Monday, November 1, 2010

ENERGY REVOLUTION KEY TO COMPLEX LIFE

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The evolution of complex life is strictly dependent on mitochondria, the tiny power stations found in all complex cells, according to a new study by Dr Nick Lane, from UCL (University College London), and Dr William Martin, from the University of Dusseldorf.

"The underlying principles are universal. Energy is vital, even in the realm of evolutionary inventions," said Dr Lane, UCL Department of Genetics, Evolution and Environment. "Even aliens will need mitochondria."

For 70 years scientists have reasoned that evolution of nucleus was the key to complex life. Now, in work published today in Nature, Lane and Martin reveal that in fact mitochondria were fundamental to the development of complex innovations like the nucleus because of their function as power stations in the cell.

"This overturns the traditional view that the jump to complex 'eukaryotic' cells simply required the right kinds of mutations. It actually required a kind of industrial revolution in terms of energy production," explained Dr Lane.

At the level of our cells, humans have far more in common with mushrooms, magnolias and marigolds than we do with bacteria. The reason is that complex cells like those of plants, animals and fungi have specialized compartments including an information centre, the nucleus, and power stations – mitochondria. These compartmentalised cells are called 'eukaryotic', and they all share a common ancestor that arose just once in four billion years of evolution.

Scientists now know that this common ancestor, 'the first eukaryote', was a lot more sophisticated than any known bacterium. It had thousands more genes and proteins than any bacterium, despite sharing other features, like the genetic code. But what enabled eukaryotes to accumulate all these extra genes and proteins? And why don't bacteria bother?

By focusing on the energy available per gene, Lane and Martin showed that an average eukaryotic cell can support an astonishing 200,000 times more genes than bacteria.

"This gives eukaryotes the genetic raw material that enables them to accumulate new genes, big gene families and regulatory systems on a scale that is totally unaffordable to bacteria," said Dr Lane. "It's the basis of complexity, even if it's not always used."

"Bacteria are at the bottom of a deep chasm in the energy landscape, and they never found a way out," explained Dr Martin. "Mitochondria give eukaryotes four or five orders of magnitude more energy per gene, and that enabled them to tunnel straight through the walls of the chasm."

The authors went on to address a second question: why can't bacteria just compartmentalise themselves to gain all the advantages of having mitochondria? They often made a start but never got very far.

The answer lies in the tiny mitochondrial genome. These genes are needed for cell respiration, and without them eukaryotic cells die. If cells get bigger and more energetic, they need more copies of these mitochondrial genes to stay alive.

Bacteria face exactly the same problem. They can deal with it by making thousands of copies of their entire genome – as many as 600,000 copies in the case of giant bacterial cells like Epulopiscium, an extreme case that lives only in the unusual guts of surgeonfish. But all this DNA has a big energetic cost that cripples even giant bacteria – stopping them from turning into more complex eukaryotes. "The only way out", said Dr Lane, "is if one cell somehow gets inside another one – an endosymbiosis."

Cells compete among themselves. When living inside other cells they tend to cut corners, relying on their host cell wherever possible. Over evolutionary time, they lose unnecessary genes and become streamlined, ultimately leaving them with a tiny fraction of the genes they started out with: only the ones they really need.

The key to complexity is that these few remaining genes weigh almost nothing. Calculate the energy needed to support a normal bacterial genome in thousands of copies and the cost is prohibitive. Do it for the tiny mitochondrial genome and the cost is easily affordable, as shown in the Nature paper. The difference is the amount of DNA that could be supported in the nucleus, not as repetitive copies of the same old genes, but as the raw material for new evolution.

"If evolution works like a tinkerer, evolution with mitochondria works like a corps of engineers," said Dr Martin.

The trouble is that, while cells within cells are common in eukaryotes, which often engulf other cells, they're vanishingly rare in more rigid bacteria. And that, Lane and Martin conclude, may well explain why complex life – eukaryotes – only evolved once in all of Earth's history.

(Photo: UCL)

University College London

NEW SPACE RESEARCH SETTLES YEARS OF SCIENTIFIC DEBATE

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New space research published in the journal Nature, has settled decades of scientific debate. Researchers from the University of California (UCLA) and British Antarctic Survey (BAS) have found the final link between electrons trapped in space and the glow of light from the upper atmosphere known as the diffuse aurora. The research will help us understand 'space weather', with benefits for the satellite, power grid and aviation industries, and how space storms affect the Earth's atmosphere from the top down.

