QUICK LAB

MODELING

Replication
Use two zipping plastic bags to model how complementary strands of DNA attach to template strands during replication.

PROCEDURE

MATERIALS
• 2 zipping bags
• scissors

1. Cut the sliding zippers off both bags. One zipper represents the template strands of a DNA molecule.
2. Cut the other zipper into four smaller pieces and unzip each of them. These represent free nucleotides. Don’t worry about which nucleotide is which in this activity.
3. Use the pieces to model replication as shown on page 237.

ANALYZE AND CONCLUDE
Evaluate What are the limitations of this model?

MAIN IDEA

Replication is fast and accurate.
FIGURE 8.9 Eukaryotic chromo-

In every living thing, DNA replication happens over and over again, and it happens remarkably fast. In human cells, about 50 nucleotides are added every second to a new strand of DNA at an origin of replication. But even at this rate, it would take many days to replicate a molecule of DNA if the molecule were like a jacket zipper, unzipping one tooth at a time. Instead, replication proceeds from hundreds of origins of replication along the chromosome, as shown in FIGURE 8.9, so the process takes just a few hours.
Another amazing feature of replication is that it has a built-in “proofreading” function to correct errors. Occasionally, the wrong nucleotide is added to the new strand of DNA. However, DNA polymerase can detect the error, remove the incorrect nucleotide, and replace it with the correct one. In this way, errors in replication are limited to about one error per 1 billion nucleotides.
Replication is happening in your cells right now. Your DNA is replicated every time your cells turn over, or replicate themselves. Your DNA has replicated trillions of