Which of the following molecules functions as a “proofreader” for a newly replicated strand of DNA?
a. DNA polymerase III
b. Primase
c. Helicase
d. Ligase

Verified step by step guidance

1. Understand the role of each enzyme listed in the options during DNA replication.
2. Recall that DNA polymerase III is the main enzyme responsible for synthesizing the new DNA strand by adding nucleotides complementary to the template strand.
3. Recognize that DNA polymerase III also has a 3' to 5' exonuclease activity, which allows it to remove incorrectly paired nucleotides immediately after they are added, effectively acting as a 'proofreader'.
4. Know that primase synthesizes RNA primers to initiate DNA synthesis, helicase unwinds the DNA double helix, and ligase joins Okazaki fragments on the lagging strand, but none of these have proofreading functions.
5. Conclude that the molecule functioning as a 'proofreader' is DNA polymerase III due to its exonuclease proofreading activity.

Key Concepts

DNA Polymerase III and Proofreading

DNA Polymerase III is the primary enzyme responsible for synthesizing new DNA strands during replication. It also has a proofreading function through its 3' to 5' exonuclease activity, which allows it to remove incorrectly paired nucleotides, ensuring high fidelity in DNA replication.

Role of Primase in DNA Replication

Primase synthesizes short RNA primers that provide a starting point for DNA polymerase to begin DNA synthesis. It does not have proofreading ability and only lays down the initial RNA sequence needed for replication to proceed.

Functions of Helicase and Ligase

Helicase unwinds the DNA double helix to allow replication machinery access to single strands, while Ligase seals nicks between Okazaki fragments on the lagging strand. Neither enzyme has proofreading capabilities during DNA replication.