The Road to Sexual Reproduction

Abstract

Abstract Living organisms are divided into two superkingdoms: the prokaryotes and eukaryotes. Prokaryotes consist of eubacteria and archaebacteria, are single-celled, and breed asexually. All other organisms comprise eukaryotes. These are larger, more complex, often multicellular, and, with some exceptions, reproduce sexually. About two billion years ago, eukaryotes evolved from prokaryotes by incorporating oxygen-using eubacteria into anaerobic archaeans. This development was accompanied by increased oxidative metabolism using the newly acquired bacteria, or mitochondria. The mitochondrial genome interacts with the nuclear genome to provide the proteins necessary for energy production. While using oxygen is more efficient when producing energy for the cell, it also results in extensive DNA damage by releasing free oxygen radicals. Meiosis is an integral part of sexual reproduction and is present in nearly all organisms that reproduce sexually. It likely evolved during the transition between the prokaryotes and eukaryotes. Of significant importance, crossing-over during an early stage of meiosis provided the capacity to repair DNA and resulted in increased levels of variability in progeny. Both of these factors may be important in the evolution of sex.