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.causing a short stretch of DNA to unwind.

^ The unwinding allows helicase and other single-strand- binding proteins to attach to the single-stranded DNA.

^ The unwinding allows helicase and other single-strand- binding proteins to attach to the single-stranded DNA.

.causing a short stretch of DNA to unwind.

Helicase

^Single-strand-binding proteins

Helicase

^Single-strand-binding proteins

112.11 E. coli DNA replication begins when initiator proteins bind to oriC, the origin of replication, causing a short stretch of DNA to unwind.

After DNA has been unwound by helicase, the single-stranded nucleotide chains have a tendency to form hydrogen bonds and reanneal (stick back together). Secondary structures, such as hairpins (see Figure 8.21), also may form between complementary nucleotides on the same strand. To stabilize the single-stranded DNA long enough for replication to take place, single-strand-binding (SSB) proteins attach tightly to the exposed single-stranded DNA (see Figure 12.12). Unlike many DNA-binding proteins, SSBs are indifferent to base sequence—they will bind to any single-stranded DNA. Single-strand-binding proteins form tetramers (groups of four) that together cover from 35 to 65 nucleotides.

Another protein essential for the unwinding process is the enzyme DNA gyrase, a topoisomerase. As discussed in

| DNA helicase binds to the lagging-strand template at each replication fork and moves in the 5'—>■ 3' direction along this strand, breaking hydrogen bonds and moving the replication fork.

^ Single-strand-binding proteins stabilize the exposed single-stranded DNA.

^ DNA gyrase relieves strain ahead of the replication fork.

Origin

Unwinding ^

DNA gyrase DNA helicase

Unwinding Single-strand-binding proteins

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