Like any monumental task, configuring chromosomes for replication requires a certain amount of preparation. For eukaryotic cells, this preparation begins early in G 1 , when DNA is made replication-ready by the recruitment of proteins to the sites along each chromosome where replication will begin. These nucleotide sequences, called origins of replication, serve as landing pads for the proteins and protein complexes that control and carry out DNA synthesis.

One of these protein complexes, called the origin recognition complex (ORC), remains perched on the replication origins throughout the cell cycle. To prepare the DNA for replication, the ORC recruits a protein called Cdc6, whose concentration rises early in G 1 . Together, these proteins load the DNA helicases that will ultimately open up the double helix at the origin of replication. Once this prereplicative complex is in place, the replication origin is loaded and ready to “fire.”

必须创建列，不必创建列族

必须创建列族，不必创建列

列族和列都要创建

列族和列都不必创建

列名

行键

时间戳

列键

正确

错误

正确

错误

正确

错误

bit

BYTE

WORD

CHAR

semi-conservative replication

semi-discontinuous replication

bi-directional replication

RNA primer required