DNA Replication

• 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.