of the University of California at Irvine. The lack of respect for species boundaries echoes what many biologists suspect the original virus world must have been like around the birth of DNA 4 billion years ago, he says.
The parallel with the ancient virus world is not perfect, since the modern viruses’ life cycles are very different from those of their ancestors. The primordial virus world was a non-cellular stage in the evolution of life, the details of which are very obscure, says Eugene Koonin, an evolutionary genomicist at the National Center for Biotechnology Information in Bethesda, Maryland. “Nowadays, viruses replicate exclusively within cells.”
Still, the finding proves that a community of viruses can move information from RNA into DNA - and that modern DNA viruses do have access to genes evolved by those in the very separate world of RNA viruses. This bolsters the argument that a similar transfer happened during early life’s RNA-to-DNA transition.
It also tells us that our modern world retains at least a trace of the uninhibited genetic free-for-all that must have preceded our current staid, cellular existence. Or as Koonin puts it: “The virus world, in its diversity and unpredictability, is still with us.”
Image Info: Hybrid viruses in this acid lake contained both RNA-derived genes and genes for DNA replication
Image Credit: Wendy White/Alamy
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Has Earth Seeded Life on Other Planets?
What’s the Latest Development?
New astronomical data suggest that life on Earth may have been carried to foreign planets as a result of ancient asteroid strikes. ”The idea is if a planet has life, like Earth, and if you hit it with an asteroid, it will create debris, some of which will escape into space,” said astronomer Mauri Valtonen of the University of Turku in Finland. Astronomers theorize that life would have been most easily carried to other planets some 4 billion years ago when our sun existed among thousands of nearby baby stars and their planets, which were just beginning to form.
What’s the Big Idea?
To search for foreign star systems where Earth’s brand of life might currently exist, Valtonen is now looking for stars that may have been born in the same nursery as our sun. So far, he has found two stars, which given their current trajectory and distance from the sun, may be its cousins. Were an asteroid strike to have thrown some Earthly amino acids into space, they could have been shielded in a piece of debris just one meter across for millions of years, until that debris landed someplace. “If it happens to land on a planet with suitable conditions,” said Valtonen, “life can start there.”
Image Credit: shutterstock.com
Lab experiments revealed that the precursor of life may have learned how to copy itself thanks to simple convection at the bottom of the ocean in tiny pores around undersea hydrothermal vents where magnesium-rich rocks react with sea water, creating a heat source that could drive miniature convection currents in nearby pores in the rock, according to research conducted in 2010 by Christof Mast and Dieter Braun of Ludwig Maximilian University of Munich, Germany. The team proposed that such convection could concentrate nucleotides, strands of DNA, and polymerase, providing a setting that would promote replication.
Sea water inside pores on or near a vent’s chimney may undergo thermal convection because the water at the wall of the pore closest to the vent’s heat source would be warmer than the water near the furthermost wall, said Mast and Braun. If the pore contained strands of DNA, nucleotides, and polymerase they would ride upward in the warm current.
The DNA strands would also be “unzipped” in the heat, splitting into two strands that each serve as templates for eventual replication. These components would then tend to shift away from the rising warmer region. In air, particles typically shift into a colder current because they are more likely to be pushed away by warmer, more energetic molecules than those on the cooler, calmer side. The researchers reckon a similar process would occur in the fluid in the vents.
Over time, the DNA templates, polymerase and nucleotides would collect at the bottom of a pore. Once there, they could become concentrated enough for the polymerase to bind new nucleotides to the single-strand DNA templates, replicating the original DNA.
To test this theory, Mast and Braun put these ingredients into tubes 1.5 millimetres long. They used a laser to heat one side of the water and create thermal convection. Sure enough, they found that the DNA doubled every 50 seconds (Physical Review Letters, vol 104, p 188102).
Fatty acids in the water may have provided a shuttle service, said Braun allowing replicated DNA to move between pores to recombine with new templates, producing a variety of configurations. Earlier, a team at Harvard University found that fatty acids driven by convection will form membranes that could trap the concentrated genetic material and transport it.
Life’s Building Blocks May Have Formed in Dust Around Young Sun
The organic molecules that were the building blocks for life on Earth could have formed in the dusty disk that surrounded our sun before the solar system had planets, a new computer model shows.
What’s more, the study suggests the process would be the same around other stars that acquired planets, which means some of those worlds, too, could be seeded with the pieces necessary for life.
Geophysicist Fred Ciesla and astrobiologist Scott Sandford showed in their computer model how the orbiting dust that provided the raw material for planets, asteroids and comets could have been exposed to the ultraviolet light needed to develop organic molecules.
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New Evidence That Comets Deposited Building Blocks of Life On Primordial Earth
ScienceDaily (Mar. 27, 2012) — New research reported in San Diego on March 27 at the 243rd National Meeting & Exposition of the American Chemical Society (ACS) provides further support for the idea that comets bombarding Earth billions of years ago carried and deposited the key ingredients for life to spring up on the planet.
Jennifer G. Blank, Ph.D., who led the research team, described experiments that recreated with powerful laboratory “guns” and computer models the conditions that existed inside comets when these celestial objects hit Earth’s atmosphere at almost 25,000 miles per hour and crashed down upon the surface. The research is part of a broader scientific effort to understand how amino acids and other ingredients for the first living things appeared on a planet that billions of years ago was barren and desolate. Amino acids make up proteins, which are the workhorses of all forms of life, ranging from microbes to people.
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Early Evolution of Life: Study of Ribosome Evolution Challenges ‘RNA World’ Hypothesis
ScienceDaily (Mar. 12, 2012) — In the beginning — of the ribosome, the cell’s protein-building workbench — there were ribonucleic acids, the molecules we call RNA that today perform a host of vital functions in cells. And according to a new analysis, even before the ribosome’s many working parts were recruited for protein synthesis, proteins also were on the scene and interacting with RNA. This finding challenges a long-held hypothesis about the early evolution of life.
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