DNA Replication during cell division in eukaryotic cells, the replicated genetic material is divided equally between two daughter cells. it is important that each cell gets an exact copy of the parent cell’s DNA. in replicating, the DNA double helix unwinds (H-bonds between the strands are broken) and each single strand acts as a template for a new strand.

The Big Picture

Semi-Conservative Replication DNA replicates semi- conservatively (each daughter strand contains 1 parent strand and 1 new strand). much of our understanding of the process has come from studying prokaryotic systems (like E. coli).

Starting Replication replication begins when proteins -DNA helicase, DNA gyrase and single stranded binding proteins) bind to a certain site on the DNA called the replication origin.

Replication Bubble the circular DNA in prokaryotes will have 1 replication origin, while in eukaryotes, there are may be several of these along a given stretch of DNA.

DNAP III replication of the DNA molecule proceeds in a 5’ to 3’ direction along each strand, thus, in opposite directions. nucleotides are added by an enzyme DNA Polymerase III, which adds dNTPs (deoxyribonucleoside triphosphates to a free 3’ end.

Adding Nucleotides DNA polymerase III needs a free 3’ end to begin, so another enzyme, primase lays down a base pair stretch of RNA primer to allow DNAP III to start.

Adding dNTPs note that the dNTPs contain 3 phosphates, while the nucleotides only contain 1. DNAP III breaks the bond between phosphates and uses the energy released to add the base to the growing strand.

Leading vs. Lagging So, one strand (the leading strand) is synthesized continuously in the 5’-3’ direction, while the other (the lagging strand) is synthesized discontinuously in the 5’- 3’ direction. lagging strand synthesis works by having RNA primers repeatedly added as the replication fork moves along.

Okazaki Fragments DNAP III forms small stretches ( base pairs long in eukaryotes) of new sequence called Okazaki fragments in the 5’-3’ direction. DNA Polymerase I removes the RNA primers from both the leading and lagging strands and replaces them with the correct dNTPs. the Okazaki fragments are joined together by another enzyme DNA ligase by creating a phosphodiester bond. as the new strands are made, two double stranded DNA molecules automatically re- form the double helix.

Quality Control because it is so important to ensure that an exact copy of the parent DNA strand is made, both DNAP I and III have a proofreading capability built in. both enzymes have exonuclease activity, that is they can remove a nucleotide added in error and replace it with the correct one.

Overall….