Why does a new DNA strand elongate only in the 5’to 3′ direction during DNA replication? The polarity of the DNA molecule prevents addition of nucleotides at the 3′ en Replication must progress toward the replication fork: DNA polymerase can add nucleotides only to the free 3′ end. DNA polymerase begins adding nucleotides at the 5′ end of the template.
The Correct Answer and Explanation is:
Correct Answer:
A 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 of the growing strand.
Explanation
DNA replication is a highly regulated process in which a double-stranded DNA molecule is copied to produce two identical DNA molecules. One important feature of DNA replication is that the new strand must elongate in the 5′ to 3′ direction. This directionality is due to the specific chemical structure and polarity of DNA strands.
Each DNA strand has a 5′ end and a 3′ end. The 5′ end has a phosphate group attached to the fifth carbon of the sugar molecule, while the 3′ end has a hydroxyl group attached to the third carbon. During replication, DNA polymerase, the enzyme responsible for synthesizing new DNA strands, requires a free 3′ hydroxyl group to attach the next nucleotide. It cannot add nucleotides to the 5′ end of the growing strand because the chemical reaction needed for bonding requires the 3′ hydroxyl group.
This means that DNA polymerase can only build the new strand by adding nucleotides to the 3′ end, causing the strand to grow in the 5′ to 3′ direction. Although the template strand is read in the 3′ to 5′ direction, the new strand is synthesized in the opposite 5′ to 3′ direction.
This directionality leads to the formation of a leading strand and a lagging strand at each replication fork. The leading strand is synthesized continuously toward the replication fork, while the lagging strand is synthesized in short segments (called Okazaki fragments) away from the fork, which are later joined together.
Therefore, the polarity of DNA and the specific action of DNA polymerase are the reasons why a new DNA strand can only elongate in the 5′ to 3′ direction during replication.
