Читать книгу Cell Biology - Stephen R. Bolsover - Страница 89

Replication of a new DNA strand starts at specific sequences known as origins of replication. The small circular chromosome of Escherichia coli has only one of these, whereas eukaryotic chromosomes, which are usually linear and much larger, have many. At each origin of replication, the parental strands of DNA untwist to give rise to a structure known as the replication fork (Figure 4.1). This unwinding permits each parental strand to act as a template for the synthesis of a new strand. The structure of the double helix and the nature of DNA replication pose a mechanical problem. How do the two strands unwind and how do they stay unwound so that each can act as a template for a new strand?