In which direction are new DNA molecules synthesized (i.e. which end is the oldest and which is the newest?)? Why are they synthesized in this direction only?
New DNA molecules are synthesized in a 5' to 3' direction, meaning that the 5' end is considered the oldest and the 3' end is the newest. This is due to the way DNA polymerase, the enzyme responsible for DNA synthesis, adds nucleotides to a growing DNA strand.
To understand why DNA synthesis occurs in this specific direction, let's first discuss the structure of DNA. DNA is composed of two strands that are antiparallel, meaning they run in opposite directions. One strand runs in a 5' to 3' direction, while the other runs in a 3' to 5' direction.
DNA polymerase, the primary enzyme involved in DNA replication, can only add nucleotides to the 3' end of an existing DNA strand. It does this by reading the template strand (parental DNA strand) and adding complementary nucleotides to the new strand. The nucleotides are connected by phosphodiester bonds, forming a new DNA strand.
Since DNA synthesis occurs only in the 5' to 3' direction, the DNA polymerase can only add nucleotides to the 3' end of the newly synthesized DNA strand. This results in the formation of a new DNA molecule that runs in the opposite direction to the template strand.
The reason for this directionality lies in the specific structure and chemistry of DNA polymerase. It has an active site that can only attach the incoming nucleotide to the 3'—OH (hydroxyl) group, which is found on the deoxyribose sugar at the end of the growing DNA strand. In other words, DNA polymerase needs an available 3'—OH group to link the nucleotides together.
Therefore, the DNA polymerase requires a primer, which provides the necessary 3'—OH group, to initiate DNA synthesis. Once the primer is in place, the DNA polymerase can extend the new DNA strand by adding nucleotides in a 5' to 3' direction based on the complementary bases of the template strand.
In summary, DNA synthesis occurs in a 5' to 3' direction because DNA polymerase can only add nucleotides to the 3' end of the growing DNA strand. This directionality is essential for accurate DNA replication and is determined by the specific chemistry of DNA polymerase and its interaction with the DNA template strand.