quinta-feira, 17 de março de 2011

ORIGEM DA GENÉTICA



Genética é a área da Biologia que estuda a herança biológica, ou hereditariedade, que a transmissão de características de pais para filhos, ao longo das gerações. A genética desenvolveu-se de maneira expressiva apenas no século XX.

Os filósofos da antiga Grécia divergiam quanto às explicações para hereditariedade. Vamos observar algumas explicações destes filósofos:

a) Alcmeon de Crotona, um discípulo de Pitágoras de Samos que viveu por volta de 500 a.C., acreditava que os homens e mulheres tinham sêmen e que este se originava no cérebro; segundo ele, o sexo das crianças era determinado pela preponderância do sêmen de um dos pais, ocorrendo hermafroditismo se os dois estivessem em igual proporção.

b) Anaxágoras de Clazomene ( 500 – 428 a.C.) postulava que o sêmen ocorria apenas nos homens e continha um protótipo de cada órgão do futuro ser; as fêmeas atuariam apenas como receptoras e nutridoras do ser pré-formado. Ele propôs também que os meninos eram gerados no lado direito do corpo e as meninas, no lado esquerdo.

c) Empédocles de Acragas ( 492-432 a. C ), o calor do útero era decisivo na determinação do sexo dos bebês: útero quente produzia homens; útero frio, mulheres.

d) Hipócrates de Cos ( 460 – 370 a.C.) ficou conhecido como “pai da medicina”. Sua hipótese foi denominada pangêsese, cada órgão ou parte do corpo de um organismo vivo produzia partículas hereditárias chamadas de gêmula, que era transmitidas aos descendentes no momento da concepção. Essa hipótese perdurou até o século XIX, tendo entre seus adeptos Jean-Baptiste Lamarck e o próprio Charles Darwin.

e) Aristóteles ( 384-322 ) escreveu um tratado sobre o desenvolvimento e a hereditariedade dos animais. Nesse livro, De generatione animalium, ele distinguiu quatro tipos de geração: abiogênese, brotamento, sexuada com cópula e sexuada sem cópula. Obs.Sobre a reprodução sexuada, ele acreditava que resultava de uma contribuição diferencial dos sexos: a fêmea fornecia a “ matéria” básica que constituía e nutria o ser em formação, enquanto o macho fornecia por meio do sêmen, a “essência”, transmitindo-lhe a alma, fonte da forma e do movimento.

f) William Harvey (1578-1657) Ele propôs que todo animal se originava de um ovo e que esse ovo produzido pela fêmea necessitava ser fertilizado pelo sêmen do macho para originar um novo ser.

No século XVIII , os defensores das idéias originais de Harvey dedicaram-se a explicar como o ovo fertilizado podia originar um novo ser. Havia então duas correntes de explicação : Pré-formismo ( teoria da pré- formação) e a Epigenética ( epigêneses). Entre os defensores do pré-formismo havia os ovistas e os espermistas.

Obs: Apesar de Harvey ter sugerido a possibilidade de ocorrer epigênese , a teoria epigenética foi proposta formalmente pelo anatomista alemão Caspar Friedrich Wolff em 1759. Wolff propõe que a nutrição e o crescimento das plantas dependem de uma força essencial, por ele denominada vis essentialis , que tem o poder de formar novos órgãos a partir de marterial amorfo. Ele acreditava que a fêmea fornecia uma unidade de matéria mais ou menos homogênea, o óvulo, enquanto o macho contribuía com a potencialidade, a vis essentialis, para seu desenvolvimento em um novo ser.

A descoberta dos Gametas

Um dos pontos fundamentais para o desenvolvimento da genética foi a constatação de que um novo ser se origina a partir da fusão de duas células, os gametas masculino e feminino. As leis da herança biológica só puderam ser formuladas após a compreensão do papel dos gametas e da fecundação na reprodução dos seres vivos.

A descoberta dos Espermatozóides

Foi em 1841 que o anatomista e fisiologista suíço Rudolf Albert von ( Albrecht) Kolliker, ao estudar a estrutura microscópica dos testículos, demostrou que os espermatozóides não eram parasitas do trato genital masculino e sim células modificadas. Kolliker observou os diversos estágios de transformação de células dos testículos em espermatozóides. Pouco tempo depois em1854 o naturalista inglês George Niwport ( 1803-1854) obteve evidências de que os espermatozóides de rã entravam no óvulo durante a fecundação

A descoberta dos Óvulos

Em 1828, o naturalista alemão Karl Ernst Von Baer descobriu, no interior de cada folículo ovarianondescrito por Graaf, um óvulo.A natureza celular dos óvulos foi estabelecida em 1829 pelo zoólogo alemão Theodor Schwann, mas foi somente em 1861 que o anatomista alemão Karl Gegenbaur demonstrou definitivamente que o óvulo dos animais vertebrados é uma única célula. Após a demonstração definitiva de que o espermatozóide e óvulos animais são células, o pré-formismo foi descartado.

51 comentários:

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