Cosmology: It all adds up

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  • The embers of the Universes primor-dial fire continue to provide cosmolo-gists with data against which to testtheir ideas. And the most reassuring newsthis year has been that their favourite theo-ries seem up to the job.

    September saw the release of new results onthe cosmic microwave background (CMB),the blanket of microwave radiation that per-vades the Universe. The microwave photonsthat make up the CMB date from just 300,000years after the Big Bang. By analysing enoughof them, cosmologists can detect faint recordsof conditions in the youthful Universe.

    For 271 days over the past two years,researchers used the Degree Angular ScaleInterferometer (DASI),an observatory at theUS Amundsen-Scott South Pole Station, tostudy microwave photons coming from twosmall patches of the Antarctic winter sky.Theimages from each day were combined into asingle super-high-resolution picture, whichrevealed a slight polarization in the micro-wave background.

    The polarization was created by electronssurfing the waves of energy that sweptthrough the early Universe, says John Carl-strom, an astronomer at the University ofChicago and leader of the DASI project, who

    announced the news at the InternationalWorkshop on Particle Physics and the EarlyUniverse, held in Chicago. The results arepublished in this issue of Nature13. As elec-trons sped along the front of the wave, theyreflected photons in a preferential direction,causing the radiation in certain areas of theUniverse to become polarized. When theelectrons eventually fell into orbit aroundprotons, the radiation escaped, taking with ita record of the energy waves.

    The polarization data back up otherresults to come from studies of the CMB.During the early 1990s, temperature differ-ences in the CMB were detected4. Cosmolo-gists suspect the differences were caused byclumps of matter in the early Universe,whichseeded the growth of the web of galaxies wesee today. According to the leading theory,these clumps of matter were created by theenergy waves so detecting evidence of thewaves was important. The polarization is aunique signature,says Carlstrom.If it wasntthere, wed have to throw out this theory ofwaves, which means wed have to throw outall of our recent interpretations of the CMB.

    NASAs Microwave Anisotropy Probesatellite, launched in June 2001, will providea more detailed all-sky picture of the polar-

    Cosmology

    It all adds up

    derived from GM grain importedillegally from Argentina. Andunapproved GM cotton varietieshave reportedly been widely planted in India,hampering attempts to monitor theenvironmental and economic impact of theofficially sanctioned crops. nPeter Aldhous

    1. Quist, D. & Chapela, I. H. Nature 414,541543 (2001).

    2. Metz, M. & Ftterer, J. Nature 416, 600601 (2002).3. Kaplinsky, N. et al. Nature 416, 601602 (2002).4. Quist, D. & Chapela, I. H. Nature 416, 602 (2002).

    5. Nature 416, 600 (2002).6. Yu, J. et al. Science 296, 7992 (2002).7. Goff, S. A. et al. Science 296, 92100 (2002).8. Sasaki, T. et al. Nature 420, 312316 (2002).9. Feng, Q. et al. Nature 420, 316320 (2002).10.Surridge, C. Nature 416, 576578 (2002).

    Holding up a mirror tophysics world view

    Atoms from the mirror world of antimatter werecaptured and analysed for the first time this year.Two teams at CERN, the European Laboratory forParticle Physics near Geneva, have created largenumbers of long-lived antihydrogen atoms, whichcan be used to test fundamental theories aboutthe Universe.

    In total, the teams produced thousands ofantihydrogen atoms by using magnetic fields tobring together antiprotons and anti-electrons.This was a considerable technical achievement initself, but the real interest lies in studying theproperties of antihydrogen. Theory suggests thatit should mirror the properties of hydrogen, but no one knows for sure, says Rolf Landua,spokesman for the ATHENA collaboration, whichannounced its results in September1.

    The standard model of fundamental particlesand forces holds that hydrogen and antihydrogen should have the same propertiesand obey the same rules, but it cant predict whythe Universe is almost devoid of antimatter.Finding a difference between matter andantimatter could lead to an explanation, andperhaps force physicists to reformulate thestandard model.

    A second CERN team, called ATRAP, has alsocaptured antihydrogen atoms2, and hassubsequently made preliminary measurements oftheir most excited energy states3. Theres still alot of work to do, says Gerald Gabrielse ofHarvard University, spokesman for ATRAP.

    This is just the start, agrees Landua.Everything is fresh; we havent exploited all ofour potential. Geoff Brumfiel

    1. Amoretti, M. et al. Nature 419, 456459 (2002).

    2. Gabrielse, G. et al. Phys. Rev. Lett. 89, 213401 (2002).

    3. Gabrielse, G. et al. Phys. Rev. Lett. 89, 233401 (2002).

    Highlight: Antihydrogen

    2002 in context

    NATURE | VOL 420 | 19/26 DECEMBER 2002 | www.nature.com/nature 731

    From the SouthPole (left) camemeasurements ofcosmic microwavesthat confirmedcosmologysleading theories,while at CERNnear Geneva(right), two teamscaptured atoms ofantihydrogen.

