Meiotic DNA Replication

Meiosis is the process by which diploid organisms produce haploid gametes capable of sexual reproduction. During meiosis, the cell performs one round of DNA replication (meiS) followed by homolog synapsis and extensive genetic recombination. Haploidization is then achieved through two subsequent nuclear divisions (meiosis I and meiosis II) without an intervening S phase. Several recent studies have found that unique properties of meiS are required to prepare the chromosomes for genetic recombination and the reductional meiosis I nuclear division. Although much of the basic replication apparatus is employed to perform meiS, studies from the budding and fission yeasts have also uncovered meiosis-specific regulators that usurp the mitotic cell cycle machinery to perform these specialized landmark events. This "same gun, different trigger" approach is employed for the initiation of DNA replication, recombination, and the meiosis I nuclear division. In addition to the correct induction and execution of meiS, this process must be precisely regulated. Not only must meiS be restricted to once and only once during the normal replication window, it must also be prevented between the two meiotic nuclear divisions. Current data indicate that blocking rereplication between meiosis I and meiosis II uses a strategy similar to mitotic cell division. However, the block to rereplication during the normal meiS window appears to apply a very different approach to solve this problem.