Chapter 8, Problem 2CT

Review Figures 8.12 and 8.13. In cells, the primers for DNA synthesis are short strands of RNA, so each newly-synthesized strand of DNA has a segment of RNA al its 5′ end. As replication proceeds, DNA polymerases remove these RNA segments and fill in the resulting gaps with DNA. However, the gaps at the very 5′ ends of the new strands cannot be filled in with DNA. Why not? DNA replication leaves exposed about 100 nucleotides al the 5′ end of each template strand, and these single-stranded ends are removed. What are the effects of this "end problem" on a cell's DNA as it continues to divide?

FIGURE 8.12 DNA replication.

Green arrows show the direction of synthesis for each strand. The Y-shaped structure where the DNA molecule is being unwound is called a replication fork.

FIGURE 8.13 Discontinuous synthesis of DNA.

This close-up of a replication fork shows that only one of the two new DNA strands is assembled continuously. The other is assembled in short segments.