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Wednesday, January 09, 2008

DNA Polymerase III polymerizes the Okazaki fragments

031. During replication of DNA, which one of the following enzymes polymerizes the Okazaki fragments?

1. DNA Polymerase I

2. DNA Polymerase II

3. DNA Polymerase III

4. RNA Polymerase I

Answer

1. DNA Polymerase III

Reference:

Harper 26th Edition Page 328

Lippincott 3rd Edition Page 403

Quality

Thinker and TRAP !!!

Status
Repeat, but with a slightly different stem

QTDF

Harper

Discussion

Before proceeding with the discussion, please read the question once again !!!! And now let us see what Harper says about DNA Polymerase

The polymerase III holoenzyme (the dnaE gene product in E.Coli) binds to template DNA as part of a multiprotein complex that consists of several polymerase accessory factors (beta, gamma, delta, delta’ and theta). DNA polymerases only synthesize DNA in the 5’ to 3’ direction and only one of the several different types of polymerases is involved at the replication fork. Because the DNA Strands are antiparallel, the polymerase functions asymmetrically. On the leading (Forward) strand, the DNA is synthesized continuously. On the lagging (retrograde) strand, the DNA is systhesized in short (1-5kb) fragments, the so called Okazaki fragments.

Explanation

1. DNA Polymerase I (pol I) completes chain synthesis between Okasaki gragments on the lagging strand.

2. DNA Polymerase II (pol II) is mostly involved in proof reading and DNA repair

3. DNA Polymerase III synthesizes the leading strand as well as the Okasaki fragments.

4. RNA Polymerase I synthesizes synthesizes the precursor of the large ribosomal RNAs(28S, 18S and 5.8S) in the nucleolus.

Comments

The main confusion seems to because of a table given in Harper

E.Coli

Mammalian

Function

I

a

Gap Filling and synthesis of lagging strand

II

e

DNA Proofreading and repair

b

DNA repair

g

Mitochondrial DNA synthesis

III

d

Processive leading strand synthesis

Tips

Please note that synthesis of lagging strand involves DNA polymerase I, but the Okasaki fragments of the lagging strands are synthesized by DNA polymerase III

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