El origen de la celula

El Origen de la Célula
[pic][pic][pic][pic][pic][pic]¿Cómo se originaron las células?
Uno de los rompecabezas más complicados del origen de la vida es cómo se formaron las primeras células y su metabolismo.
Pudiera pensarse que las primeras células fueran como los organismos más pequeños y simples que viven hoy en día, los microbios conocidos como micoplasmas.
Las células de los micoplasmasson realmente diminutas, más de mil millones de veces menores que un protozoo, y albergan tan sólo una fracción del ADN y de las proteínas normalmente presentes en una célula.
Pero todos los micoplasmas son parásitos, versiones menores y simplificadas de microorganismos mayores de vida libre, y sólo pueden crecer y reproducirse en el interior de otras células, por lo general de mamíferos, un modode vida claramente imposible para las primeras formas de vida.
Las bacterias comunes ofrecen un modelo alternativo, pero los microbios de vida libre son demasiado complejos, compuestos de cientos de polímeros diferentes (entre ellos, unos quinientos tipos de ARN), más de un millar de enzimas y decenas de miles de moléculas.
Las primeras células debieron ser considerablemente más simples.
Paraconocer cómo eran las primeras células es necesario levantar el velo evolutivo que separa la vida actual de sus principios.
Esta tarea apenas ha comenzado.
Los avances en la comprensión de la historia temprana de la vida sólo se producen progresivamente, así que hasta que no se disponga de un modelo del origen de los monómeros y los polímeros parecería ocioso atacar esta compleja cuestión.
Peroaunque apenas se conozca nada con certeza acerca del origen de la vida celular, sí está claro que ocurrió según una secuencia de tres estadios:
1. Es probable que ya existieran sistemas químicos que la mayoría de científicos describirían como vivos antes de que fueran empaquetados en células. La evidencia fósil de este estadio precelular pudiera no llegar a descubrirse nunca, puesto que sucomposición química sería demasiado frágil para conservarse, pero se cree que durante un cierto periodo la Tierra primitiva estuvo poblada por moléculas vivas, «genes desnudos primordiales» de ácidos nucleicos capaces de autorreproducirse.
2. A causa de su composición química, estos y otros compuestos orgánicos del caldo primordial habrían acabado por agregarse en diminutos lóbulosprotocelulares, ancestros potenciales de células con pared celular. Las pruebas empíricas de este estadio parecen haberse evadido también de la historia, si bien es posible que algún día se hallen células fósiles más antiguas que las más antiguas conocidas en la actualidad.
3. Las primeras células estarían impulsadas por una forma sencilla de metabolismo que estableció las bases para la evolución posteriorde una maquinaria metabólica más compleja.
Consideremos cada uno de estos estadios por separado.
El problema de la gallina y el huevo
La vida puede multiplicarse solamente si la información química almacenada en los ácidos nucleicos de los cromosomas se copia y se transmite a la descendencia. La mayor parte de esta información dirige la síntesis de proteínas enzimáticas, entre las cuales seencuentra un tipo especial (las polimerasas) que se necesitan para copiar los propios ácidos nucleicos.
Así pues, ¿qué fue primero, los ácidos nucleicos necesarios para sintetizar las polimerasas, o los enzimas necesarios para sintetizar los ácidos?
Durante muchos años, los intentos por resolver este problema, que recuerda al de la gallina y el huevo, se centraron en el ADN y el papel que desempeñaen la fabricación de proteínas enzimáticas. Pero como el propio ADN es un producto de la evolución, una versión joven y más avanzada de un ácido ribonucleico antecesor más primitivo, la atención pasó a centrarse sobre el ARN como almacén de información genética primigenio de la vida.
A principios de los años ochenta, este cambio de enfoque recibió su recompensa con el descubrimiento de los…

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