Why DNA Replication Occurs in the 5′ to 3′ Direction: The Role of DNA Polymerase and Molecular Structure

Why does a new DNA strand elongate only in the 5′ to 3′ direction during DNA replication? A) DNA polymerase begins adding nucleotides at the 5′ end of the template.B) The polarity of the DNA molecule prevents addition of nucleotides at the 3′ end.C) Replication must progress toward the replication fork.D) DNA polymerase can add nucleotides only to the free 3′ end.

The correct answer is D) DNA polymerase can add nucleotides only to the free 3′ end

The correct answer is D) DNA polymerase can add nucleotides only to the free 3′ end.

DNA replication is a process that occurs during cell division, where a copy of the DNA molecule is created. During replication, a new DNA strand is synthesized by adding nucleotides to an existing template strand. The direction of the new DNA strand elongation is important because it determines the accuracy and efficiency of DNA replication.

DNA polymerase, the enzyme responsible for adding nucleotides to the growing DNA strand, can only attach new nucleotides to the free 3′ end of the existing DNA strand. This is because the enzyme has a specific active site that can only interact with the 3′ hydroxyl group (-OH) of the last nucleotide on the existing DNA strand.

The structure of DNA also contributes to the 5′ to 3′ directionality of DNA replication. DNA is a double-stranded molecule with two antiparallel strands. One strand runs in the 5′ to 3′ direction, while the other runs in the 3′ to 5′ direction. The two strands have opposite orientations, meaning that the 5′ end of one strand pairs with the 3′ end of the other strand. This arrangement forms the famous double helix structure of DNA.

During DNA replication, an enzyme called helicase unwinds the double helix, creating a replication fork. At the replication fork, the two strands separate, and each strand acts as a template for the synthesis of a new complementary strand. Because DNA polymerase can only add nucleotides to the free 3′ end, the synthesis of the new DNA strand must occur in the 5′ to 3′ direction.

In summary, the new DNA strand elongates only in the 5′ to 3′ direction during DNA replication because DNA polymerase can only add nucleotides to the free 3′ end. The polarity and structure of DNA molecules further support this directionality.

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