DNA replication in eukaryotes and prokaryotes starts when a complex of proteins binds to an origin of replication (Figure 1A) and separates the two strands of the double helix to form a replication bubble with replication forks at each end (Figure 1B). In order for replication to proceed, the enzyme topoisomerase binds to the DNA immediately in front of the advancing replication forks.

Figure 1. DNA replication starts when the two strands of the double helix separate at an origin of replication. Topoisomerase binding sites are indicated.

 

(a) Describe how the structure of a prokaryotic chromosome differs from the structure of a eukaryotic chromosome.

(b) Explain why DNA replication cannot proceed to the regions of the chromosome labeled as I in Figure 1B unless topoisomerase binds ahead of each advancing replication fork in region II.

(c) Use the template of a replication fork to draw arrows that represent both continuous and discontinuous DNA synthesis. Draw one long arrow to show continuous DNA synthesis and three arrows to show discontinuous DNA synthesis. The arrows should point in the direction of nucleotide addition, and the three arrows showing discontinuous synthesis should be numbered 1, 2, and 3, in the order of fragment synthesis.

 

(d) Referring to Figure 1B, explain why any newly synthesized strand of DNA is the result of both continuous and discontinuous DNA synthesis.

Origin of Replication 5' A XXXXXXXXXxxxxxxxx 3' Replication Bubble 5' B XXXXXXXXII good xxxxxx 3' Replication Forks Topoisomerase Binding Sites