(20)
2. Angive at et prokaryot kromosom har et enkelt replikation-origin (OriC) mens eukaryote kromosomer indeholder mange replikation-origins.
Devlin, s.177-178, fig.4.14
Stryer, s.760, s.764 

Origins of replication (OriC) are specific sites on the DNA molecules from where the replicating begins and proceeds in both directions (bidirectional).

Known origins of replication contain:

• multiple, short, repeated sequences that bind specific proteins

• A=T rich regions on which the initial separation of parental strands occurs

 A prokaryotic DNA-helix has only one origin of replication (fx. E.Coli, 245bp-region acts as OriC. It contains four repeats of a sequence that together act as a binding site for an initiation protein called DnaA. It also contains a tandem array of three nearly identical 13-nucleotide sequences rich in A=T)

In eukaryotic cells, there are thousands of origins. These OriC are tandemly arrayed along the chromosome, placed ca. 50.000-100.000 bp apart. In human beings, replication requires about 30.000 origins of replication, with each chromosome containing several hundred.

Adjacent OriC tend to function together.  The replication proceeds bidirectionally from an OriC - there are two replication forks created, two DNA polymerases III move in opposite directions,, each synthetising one leading and one lagging strand. This basically means that each new DNA-strand is synthesized partially as leading and partially as a lagging strand.

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