We saw in Figure 4-4 that an A-T base pair is held together by fewer hydrogen bonds than a G-C base pair. This DNA contains short sequences that attract initiator proteins, as well as stretches of DNA that are especially easy to open. In simple cells like those of bacteria or yeast, origins are specified by DNA sequences several hundred nucleotide pairs in length. The positions at which the DNA helix is first opened are called replication origins ( Figure 5-29). As we shall see, the process of DNA replication is begun by special initiator proteins that bind to double-stranded DNA and pry the two strands apart, breaking the hydrogen bonds between the bases. To be used as a template, the double helix must first be opened up and the two strands separated to expose unpaired bases.

DNA Synthesis Begins at Replication OriginsĪs discussed previously, the DNA double helix is normally very stable: the two DNA strands are locked together firmly by a large number of hydrogen bonds formed between the bases on each strand.