Course: SAT 170 Introductory Biotechnology

Unit 3 Reading and Multimedia Assignment

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Page 10 of 13
Elongation of Growing DNA Strand

Each strand of DNA has directionality and requires replication to move in opposite directions, or antiparallel, to the parent strand.

DNA strands are antiparallel, meaning that they are oriented in opposite directions. The different ends are identified by the carbon to which the adjacent deoxyribose sugar is attached. Each strand has a 5' end and a 3' end and they oppose each other in the double helix molecule. DNA polymerase synthesizes a new strand of DNA by extending the 3' end of an existing nucleotide chain, adding new nucleotides matched to the template strand one at a time via the creation of phosphodiester bonds. Thus, DNA is synthesized only in the 5' to 3' direction, allowing antiparallel elongation. The energy for this process of DNA polymerization comes from two of the three total phosphates attached to each unincorporated base. When a nucleotide is being added to a growing DNA strand, two of the phosphates are removed and the energy produced creates a phosphodiester bond that attaches the remaining phosphate to the growing chain. The energetics of this process also help explain the directionality of synthesis. If DNA were synthesized in the 3' to 5' direction, the energy for the process would come from the 5' end of the growing strand rather than from free nucleotides.

The leading strand is the template strand of the DNA double helix on which the replication fork moves in the 3' to 5' direction, as shown in the above figure. This allows the new strand to be synthesized 5' to 3' in the same direction as the movement of the replication fork. On the leading strand, a polymerase reads the DNA and continuously adds nucleotides to it.

DNA polymerase reads in the 3' to 5' direction, therefore nucleotides are synthesized (or attached to the template strand) in the 5' to 3' direction; however, one of the parent strands of DNA is 3' to 5' while the other is 5' to 3'. To solve this problem, replication occurs in opposite directions. Heading towards the replication fork, the leading strand is synthesized in a continuous fashion, only requiring one primer. On the other hand, the lagging strand, heading away from the replication fork, is synthesized in a series of short fragments known as Okazaki fragments, consequently requiring many primers.