The structure of DNA allows it to carry out two vital functions for the cell :

  • Encoding the information need to build and regulate the cell, and
  • Transmission of this information from generation to generation

In order for the genetic information to be passed on, it must be copied. DNA replication occurs during the S (synthesis) phase of the cell cycle. It only proceeds if the G1 checkpoint is passed, which ensures that the chromosomes have properly segregated during mitosis.

In simple terms, DNA replication involves the separation of the two strands of the DNA molecule and the construction of complementary strands for each one, using the A - T, G - C binding rules.

Simplified diagram of base matching for DNA replication

Because the two new strands of DNA each contain one of the original parental strands, the process of DNA replication is said to be semi-conservative (ie. half of the new DNA molecule are strands “saved” from the parental molecule).

Naturally, the process of replication is a more complicated process than simply matching nucleotide bases. Copying DNA involves the interplay of a series of enzymes and regulatory processes, all kept in check by stringent error checking and repair mechanisms.

DNA replication begins when the enzyme helicase “unwinds” a small portion of the DNA helix, separating the two strands. This point of separation is called the replication fork. The two strands are kept separated by single stranded binding proteins (SSB) which bind onto each of the strands. A group of enzymes called the DNA polymerases are responsible for creating the new DNA strand, however they cannot start the new strand off, only extend the end of a pre-existing strand. Therefore, before the DNA polymerases can start synthesizing the new strand, the enzyme primase attaches a short (~60 nucleotides) sequence of RNA called a primer. The DNA polymerases then extend this primer, moving along each strand from the 3’ end to the 5’ end and adding nucleotides to the 3’ hydroxyl group of the previous nucleotide base. The order of nucleotides is retained by matching complementary nucleotides on the template strand.

Complex representation of DNA replication, showing actions of enzymes, leading and lagging strands and Okazaki fragments

 

It’s important to realize that the polymerases can only operate in one direction. This works out for one of the DNA strands (the leading strand) – the polymerase moves along the strand in the same direction as the replication fork. However the other strand (the lagging strand) runs in the opposite direction. As a result the complementary strand to the lagging strand is made in short sections called Okazaki fragments. These sections are then later joined together by the enzyme DNA ligase.

Once the complementary strand of DNA has been synthesized, the primers are removed by the enzyme RNAse H and the remaining gaps filled with lengths of DNA by DNA polymerase.

Some excellent animations of DNA replication can be found here :

  • Interactive Concepts in Biochemistry has a tutorial which takes you through the process step by step (note : these open to external websites).
  • This animation (via www.FreeScienceLectures.com) is a computer generated movie showing what the process would look like on a molecular level (note : this opens to an external website and is a You Tube video, which may be unavailable to some users).