DNA Structure and Replication

ID #2238

Does DNA polymerase 1 move continuously along the lagging strand or does it patch up a bit, disconnect, reconnect somewhere else and patch up a bit? It seems like it would be an 'endonuclease' if it disconnected and reconnected, but the book says it goes 5' to 3'. Does the polymerase remove 1 RNA base and replace it with DNA or will it remove all RNA's and then go back and replace them with DNA? I'm not really sure how the DNA polymerase I works at the telomere ends. From figure 15.12 of the Freeman textbook, does polymerase latch on at the very far 5' end (the last gold rna base) and work to the left? But this DNA Pol I won't be able to replace the last primer with DNA bases because there is no available OH group to add DNA bases to, is that right?


DNA Pol I is an exonuclease because, if you look at one of the slides from lecture 15, you can see that there is a nick (which effectively works like an end) in the backbone between the DNA bases and the start of the RNA primer. The DNA Polymerase I equivalent in eukaryotes has 5' to 3' exonuclease activity, so it will start removing the RNA primer at the 5' end. This is the farthest right gold base. Normally, the enzyme will systematically remove each RNA base at the 5' end and replace it with a DNA base, and then it will move on to the next RNA base and will replace it. This occurs until the entire RNA primer has been removed and replaced with DNA. In telomeres, however, there is no free 3'OH group required for DNA synthesis so the Pol I equivalent can remove the primer but cannot replace it with DNA. You are correct- without a 3'OH group, Pol I cannot synthesize.


As an aside, I used the term "equivalent" when describing the activity of the eukaryotic DNA Polymerase that removes primers. This is because eukaryotic DNA Polymerases are slightly different than those found in bacteria and are named differently. Nucleic acid synthesis is universal in eukaryotes and prokaryotes, but Prof. Mehrtens used E. coli names in lecture. There is a polymerase in eukaryotes that performs the same function as DNA Pol I in E. coli, but it has a different name (and the name is probably Greek, since most eukaryotic DNA polymerases are named after Greek letters).

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