DNA REPLICATION

• DNA replication is the biological process of producing two identical replicas of DNA from on original DNA molecule.

• DNA is made up of a double helix of two complementary strands.

DNA Replication is Semi-Conservative

• DNA replication of one helix of DNA results in two identical helices. If the original DNA helix is called the "parental" DNA, the two resulting helices can be called "daughter" helices.

• DNA creates "daughters" by using the parental strands of DNA as a template or guide. 

• Each newly synthesized strand of DNA (daughter strand) is made by the addition of a nucleotide that is complementary to the parent strand of DNA.

• In this way, DNA replication is semi-conservative, meaning that one parent strand is always passed on to the daughter helix of DNA.

The Semi-Conservative Nature of DNA Replication

DNA Replication Process

The Three Steps of DNA Replication

Step 1 : Initiation

• The first step in DNA replication is the separation of the two DNA strands that make up the helix that is to be copied. 

• DNA Helicase untwists or “unzipped” the helix at locations called replication origins. 

• The replication origin forms a Y shape, and is called a replication fork. 

• The replication fork moves down the DNA strand, usually from an internal location to the strand's end. 

• The result is that every replication fork has a twin replication fork, moving in the opposite direction from that same internal location to the strand's opposite end.

• Single-stranded binding proteins (SSB) work with helicase to keep the parental DNA helix unwound. 

• It works by coating the unwound strands with rigid subunits of SSB that keep the strands from snapping back together in a helix. 

• The SSB subunits coat the single-strands of DNA in a way as not to cover the bases, allowing the DNA to remain available for base-pairing with the newly synthesized daughter strands.

• The cell prepares for the next step, elongation, by creating short sequences of RNA called primers that provide a starting point of elongation.

Replication fork 

Step 2 : Elongation

• Enzyme DNA polymerases are responsible creating the new strand by a process called elongation. 

• Five different known types of DNA polymerases in bacteria and human cells which is polymerase I, II, III, IV and V. 

• Polymerase III is the main replication enzyme, while polymerase I, II, IV and V are responsible for error checking and repair. 

• The enzyme DNA polymerase controls elongation, which can occur only in the leading direction.

• DNA is directional in both strands, signified by a 5' and 3' end. This notation signifies which side group is attached the DNA backbone. 

• The 5' end has a phosphate (P) group attached, while the 3' end has a hydroxyl (OH) group attached. 

• This directionality is important for replication as it only progresses in the 5' to 3' direction.

• However, the replication fork is bi-directional; as in the picture, the leading strand is synthesized continuously in the 5’ to 3’ direction toward the replication fork while lagging strand is synthesized discontinuously (Okazaki fragments) also in the 5’ to 3’ direction, but away from the replication fork. 

DNA Replication : Elongation

Step 3 : Termination

• Once all of the bases are matched up (A with T, C with G), an enzyme called exonuclease strips away the primer(s). 

• Then, these primers are replaced with appropriate bases. Another exonuclease “proofreads” the newly formed DNA to check, remove and replace any errors. 

• Another enzyme called DNA ligase joins Okazaki fragments together forming a single unified strand. 

• The ends of the linear DNA present a problem as DNA polymerase can only add nucleotides in the 5′ to 3′ direction.

• The ends of the parent strands consist of repeated DNA sequences called telomeres. Telomeres act as protective caps at the end of chromosomes to prevent nearby chromosomes from fusing. 

• A special type of DNA polymerase enzyme called telomerase catalyzes the synthesis of telomere sequences at the ends of the DNA. 

• Once completed, the parent strand and its complementary DNA strand coils into the familiar double helix shape. 

• In the end, replication produces two DNA molecules, each with one strand from the parent molecule and one new strand.

DNA Replication : Initiation, Elongation, and Termination Steps

Replication Enzymes

DNA replication would not occur without enzymes that catalyze various steps in the process. Enzymes that participate in the eukaryotic DNA replication process include:

DNA Helicase

• unwinds and separates double stranded DNA as it moves along the DNA. It forms the replication fork by breaking hydrogen bonds between nucleotide pairs in DNA.

DNA Primase

• a type of RNA polymerase that generates RNA primers. Primers are short RNA molecules that act as templates for the starting point of DNA replication.

DNA Polymerases

• synthesize new DNA molecules by adding nucleotides to leading and lagging DNA strands. 

• 5 type of DNA polymerase :

          🍀DNA-Pol I: repair and patching of DNA
          🍀DNA-Pol III: responsible for the polymerization of the newly formed DNA strand
          🍀DNA-Pol II, IV, and V: proofreading and repair enzymes

Topoisomerase or DNA Gyrase

• unwinds and rewinds DNA strands to prevent the DNA from becoming tangled or supercoiled.

DNA Ligase

• joins DNA fragments together by forming phosphodiester bonds between nucleotides.

Exonucleases

• group of enzymes that remove nucleotide bases from the end of a DNA chain.