Origins of Replication

Origins of replication are sequences where DNA replication is initiated within a genome

  • Most bacteria have a single, circular molecule of DNA and possess only a single origin of replication
  • Eukaryotic chromosomes are usually significantly larger and hence possess many origins of replication along their length
  • DNA synthesis may occur bi-directionally from an origin of replication

DNA Synthesis at an Origin of Replication

origin of replication

When DNA replication occurs bi-directionally, the two replication forks move in opposite directions to create a replication bubble

  • As replication bubbles expand in both directions, they will eventually fuse together as the intervening regions are copied
  • This functions to greatly limit the time required for DNA replication to occur

If human chromosomes did not possess multiple origins of replication, it would take roughly one month to copy a chromosome

  • An average human chromosome is 150 million base pairs in length and DNA polymerase copies at a rate of ~ 50 bp / sec
  • 150,000,000 ÷ 50 = 3,000,000 seconds (÷ 60) = 50,000 minutes (÷ 60) = 833 hours (÷ 24) = ~ 35 days