single-strand binding protein
/ SSB /
When you peel apart a strip of Velcro, the two halves want to snap right back together. Freshly separated DNA strands behave the same way — left alone, the unzipped single strands would immediately re-pair or tangle into hairpins by pairing with themselves. Single-strand binding proteins are the molecular splints that hold the opened strands apart and straight until the copying machinery can use them.
Right after helicase unwinds the helix, many copies of single-strand binding protein (SSB in bacteria; the equivalent in eukaryotes is RPA, replication protein A) coat the exposed single strands. By binding cooperatively along the bare DNA, they keep it from re-annealing into a double helix and from folding back on itself, and they protect it from nucleases that would chew up loose single-stranded DNA. They hold the template open and accessible but step aside as the polymerase advances, since the polymerase displaces them as it lays down the new strand.
Without them, the fork would stall: the strands the helicase just opened would close right back up, and self-pairing would block the polymerase. SSB and RPA are not just passive coatings — they also help recruit and coordinate other replication and repair proteins, acting as a hub on single-stranded DNA. The same proteins are central in DNA repair and recombination, wherever single-stranded DNA is exposed.
On the lagging-strand template, long stretches of single-stranded DNA are exposed before each Okazaki fragment is made. SSB coats those stretches, keeping them open and untangled so primase and polymerase can work on them in turn.
SSB keeps separated strands open until they are copied.
SSBs bind any single-stranded DNA regardless of its sequence — they are not sequence-specific; their job is structural, holding strands open, not reading the code.