During DNA replication, why is the leading strand synthesised continuously in the 5' -> 3' direction by DNA pol III?
Techincally the replication can only occur in a 5'→3' direction.
DNA Pol III adds a DNA nucleotide to the 3' end of the RNA primer and then continues adding DNA nucleotides complementary to the template strand. The two ends of a DNA strand are different, in that they are antiparallel to each other. This directionality is important for the synthesis of DNA, because DNA polymerase can only add nucleotides to the 3’ end of a growing DNA strand (not the 5’ end). Thus, a new DNA strand can only be made in the 5’ to 3’ direction. This is also because the 3' end has a hydroxyl group (-OH) which can form a phosphodiester bond with the 5' end of the next nucleotide on the chain.
The leading strand is synthesizes in the 5' to 3' because of the phosophate groups. 5' end of the dioxyribose sugar has three phosphates that have the energy necessary to form the phosphodiester bond between the two sugars (remember the backbone of DNA is made up of phosphates and sugars). The phosphodiester bond forms by getting rid of two of the three phosphate groups on the 5' (or 5th carbon) of the dioxyribose sugar. The 3' end contains and -OH group that does not contain phosphate groups. Without the phosphate groups there, the bond will not form. So the DNA can not be synthesizes in the 3' to 5' direction because there are no phosophate groups available on the 3' end. This also why the lagging strand (which is synthesized from 3' to 5') has Okazaki fragments allow the DNA strand to be synthesized in the 5' to 3' direction. Phosphate groups are critical to phosphosdiester bond formation.