Scientists have long understood that the 'diffuse aurora' is caused by electrons striking the upper atmosphere. However, the electrons are normally trapped much higher up in the Earth's magnetic field through a long chain of events starting with the Sun. The problem is to understand how these electrons reach the atmosphere.

Since the 1970s scientists have debated whether very low frequency (VLF) radio waves could scatter the trapped electrons into the atmosphere. Two types of VLF waves were identified in space as the possible cause of the 'diffuse aurora', but despite years of argument and research no conclusive result had been possible. The new research shows, without doubt, that VLF waves known as 'chorus' are responsible; so-called since the signals detected by ground-based recording equipment sound like the bird's dawn chorus when played back through a loud speaker.

Through detailed analysis of satellite data the authors were able to calculate the effects on the trapped electrons and identify which radio waves were causing the scattering.

Lead author Professor Richard Thorne from UCLA says, "The breakthrough came when we realised that the electrons being lost from space to the Earth's atmosphere were leaving a signature, effectively telling a story about how they were being scattered. We could then analyse our satellite data on the two types of VLF waves and by running calculations on them – including the rate at which the electrons were being lost to the Earth's atmosphere – we could clearly see that chorus waves were the cause of the scattering."

Professor Richard Horne from British Antarctic Survey says, "Our finding is an important one because it will help scientists to understand how the diffuse aurora leads to changes in the chemistry of the Earth's upper atmosphere, including effects on ozone at high altitude, which may affect temperature right through the atmosphere.

"We are also including the VLF waves into computer models to help predict 'space weather' which not only affects satellites and power grids, but also the accuracy of GPS navigation and high frequency radio communications with aircraft on polar routes."

The 'diffuse aurora', is not the same as the 'discrete aurora' known as the northern and southern lights. 'Discrete aurora' look like fiery moving curtains of colourful light and can be seen by the naked eye, whereas the diffuse aurora is much fainter but more extensive. The 'diffuse aurora', which typically accounts for three-quarters of the energy input into the upper atmosphere at night, varies according to the season and the 11 year solar cycle.

(Photo: BAS)

British Antarctic Survey

SEE NO SHAPE, TOUCH NO SHAPE, HEAR A SHAPE?

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Scientists at The Montreal Neurological Institute and Hospital – The Neuro, McGill University have discovered that our brains have the ability to determine the shape of an object simply by processing specially-coded sounds, without any visual or tactile input. Not only does this new research tell us about the plasticity of the brain and how it perceives the world around us, it also provides important new possibilities for aiding those who are blind or with impaired vision.

Shape is an inherent property of objects existing in both vision and touch but not sound. Researchers at The Neuro posed the question 'can shape be represented by sound artificially?' "The fact that a property of sound such as frequency can be used to convey shape information suggests that as long as the spatial relation is coded in a systematic way, shape can be preserved and made accessible - even if the medium via which space is coded is not spatial in its physical nature," says Jung-Kyong Kim, PhD student in Dr. Robert Zatorre's lab at The Neuro and lead investigator in the study.

In other words, similar to our ocean-dwelling dolphin cousins who use echolocation to explore their surroundings, our brains can be trained to recognize shapes represented by sound and the hope is that those with impaired vision could be trained to use this as a tool. In the study, blindfolded sighted participants were trained to recognize tactile spatial information using sounds mapped from abstract shapes. Following training, the individuals were able to match auditory input to tactually discerned shapes and showed generalization to new auditory-tactile or sound-touch pairings.

"We live in a world where we perceive objects using information available from multiple sensory inputs," says Dr. Zatorre, neuroscientist at The Neuro and co-director of the International Laboratory for Brain Music and Sound Research. "On one hand, this organization leads to unique sense-specific percepts, such as colour in vision or pitch in hearing. On the other hand our perceptual system can integrate information present across different senses and generate a unified representation of an object. We can perceive a multisensory object as a single entity because we can detect equivalent attributes or patterns across different senses." Neuroimaging studies have identified brain areas that integrate information coming from different senses – combining input from across the senses to create a complete and comprehensive picture.

The results from The Neuro study strengthen the hypothesis that our perception of a coherent object or event ultimately occurs at an abstract level beyond the sensory input modes in which it is presented. This research provides important new insight into how our brains process the world as well as new possibilities for those with impaired senses.

McGill University

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