DNA Structure and Replication

ID #2237

I'm confused with how the telomeres work as described in the book. It says on the lagging strand, there will be a gap when the primer is removed that can't be filled. Why can't it be filled? Wouldn't polymerase 1 replace the RNA with DNA? What's different about this primer from the others? Also, does the first primer on the leading strand cause a problem as well?

Look at Figure 15.12 from your Freeman textbook, and think through the process of how DNA pol I creates a phosphodiester linkage between two DNA nucleotides after removing the RNA primer. The reason why the telomere primer on the lagging strand shown in gold labeled as "last primer" in the figure can't be filled by DNA Pol I is that the polymerase requires a free 3'OH group for the attachment of the first DNA nucleotide. The position of the first DNA nucleotide is indicated in the figure by the furthest right gold bar near the 5' mark on the lagging strand. After cutting out the RNA primer, the first DNA nucleotide would need to be attached to the DNA nucleotide preceding it, but it can't be because there is no DNA nucleotide preceding it since its all the way at the end of the linear strand. So the RNA primer is removed by DNA pol I but DNA pol I is not able to replace it with DNA, leaving a gap. The same is not true on the leading strand because the primer is placed near the ori, not near the end of the linear strand. 

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