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遗传代谢,孰先孰后

已有 3480 次阅读 2010-1-12 15:30 |个人分类:生命起源|系统分类:科普集锦

生命体的两个最基本特征是遗传和代谢,广义上当然还包括生长发育和应激反应。在生命的起源和进化过程中,二者究竟孰先孰后,孰轻孰重,倒是有一番争议和一场拉锯战。

NASA将生命定义为适用于Darwinian进化的自持化学系统,即无论是遗传还是代谢,都必须具有一定分子特征的物质基础。

DNARNA遗传物质确认之前,人们更倾向于代谢first”,甚至认为参与代谢的蛋白就是遗传物质。其中,前苏联学者Oparin曾提出了团聚体假说,他将蛋白质、多肽、核酸和多糖等放在合适的溶液中,发现它们能自动地浓缩聚集为分散的球状小滴,这些小滴就是团聚体。奥巴林认为,团聚体可以表现出合成、分解、生长、生殖等生命现象。例如,团聚体具有类似于膜的边界,其内部的化学事件及特征有别于外部的溶液环境。团聚体能从外部溶液中吸入某些分子作为反应物,还能在酶的催化作用下发生特定的生化反应,反应的产物也能从团聚体中释放出去。

然而,随着中心法则的确立,人们对作为遗传物质的DNARNA更加着迷,甚至极端的有人认为基因决定了生命的一切,可以解释一切。遗传才是王道。

可是,抛开DNA合成过程中核糖与碱基的结合、磷酯键的形成、链延伸的热力学和动力学障碍及其自身的稳定性不说,DNA的复制本就是一个异常复杂的、需要辅助(酶)的准确的单体聚合的精细活。这些在前生无条件下能否实现是个问题。近些年的研究表明,许多被称为"compound genomes" “composomes“化合物都具有存储信息、复制并遗传给子代的特性,有些甚至能够自催化(合成)和自组装。如果以这些具有一定代谢特征的分子中的一类或几类为模板,在后期实现生物分子,如DNA,的合成、复制和遗传也不失为一种选择。这样看来,遗传并不局限于DNARNA,基于DNARNA的遗传并不是不可或缺的。

但是,最近匈牙利的Vera Vasas等运用模拟和网络近似分析,认为上述composomes在演变至一个临界大小时会发生分解,不满足Darwinian进化特点,从而再次驳斥了代谢first”学说。

看来掐架还得继续,不过最终会不会也掐出个“二象性”的结论呢?

 

Vera Vasasa, Eörs Szathmáry and Mauro Santosa.

Lack of evolvability in self-sustaining autocatalytic networks: A constraint on the metabolism-first path to the origin of life.

PNAS, January 4, 2010 DOI: 10.1073/pnas.0912628107

 

下面是sciencedaily上的网评:

A new study published in Proceedings of National Academy of Sciences rejects the theory that the origin of life stems from a system of self-catalytic molecules capable of experiencing Darwinian evolution without the need of RNA or DNA and their replication.

 

The research, which was carried out with the participation of Mauro Santos, researcher of the Department of Genetics and Microbiology at Universitat Autònoma de Barcelona (UAB), has demonstrated that, through the analysis of what some researchers name "compound genomes," these chemical networks cannot be considered evolutionary units because they lose properties which are essential for evolution when they reach a critical size and greater level of complexity.

The U.S. National Aeronautics and Space Administration (NASA) defines life as a "self-sustaining chemical system capable of Darwinian evolution." The scientific theories on the origin of life revolve around two main ideas: one focuses on genetics -- with RNA or DNA replication as an essential condition for Darwinian evolution to take place -- and the other focuses on metabolism. It is clear that both situations must have begun with simple organic molecules formed by prebiotic processes, as was demonstrated by the Miller-Urey experiment (in which organic molecules were created from inorganic substances). The point in which these two theories differ is that the replication of RNA or DNA molecules is a far too complex process which requires a correct combination of monomers within the polymers to produce a molecular chain resulting from the replication.

Until now no plausible chemical explanation exists for how these processes occured. In addition, defenders of the second theory argue that the processes needed for evolution to take place depend on primordial metabolism. This metabolism is believed to be a type of chemical network entailing a high degree of mutual catalysis between its components which, in turn, eventually allows for adaptation and evolution without any molecular replication.

In the first half of the 20th century, Alexander Oparin established the "Metabolism First" hypothesis to explain the origin of life, thus strengthening the primary role of cells as small drops of coacervates (evolutionary precursors of the first prokaryote cells). Dr Oparin did not refer to RNA or DNA molecules since at that time it was not clear just how important the role of these molecules was in living organisms. However he did form a solid base for the idea of self-replication as a collective property of molecular compounds.

Science more recently demonstrated that sets of chemical components store information about their composition which can be duplicated and transmitted to their descendents. This has led to their being named "compound genomes" or composomes. In other words, heredity does not require information in order to be stored in RNA or DNA molecules. These "compound genomes" apparently fulfil the conditions required to be considered evolutionary units, which suggests a pathway from pre-Darwinian dynamics to a minimum protocell.

Researchers in this study nevertheless reveal that these systems are incapable of undergoing a Darwinian evolution. For the first time a rigorous analysis was carried out to study the supposed evolution of these molecular networks using a combination of numerical and analytical simulations and network analysis approximations. Their research demonstrated that the dynamics of molecular compound populations which divide after having reached a critical size do not evolve, since during this process the compounds lose properties which are essential for Darwinian evolution.

Researchers concluded that this fundamental limitation of "compound genomes" should lead to caution towards theories that set metabolism first as the origin as life, even though former metabolic systems could have offered a stable habitat in which primitive polymers such as RNA could have evolved.

Researchers state that different prebiotic Earth scenarios can be considered. However, the basic property of life as a system capable of undergoing Darwinian evolution began when genetic information was finally stored and transmitted such as occurs in nucleotide polymers (RNA and DNA).

 



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