Genética

Genética

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relatively greater numbers ofsmall bodies7 than expected.

Funato and colleagues’theory1combines aspects ofboth ofthese mechanisms6,7.They suggest that objects ofdisparate masses initially form a close binary system through gravitational instability or through collisions.Then the pair encounters another KBO that typically has a mass similar to that ofthe larger ofthe companions (Fig.1c). Following an elaborate gravitational dance, the two massive objects pair up and the small object is ejected from the system.This clever idea produces binaries whose attributes match those ofthe observed binary population ofthe Kuiper belt.

But caution should be the watchword when theoreticians interpret observational data before developing formation models. To begin with,binary KBOs must have been more numerous in the past8:there is no known mechanism that is actively creating binary systems at present,and yet binaries can be eliminated whenever energetic impacts shatter satellites and disperse the fragments. Furthermore, less catast rophic collisions and close flybys ofother KBOs might loosen the orbital embrace ofpairs. These processes modify the character ofany surviving pairs because they preferentially affect those binaries that contain small companions or whose orbits are weakly bound.

Moreover,because the Kuiper belt is so distant and its denizens are so small,selection effects can dramatically influence the frequency ofdiscoveries and mislead us about the nature ofthe pairs.Typical KBOs lie near the observational limit ofmany telescopes, and close binaries would not be resolved as two individual objects.Hence,it is no surprise that most ofthe binaries discovered are widely separated pairs ofequally bright objects.

The binary population ofthe Kuiper belt had been estimated at 1–2% ofthe total3,but this is likely to be a severe underestimate. Two systematic searches using data from the Hubble Space Telescope,which can detect faint,close companions more efficiently than can most ground-based telescopes, have revised that estimate upwards:one2 finds that (4 2)% ofKBOs are binaries,the other (7 5)% (ref.9).The latter study has been combined with a search10using adaptive optics on the world’s largest telescope, Keck I,in Hawaii.The conclusion reached is that about 10% ofKBOs are binary,but that there are no wide binaries with faint companions.These are probably still lower limits,as some closely bound or very faint objects would have gone undetected.

The orbits ofthis increasing collection of binaries must now be determined.This information will be crucial in working out the likely origins ofthese objects,and in confirming the degree to which the mechanism proposed by Funato et al.1is part of the picture.■ Joseph A.Burns is in the Departments ofTheoretical and Applied Mechanics,and ofAstronomy, Cornell University,Ithaca,New York 14853,USA. e-mail: jab16@cornell.edu

Nature 427, 518–520 (2004). 2.Noll,K.Earth Moon Planets(in the press). 3.Merline,W.J.et al.in Asteroids I(eds Bottke,W.F.,Cellino,A.,

Paolicchi,P.& Binzel,R.P.) 289–312 (Univ.Arizona Press, Tucson, 2003). 4. Michel, P., Benz, W., Tanga, P. & Richardson, D. C. Science 294, 1696–1699 (2001). 5. Stern, S. A. Astron. J. 124, 2300–2304 (2002). 6. Weidenschilling, S. J. Icarus 160, 212–215 (2002). 7. Goldreich, P., Lithwick,Y. & Sari, R. Nature 420, 643–646 (2002). 8.Petit,J.-M.& Mousis,O.Icarus(in the press);preprint at http://arxiv.org/abs/astro-ph/0305156 (2003). 9.Trujillo,C.A.& Brown,M.E.IAU Circ.No.7787 (2002). 10.Schaller, E.L.& Brown,M.E.Bull.Am.Astron.Soc.35, 993 (2003).

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Figure 1Doubling up.a,A binary can form through the collision oftwo objects.In the resulting system,one partner tends to be much smaller than the other and the bodies are relatively close to each other.In the Kuiper belt,however,binary partners are usually similar in size and farther apart. b,Alternatively,a binary may form through gravitational capture as one object passes another,with background masses tugging at the pair.In this case,partners ofany relative size can link up,and again the system is unlike a Kuiper-belt binary.c,Funato et al.propose a mechanism that combines elements ofaand b.They consider an existing ‘small-partner’binary,which may have formed through collision,disrupted by the subsequent gravitational capture ofa larger object.The original small partner is ejected,leaving a widely separated system oftwo larger objects — exactly the characteristics ofthe known Kuiper-belt binaries.

b c

Cell division

Guardian spirit blesses meiosis Robin Allshire

During egg and sperm production, the two copies of a duplicated chromosome must be bound together until it is time for their separation. A protein that protects this chromosomal glue has now been discovered.

It is estimated that about 20% ofhuman eggs have an abnormal number ofchromosomes,and it is well known that the incidence offetuses with three copies of some chromosomes — rather than the usual two — increases markedly with the age ofthe mother1,2.The fact that human eggs can arrest in the early stages oftheir creation for up to 45 years is clearly a factor. For this entire period,to prevent abnormal chromosome numbers,the two copies ofa duplicated chromosome (‘sister chromatids’) need to remain tethered to each other,and the chromosome needs to be

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© 2004Nature PublishingGroup connected to its opposite number,until they are told to separate.On page 510 of this issue,Kitajima et al.3reveal how the molecular tether between sister chromatids is kept in place.

There are two ways in which cells can multiply:mitosis and meiosis.Mitosis occurs in all the body’s tissues,and generates more-or-less identical daughter cells;it serves the purpose of,for example,replacing cells lost through general wear and tear. Meiosis occurs only in reproductive tissue and generates reproductive cells.

The starting point for both mitosis and meiosis is a cell with two copies (homologues) ofeach chromosome,one from the mother and one from the father.These chromosomes are duplicated,producing homologous pairs ofsister chromatids.During mitosis (Fig.1),the two sister chromatids ofeach pair are pulled apart to opposite poles ofthe cell,and the cell splits into two. So,each daughter cell again has two copies ofeach chromosome.