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  • ization sometime early next year. But thepolarization may retain a secret. The energywaves are thought to have been triggered by arapid enlargement of the early Universe,known as inflation. This would also have created gravity waves, which would lead totiny whirlpools in the CMB polarization.Detecting these whirlpools would providethe best evidence yet for inflation.Measuring

    such vortices will require more sensitiveinstruments, however. No one knows forsure exactly how to do it yet, says Lyman Page, a cosmologist at Princeton Universityin New Jersey. nGeoff Brumfiel1. Leitch, E. M. et al. Nature 420, 763771 (2002).2. Kovac, J. M. et al. Nature 420, 772787 (2002).3. Zaldarriaga, M. Nature 420, 747748 (2002).4. Smoot, G. et al. Astrophys. J. Lett. 396, L1L4 (1992).

    They can be thought of as biologysdark matter: tiny RNAs that dontencode any protein. But 2002 has seenan avalanche of discoveries about their rolesin influencing gene activity lending somecredence to the radical idea that small RNAshover above the genome, providing a matrixof regulatory control.

    MicroRNAs (miRNAs) are about 22nucleotides long, and were first identified inthe nematode Caenorhabditis elegans1,2. InC. elegans, they regulate the activity of specificgenes by binding to messenger RNAs (mRNAs)and preventing their translation to proteins.

    Until recently, many experts thought thatsuch examples were interesting anomalies.But they are now realizing that miRNAs areinvolved in gene regulation in a wide varietyof organisms, and researchers are findinglinks between miRNAs and the phenomenonof RNA interference (RNAi). The lattermechanism, which is exploited by biologistsfor gene-silencing studies, is thought to be anatural defence against invading viruses. Ituses an enzyme called Dicer to cut double-stranded viral RNA into small interferingRNAs (siRNAs), again about 22 nucleotideslong. These then bind to other viral RNA,

    targeting it for destruction.Dicer also creates miRNAs, by cutting

    them from longer, hairpin-shaped RNAstranscribed from the genome, but that waswhere the similarity was thought to stop. InAugust, however, it emerged that miRNAscan also function in the RNAi pathway andcause their target mRNAs to be degraded, ifthey perfectly match its target sequence3.

    Speculation was mounting that miRNAsrepresented an unexplored layer of generegulation but, without knowing the identityof their targets, their role remained unclear.This again changed in the summer, when themRNA targets of miRNAs were identified inthe weed Arabidopsis thaliana most ofthose studied seem to be involved in earlyplant development4,5. This was remarkableprogress, given that miRNAs were onlyidentified in plants a few months earlier6,7.

    At the same time, new functions forsmall RNAs were being found. Tiny RNAsnow appear to do a whole lot more than justtarget mRNAs. They seem to be associatedwith various epigenetic phenomena theinheritance of features that do not involvegenetic sequence changes. For instance, astartling connection has been made between

    small RNAs and the silencing of geneactivity in tightly packed regions of thegenome: deleting genes that encodecomponents of the RNAi pathway led to aloss of gene silencing in the fission yeastSchizosaccharomyces pombe8,9.

    Small RNAs also seem to be capable ofreshaping the genome. The ciliated protozoanTetrahymena thermophila possesses twonuclei, the larger of which loses roughly 15%of its DNA during the cells development and this seems to be guided by small RNAs10.

    Finally, some researchers are taking a leadfrom the presumed natural function of RNAiby exploring the use of small RNAs as anti-viral agents. siRNAs targeted at viral genescan inhibit the replication of HIV-1 or polio-virus in cultured cells1113. Whats more,siRNAs targeted at the host receptors used byHIV to enter the cell can also block infection13.

    Of course, results in cultured cells dontalways translate to the clinic. The challengewill be in the delivery, says Andy Fire of theCarnegie Institution of Washington inBaltimore, co-discoverer of RNAi in C. elegans.But there are early signs that siRNAs targetedat human hepatitis C virus can function wheninjected into mice14. These small nucleotidestrings could yet be big news for medicine. nCarina Dennis1. Lee, R. C., Feinbaum, R. L. & Ambros, V. Cell 75, 843854 (1993).2. Reinhart, B. J. et al. Nature 403, 901906 (2000).3. Hutvgner, G. & Zamore, P. A. Science 297, 20562060 (2002).4. Llave, C., Xie, Z., Kasschau, K. D. & Carrington, J. C. Science

    297, 20532056 (2002).5. Rhoades, M. W. et al. Cell 110, 513520 (2002).6. Llave, C., Kasschau, K. D., Rector, M. A. & Carrington, J. C.

    Plant Cell 14, 16051619 (2002).7. Reinhart, B. J., Weinstein, E. G., Rhoades, M. W., Bartel, B. &

    Bartel, D. P. Genes Dev. 16, 16161626 (2002).8. Volpe, T. A. et al. Science 297, 18331837 (2002).9. Hall, I. M. et al. Science 297, 22322237 (2002).10.Mochizuki, K., Fine, N. A., Fujisawa, T. & Gorovsky, M. A. Cell

    110, 689699 (2002).11.Gitlin, L., Karelsky, S. & Andino, R. Nature 418, 430434 (2002).12. Jacque, J-M., Triques, K. & Stevenson, M. Nature 418,

    435438 (2002).13.Novina, C. D. et al. Nature Med. 8, 681686 (2002).14.McCaffrey, A. P. et al. Nature 418, 3839 (2002).

    2002 in context

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    Small RNAs

    The genomes guiding hand?

    Worming out thetruth: microRNAs,which were firstidentified in thenematodeCaenorhabditiselegans (far left),were this yearfound to have anunexpectedlyextensive role ingene regulation.

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