Origin and function Of Meiosis and Mitosis

Meiosis is ubiquitous among eukaryotes. It occurs in single-celled organisms such as yeast, as well as in multicellular organisms, such as humans. Eukaryotes arose from prokaryotes more than 1.5 billion years ago, and the earliest eukaryotes were likely single-celled organisms. To understand meiosis in eukaryotes, it is necessary to understand 
 (1) How meiosis arose in single celled eukaryotes, 
 (2) The function of Meiosis and Mitosis

    1)  Origin
There are two conflicting theories on how meiosis arose. One is that meiosis evolved from bacterial sex (called transformation) during the evolution of eukaryotes. The other is that meiosis arose from mitosis.

• Theory that meiosis evolved from bacterial sex(transformation)
• Theory that meiosis evolved from mitosis
• Sharing of components during the evolution of meiosis and mitosis  
   

     2) Function

Single-celled eukaryotes (protists) generally can reproduce asexually (vegetative reproduction) or sexually, depending on conditions. Asexual reproduction involves mitosis, and sexual reproduction involves meiosis. When sex is not an obligate part of reproduction, it is referred to as facultative sex. Present-day protists, generally, are facultative sexual organisms, as are many bacteria. The earliest form of sexual reproduction in eukaryotes was probably facultative, like that of present-day protists. To understand the function of meiosis in facultative sexual protists, we next consider under what circumstances these organisms switch from asexual to sexual reproduction, and what function this transition may serve.

• Stress induces the eukaryotic sexual cycle in protists
• Stress induces sex in bacteria
• Theory that DNA repair is the adaptive advantage of meiosis
• Theory that genetic diversity is the adaptive advantage of sex