The enzyme DNA polymerase in E. coli is a DNA dependent polymerase and also has the ability to proof-read the

NCERT Exemplar · Q31

Q. The enzyme DNA polymerase in E. coli is a DNA dependent polymerase and also has the ability to proof-read the DNA strand being synthesised. Explain. Discuss the dual polymerase.

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Concept understanding — Types Of Polymerases

You have probably never heard the word "polymerase" before, but you already understand what it does. Think of a polymer as a long chain — like a necklace made of many identical or similar beads. In biology, the most famous polymers are DNA and RNA, which are chains of smaller units called nucleotides. A polymerase is simply the enzyme (a protein machine) that links those beads together to build the chain.

In the context of your syllabus, the term "polymerases" almost always refers to DNA polymerases and RNA polymerases. These are the two main types you need to know. They are not interchangeable; each has a specific job and a specific set of rules.

1. DNA Polymerase — The Copyist

DNA polymerase is the enzyme responsible for DNA replication — making an exact copy of the entire DNA molecule before a cell divides. Imagine you have a master blueprint (the original DNA). DNA polymerase reads that blueprint and builds a matching second copy.

Key features to remember:

It can only add new nucleotides to an existing strand (it needs a "primer" to start).
It works in one direction only: from the 5' end to the 3' end of the new strand.
It has a proofreading ability. If it accidentally puts the wrong bead on the chain, it can cut it out and replace it. This is why DNA replication is so accurate.

Important

In NCERT, you will encounter three types of DNA polymerases in prokaryotes (like E. coli): DNA polymerase I, II, and III. Of these, DNA polymerase III is the main enzyme that does the bulk of replication. DNA polymerase I helps remove the RNA primer and fill in gaps. DNA polymerase II is mainly involved in repair.

2. RNA Polymerase — The Transcriber

RNA polymerase does a different job. It is responsible for transcription — copying a specific gene from DNA into a messenger RNA (mRNA) molecule. Think of it as a photocopier that makes a working copy of just one page of the blueprint, not the whole book.

Key differences from DNA polymerase:

It does not need a primer to start. It can begin building the RNA chain from scratch.
It uses DNA as a template but builds an RNA strand (which uses uracil instead of thymine).
It is less accurate than DNA polymerase — it has no proofreading ability. This is acceptable because RNA molecules are temporary and get replaced. …

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E. coli DNA polymerase is a DNA-dependent enzyme that synthesizes new DNA strands using a DNA template and possesses a crucial proofreading ability to correct errors during synthesis.

DNA replication, the process by which a cell makes an exact copy of its DNA, is fundamental to life. In organisms like E. coli, this intricate process is orchestrated by a complex machinery of enzymes, with DNA polymerase playing the central role. Understanding its functions is key to grasping the fidelity of genetic information transfer.

The primary function of E. coli DNA polymerase is its DNA-dependent polymerase activity. This means the enzyme requires an existing DNA strand to serve as a template, guiding the synthesis of a new complementary DNA strand. It reads the sequence of nucleotides on the template strand and adds corresponding complementary nucleotides to the growing new strand. This synthesis always proceeds in a specific direction: from the $5'$ end to the $3'$ end of the newly forming strand. This directional synthesis is crucial for the overall replication process, as it dictates how the leading and lagging strands are synthesized.

Important

DNA polymerase synthesizes new DNA strands exclusively in the $5' \to 3'$ direction, adding nucleotides to the $3'$-hydroxyl end of the growing strand.

While DNA polymerase is remarkably efficient, synthesizing thousands of nucleotides per second, its accuracy is equally vital. Even a single incorrect nucleotide incorporation can lead to a mutation, potentially altering gene function and impacting the cell or organism. To prevent such errors, E. coli DNA polymerase possesses a critical proofreading ability.

This proofreading function is carried out by an associated $3' \to 5'$ exonuclease activity. An exonuclease is an enzyme that cleaves nucleotides one by one from the end of a polynucleotide chain. In this case, if DNA polymerase accidentally adds an incorrect nucleotide that does not properly base-pair with the template, the enzyme pauses. Its $3' \to 5'$ exonuclease activity then excises the wrongly incorporated nucleotide from the $3'$ end of the newly synthesized strand. Once the incorrect nucleotide is removed, the polymerase activity resumes, adding the correct nucleotide and continuing synthesis. This 'backtracking and correcting' mechanism significantly enhances the fidelity of DNA replication, ensuring that the genetic information is copied with very high accuracy. …

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Q.The enzyme DNA polymerase in E. coli is a DNA dependent polymerase and also has the ability to proof-read the DNA strand being synthesised. Explain. Discuss the dual polymerase.

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