<P> Termination requires that the progress of the DNA replication fork must stop or be blocked . Termination at a specific locus, when it occurs, involves the interaction between two components: (1) a termination site sequence in the DNA, and (2) a protein which binds to this sequence to physically stop DNA replication . In various bacterial species, this is named the DNA replication terminus site - binding protein, or Ter protein . </P> <P> Because bacteria have circular chromosomes, termination of replication occurs when the two replication forks meet each other on the opposite end of the parental chromosome . E. coli regulates this process through the use of termination sequences that, when bound by the Tus protein, enable only one direction of replication fork to pass through . As a result, the replication forks are constrained to always meet within the termination region of the chromosome . </P> <P> Within eukaryotes, DNA replication is controlled within the context of the cell cycle . As the cell grows and divides, it progresses through stages in the cell cycle; DNA replication takes place during the S phase (synthesis phase). The progress of the eukaryotic cell through the cycle is controlled by cell cycle checkpoints . Progression through checkpoints is controlled through complex interactions between various proteins, including cyclins and cyclin - dependent kinases . Unlike bacteria, eukaryotic DNA replicates in the confines of the nucleus . </P> <P> The G1 / S checkpoint (or restriction checkpoint) regulates whether eukaryotic cells enter the process of DNA replication and subsequent division . Cells that do not proceed through this checkpoint remain in the G0 stage and do not replicate their DNA . </P>

Where do nucleotides come from during dna replication