Chapter 13, Problem 1Q

Summary Introduction

To analyze:

The time taken by the DNA polymerase to complete the replication process of the entire DNA molecule of the shortest chromosome, by moving uninterruptedly from one end to the other end. Also figure out whether DNA replication involves one starting point or follows an alternate hypothesis.

Given:

The DNA polymerase proceeds uninterruptedly on the DNA strand, from one end to the other end to synthesize the new DNA strand. Addition of nucleotides on the new DNA strand occurs at the rate of 50 base pairs per second (bp/s). The replication is limited to the S or the synthesis phase of the cell cycle. The shortest chromosome is 21^st chromosome in humans and its length is $50\times {10}^6$ bps.

Introduction:

The division of cells in a cell cycle involves two phases: interphase and mitotic phase. Replication of the genetic material takes place during synthesis phase or S phase of the interphase. DNA polymerase is responsible for DNA replication in both eukaryotes and prokaryotes.

Explanation of Solution

The rate at which DNA polymerase proceeds the formation of new DNA strand is 50 bp/s. Therefore, to replicate a DNA strand of $50\times {10}^6$ bps, the enzyme DNA polymerase will take 10⁶ seconds, which is approximately 278 hours.

The synthesis phase of a mammalian cell is 8 hours long, and the entire replication process occurs during this time. This indicates that there must be multiple DNA polymerase complexes involved in this process. Hence more than one replication origin sites must be there, where the enzyme would attach itself to synthesize a new strand.

Conclusion

Therefore, it can be concluded that there are many replication origin sites where DNA polymerase enzyme work simultaneously, to complete the synthesis of new DNA strand.