The evolution of meiosis from mitosis

Abstract

Podrijetlo mejoze bilo je i ostaje jedno od najintrigantnijih pitanja biologije. Činjenica da bez mejoze ne bi bilo spolnog razmnožavanja, koje je osnovni način stvaranja potomstva i opstanka vrste kroz vrijeme za većinu eukariotskih organizama, održava zanimanje za sve aspekte ovog tipa stanične diobe. Njena sličnost s mitozom, kao i veća složenost, dovode do zaključka da je mejoza evoluirala iz mitoze. Značajne prednosti pred selekcijskim pritiscima, koje nedvojbeno imaju organizmi koji se spolno razmnožavaju, uglavnom se objašnjavaju izmjenom dijelova homolognih kromosoma (crossing-over) u profazi I, koja zatim dovodi do novih kombinacija alela, veće varijabilnosti unutar vrste te veće sposobnosti preživljavanja u promjenjivim uvjetima okoliša. U ovom seminarskom radu pokušala sam dati pregled tijeka mitotske, odnosno, mejotske diobe, njihovu usporedbu, naglasiti razlike te pružiti jasan slijed zaključaka na temelju dedukcije Wilkinsa i Hollidayja - slijedeći međusobne uzorčno-posljedične veze između noviteta u mejozi, zaključeno je da je za nastanak mejoze kao redukcijske diobe bio presudan jedan jedini novitet: sparivanje homolognih kromosoma čitavom njihovom duljinom. Razmatram hipotezu o rekombinacijskom popravku DNA koji je omogućio opstanak takvog noviteta, njegove prednosti i mane pred selekcijskim pritiscima te povezanost prvotnih paraspolnih ciklusa s konačnim rezultatom - spolnim razmnožavanjem. Zaključujem predlaganjem nekoliko načina laboratorijskog testiranja ove hipoteze.The origin of meiosis was and remains to be one of the most intriguing research fields of biology. The fact that sexual reproduction, that is the main strategy by which the majority of eucaryotic organisms create offspring and persist as distinct species over time, would have been impossible without the development of meiosis keeps the continuing interest in this form of cellular division. Its similarity, as well as its higher complexity over mitosis, lead to the conclusion that meiosis had evolved from mitosis. Significant advantages of sexually reproducing organisms when they are faced with selection pressures, are mostly explained by the act of crossing-over during prophase I, which then brigs about new alelic combinations, higher intraspecial variability and the capability to survive in a changing environment. In this paper, I have tried to give an overview of the courses of the mitotic and meiotic division and their comparison. I have also tried to emphasize the differences and give a clear sequence of conclusions based on Wilkins' and Holliday's deduction - by following the causal connections between meiotic novelties, it has been concluded that the dawn of meiosis had been possible thanks to a single novelty: the pairing of homologous chromosomes along their entire length. I look into the hypothesis of recombinational DNA repair that had enabled the persistence of such a novelty, its advantages and disadvantages under selection pressures and the connection of initiative parasexual cycles to the final result - sexual reproduction. I finally conclude by proposing a few ways to test the hypothesis in the laboratory