DNA replication is a process by which DNA molecule is copied to produce two identical DNA molecules; each one is composed of both old and newly synthesized strand. This is why it’s also called a semi-conservative process.
Why Is DNA Replication Necessary?
The information stored in DNA is essential for life. Most cells grow, perform the activities needed to survive, and divide into new cells through the cell cycle. Cell division is of great significance in cell cycle because during which the cell passes its genetic information to its daughter cells.
The division ensures that new cells are available to replace the older cells within an organism whenever those cells die. DNA replication is essential to maintain the number of chromosomes. Without DNA replication, daughter cells would receive half the number of chromosomes.
When Does Replication Occur?
DNA replication occurs during the S-stage of interphase. The cell copies its DNA in preparation for mitosis during interphase. S (Synthesize) phase deals with the synthesis of DNA and doubling of chromosome number.
Essential Tools For DNA Replication:
- DNA polymerase: It adds complementary nucleotides to the growing DNA chain.
- Primer– It is a short single strand of DNA which serves as a starting point. Enzyme primase makes primers which are generally five to ten nucleotides long.
- DNA helicase: It is needed to unwind the DNA.
- Exonuclease: It proof-reads, removes mismatched nucleotides and corrects the error.
- Ligase: It joins DNA strands together & seals the gap.
- DNA helicase unwinds the DNA by breaking the hydrogen bonds that hold the bases which are complementary to each other.
- This creates a Y like structure, which is called a replication fork. The replication fork will move in opposite directions as replication proceeds. Now, these two separated strands act as templates in the making of new DNA strands.
- One of the strands run from 3’ to 5’ which is called as leading strand & another one runs from 5’ to 3’ is called lagging strand. Leading strand replicates continuously (because the DNA polymerase is moving in the same direction as the replication fork) whereas lagging strand replicates in a discontinuous manner (DNA polymerase is moving away from the fork).
- In a continuous replication, primer goes and binds to leading strand acting as a starting point. Then comes, the DNA polymerase that adds on complementary nucleotides in 5’ to 3’ direction.
- In a discontinuous replication, primers bind at various points along the lagging strand. Okazaki segments are then added from 5’ to 3’ direction.
- Once all the bases are matched up, exonuclease proofreads and removes mismatched nucleotides which leads to gaps in between. These gaps are further filled with more complementary nucleotides.
- Proofreading is done again to avoid mismatches. DNA is replicated with the very high fidelity & the accuracy of replication relies heavily on the ability of DNA polymerases to efficiently select correct nucleotides and excise mistakenly incorporated nucleotides using exonucleases. Otherwise, it causes a permanent change in the DNA sequence.
- Further, ligase working as a superglue, sticks the strands, forming a double helix structure. Replication is incomplete with two copies of DNA.
Read More- Surrogacy | Assisted Reproductive Techniques
- A DNA clamp is a protein fold serves as a processivity-promoting factor in DNA replication. It is a critical component of the DNA polymerase III holoenzyme. It binds DNA polymerase and prevents this enzyme from dissociating from the template DNA strand. The interaction between clamp and polymerase is stronger and specific as compared to the direct interactions between the polymerase and the template DNA strand. As one rate-limiting steps in the DNA synthesis reaction is the association of the polymerase with the DNA template. Sliding clamp increases the number of nucleotides which the polymerase can add to the growing strand per event. Sliding camp can highly increase the rate of DNA synthesis which is nearly up to 1,000-fold compared with a non-processive polymerase.
- Topoisomerases participate in the overwinding or underwinding of DNA. The problem arises due to the double-helical nature of DNA. To prevent and correct these types of topological issues which are caused by the double helix nature, topoisomerases bind to DNA and further cut the phosphate backbone of either one or both the DNA strands. This break in between allows the DNA to be untangled and at the end of these processes, the DNA backbone is resealed again.
How Quick Can DNA Replication Occur?
In the prokaryotes, replication can occur at the rate of 1,000 nucleotides per second. Whereas in eukaryotes, DNA replicates at the rate of 50 nucleotides per second. It’s quick because multiple polymerases are synthesizing two new strands at the same time by using each unwound strand from original DNA acting as a template.
Difference between Replication in Prokaryotes and Eukaryotes
- he rate of replication in prokaryotes is higher as compared to eukaryotes.
- The prokaryotic DNA is nucleoid which is DNA protein complex. Thus, replication occurs in the cytoplasm of the cell.
- Although in the case of eukaryotes, replication takes place in the nucleus.
- In prokaryotes, DNA replication being the first step of cell division occurs through binary fission or budding. In eukaryotes, it happens during the synthesis (S) phase.
- The prokaryotic DNA molecules contain a single origin of replication and a single replicon.
- Eukaryotic DNA is comparatively large and is organized into linear chromosomes. As the high amount of material has to be copied, it contains multiple origins of replication on every chromosome. DNA replication can initiate at each origin separately and terminate at the corresponding termination sites.
- Although a similar set of enzymes are involved in prokaryotic and eukaryotic DNA replication. Besides, eukaryotes contain DNA polymerase γ & it is involved in mitochondrial DNA replication. Topoisomerases regulate the winding and unwinding of DNA during the movement of the replication fork. Prokaryotes, generally use type II topoisomerase called DNA gyrase whereas most eukaryotes utilize type I topoisomerases.