The following discussion of the process of replication will focus on bacterial systems, where replication has been most thoroughly studied and is best understood. Although many aspects of replication in eukaryotic cells are similar to those of prokaryotic cells, there are some important differences. We will compare bacterial and eukaryotic replication later in the chapter.
Initiation The circular chromosome of E. coli has a single replication origin (oriC). The minimal sequence required for oriC to function consists of 245 bp that contain several critical sites. Initiator proteins bind to oriC and cause a short section of DNA to unwind. This unwinding allows helicase and other single-strand-binding proteins to attach to the polynucleotide strand (I Figure 12.11).
Unwinding Because DNA synthesis requires a single-stranded template and double-stranded DNA must be unwound before DNA synthesis can take place, the cell relies on several proteins and enzymes to accomplish the unwinding. DNA helicases break the hydrogen bonds that exist between the bases of the two nucleotide strands of a DNA molecule. Helicases cannot initiate the unwinding of double-stranded DNA; the initiator proteins first separate DNA strands at the origin, providing a short stretch of single-stranded DNA to which a helicase binds. Helicases bind to the lagging-strand template at each replication fork and move in the 5':3' direction along this strand, thus also moving the replication fork (I Figure 12.12).
ffi Initiator proteins bind to oriC, the origin of replication,.
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