• DNA replication is semiconservative.
• This means that when a new double strand is created, it contains one strand from the original DNA, and one newly synthesized strand.
• It is similar for both Prokaryotes and Eukaryotes.
• The process of DNA replication is governed by a group of proteins called a replisome.
DNA Replication Diagram
• It is made up of a number of subcomponents that each provide a specific function during the process of replication.
• Replication doesn’t begin at the end of chromosomes, but toward the middle at a site called the origin of replication.
• A single eukaryotic chromosome contains multiple origins on each chromosome, while replication in prokaryotes usually takes place for a single origin on the circular chromosome.
• From the origin, two replisomes proceed in opposite directions along the chromosome making replication a bidirectional process.
• The point where a replisome is attached to the chromosome is called the replication fork.
DNA Replication Fork
• Each chromosome of eukaryotic DNA is replicated in many discrete segments called replication units or replicons.
• A replicon is a DNA molecule or RNA molecule, or a region of DNA or RNA, that replicates from a single origin of replication.
• For most prokaryotic chromosomes, the replicon is the entire chromosome.
• For eukaryotic chromosomes, there are multiple replicons per chromosome.
• The definition of replicons is somewhat confused with mitochondria, as they use unidirectional replication with two separate origins.
"Exonucleases are enzymes that work by cleaving nucleotides one at a time from the end of a polynucleotide chain"
• DNA helicase uses the energy of ATP hydrolysis to actively unwind the two strands.
• DNA polymerase, the enzyme that builds the new DNA, cannot initiate a strand from two nucleotides, but can only add nucleotides to an existing strand.
• Therefore it requires an RNA primer to get started.
• It reads the parental strand in the 3’ → 5’ direction.
• Since it can only add new subunits to the 3’ end of the chain, it must create the new complementary strand in the 5’ → 3’ direction
• Besides being a polymerase, one of the subunits in DNA polymerase has 3’ → 5’ exonuclease (it removes nucleotides from the strand) capabilities.
• Exonucleases are enzymes that work by cleaving nucleotides one at a time from the end of a polynucleotide chain
• There are 3 types of polymerase molecules This enzyme automatically proofreads each new strand, and makes repairs when it discovers any mismatched nucleotides.
• Primase, an RNA polymerase, creates an RNA primer approximately 10 ribonucleotides long to initiate the strand.
• Each nucleotide added to the new strand requires the removal of a pyrophosphate group (two phosphates binded together) from a DNTP (deoxynucleotide triphosphate).
• Some of this energy derived from the hydrolysis of the pyrophosphate is used to drive replication.
• SSB tetramer proteins (also called helix destabilizer proteins) prevent the two strands from reattaching after the helix is opened.
• The interrupted strand is called the lagging strand; the continuous new strand is called the leading strand.
• The lagging strand is made from a series of disconnected strands called Okazaki fragments.
• Okazaki fragments are about 100-200 nucleotides long in eukaryotes and about 1000-2000 nucleotides long in prokaryotes.
"SSB tetramer proteins (also called helix destabilizer proteins) prevent the two strands from reattaching after the helix is opened"
• DNA Ligase moves along the lagging strand and ties the Okazaki fragments together to complete the polymer.
• Since the formation of one strand is continuous and the other fragmented, the process of replication is said to be semidiscontinuous.
• The ends of eukaryotic chromosomal DNA possess telomeres.
• Telomeres are repeated 6 nucleotide units from 100-1000 units long that protect the chromosomes from being eroded through repeated rounds of replication.
• Telomerase catalyze the lengthening of telomeres at the 3’ ends of DNA strands.
• Eukaryotic chromosomes contain linear DNA molecules; Prokaryotic chromosomes are usually circular.
• So telomeres aren’t required by Prokaryotes that have circular DNA.
DNA Replication Citations
- Exploiting DNA Replication Stress for Cancer Treatment. Cancer Res . 2019 Apr 15;79(8):1730-1739.
- DNA replication origins-where do we begin? Genes Dev . 2016 Aug 1;30(15):1683-97.
- Mechanisms of DNA replication termination. Nat Rev Mol Cell Biol . 2017 Aug;18(8):507-516.
- Origins of DNA replication. PLoS Genet . 2019 Sep 12;15(9):e1008320.