Meiosis,by contrast,is divided into two stages (Fig.2).In meiosisI,the two sister chromatids ofa pair are held together,and the maternal pair is separated from the paternal pair.In meiosis I,the sister chromatids are finally separated.The result is four cells,each with halfthe usual number of chromosomes.The cell’s genetic complement is restored when it meets a complementary reproductive cell.

During the early stages ofmeiosisI,the homologous pairs ofsister chromatids are held together along their arms;the two sister chromatids ofa pair are also glued together at specialized regions called centromeres. This bonding is achieved in part by cohesin,a complex ofcohesive proteins.It is well established that cohesion along the arms must be different from cohesion at centromeres, because later in meiosisI (at the onset of so-called anaphaseI),arm cohesion is released but centromere cohesion remains intact,allowing homologous pairs to travel to opposite poles4,5.Whatever is responsible for this difference must be lost by anaphase I ofmeiosis I,when sister chromatids,in turn,separate4.

Chromosome segregation at anaphases I and I involves two regulated bursts ofa specific pr otein-degrading ac tivity, called separase.This cleaves one subunit (Rec8) of cohesin,first at the arms and later at the centromeres5.But why is Rec8 cleaved only at the arms during anaphase I? How is the Rec8 at centromeres maintained? This is the problem that Kitajima et al.3address,using fission yeast, Schizosaccharomyces pombe,a s their organism ofchoice.

It was known5–7that Rec8 is normally expressed only during meiosis,when it takes over from its mitotic counterpart,Rad21. But when mitotic cells are manipulated, Rec8 can largely replace Rad21 (ref.6),hinting that the expression ofRec8 is not,by itself,enough to impose the properties of meiosisI on a normal mitotic division.So it was suggested4,5,8that perhaps the difference is that there is a factor that protects centromeric Rec8 during meiosisI.For various reasons,a good candidate for this factor was the fruitfly Mei-S332 protein8.But this molecule fell out offavour,mainly because ofa lack ofapparent counterparts in other organisms.

Kitajima et al.3reasoned that ifa protector exists in the form ofa single protein,then it might usually be expressed only during meiosis,and,ifforcibly expressed with Rec8 in mitotic cells,it might protect cohesion at centromeres.This would kill the cells, because sister chromatids would be unable news and views

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Figure 2Meiosis.In meiosisI the centromeres ofa pair ofsister chromatids work in unison so that they face,and attach to,the same spindle pole.A cohesin complex containing the Rec8 protein holds sister chromatids together at centromeres and along chromosome arms.AnaphaseI is triggered by cleavage ofarm Rec8 by separase,allowing the homologous pairs to disengage and move to opposite poles.Katajima et al.have found that Sgo1 protects Rec8 at centromeres during anaphaseI,so that sister chromatids remain tethered.Sgo1 is degraded after anaphaseI (telophase I),so that,at metaphase I,centromeric Rec8 is no longer protected from separase.Cleavage ofthis centromeric Rec8 at anaphase I allows sister chromatids to separate.

Figure 1Mitosis.After chromosomes are duplicated,producing pairs ofsister chromatids,the chromatids are kept together by a cohesin complex containing the Rad21 protein.When the centromeres attach to the spindle during metaphase,pulling forces generate tension,which activates the separase enzyme.This cleaves Rad21,resulting in the release ofcohesion and allowing sister chromatids to move to opposite spindle poles during anaphase.

Metaphase Anaphase

Separase cleaves Rad21

Cohesin, containing Rad21

Spindle

Sister chromatid Centromere

Mitosis

Separase cleaves Rec8 on armsDegradation of Sgo1

Meiosis I

Meiosis I

Metaphase IAnaphase I

Separase cleaves Rec8 at centromeres

Metaphase IAnaphase ITelophase I

Cohesin, containing Rec8

Sgo1

Sgo-protected Rec8 cohesin

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to separate efficiently.So the authors screened for genes that were lethal only when expressed with Rec8 in mitotic fission yeast.They isolated one such gene,which is usually expressed only in meiosis, and named its encoded protein Sgo1, short for ‘Sugoshin’— ‘guardian spirit’ in Japanese.

Katajima et al.found that expressing Sgo1 with Rec8 in mitotic cells allowed Rec8 to persist at mitotic centromeres.Moreover, Sgo1 associates with centromeres in early meiosis and remains there until anaphaseI, as expected (Fig.2).Deleting the gene results in normal chromosome segregation in meiosisI but random segregation ofsister chromatids in meiosis I,because ofpremature loss ofcohesion at anaphaseI (because all Rec8 is now cleaved).The authors further show that Sgo1 and Rec8 associate in the same complex.These data are consistent with a role for Sgo1 in protecting centromeric Rec8 from separase at the onset of anaphaseI.

How is Sgo1 recruited to centromeres?

Here,too,Kitajima et al.suggest an answer. During both meiosis and mitosis,a ‘checkpoint’monitors the attachment ofcentromeres to the spindle — the apparatus that physically separates chromosomes.This checkpoint also monitors the physical tension between sister centromeres.By doing so it ensures that sister chromatids make the correct attachments and achieve the required arrangement on the spindle9. Previous analyses offission yeast10had shown that one checkpoint component, Bub1,is required to maintain Rec8 at centromeres after anaphaseI.

Kitajima et al.show that Bub1 is,in fact, required to recruit Sgo1 to centromeres,suggesting a connection between the protection ofRec8 and the spindle checkpoint.One news and views

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