Everything about DNA Replication
Cell division is a constant process. On the skin, it occurs very often because new skin layers must be constantly reproduced to replace the lost ones. On the brain and nervous system, cell division also occurs, though it takes place much less frequently than skin cell division. Whenever a parent cell divides, the daughter cells will inherit everything that the parent cell has, including a complete set of DNA molecules. Cell division thus needs to be accompanied by the duplication of DNA’s double helix polynucleotide strands with all hydrogen bonds that bind them. This duplication process of DNA is called DNA replication and DNA replication is carried out by a series of enzymes.
Duplication or Replication
Before going deeper into the concept of DNA replication, it is important to understand the term first. Why the process is called DNA replication instead of duplication? Although there is little difference between DNA duplication and DNA replication and although both terms are often used interchangeably, DNA replication is the accepted term for the process of the formation of the daughter DNA double-helix strands from each of the parent DNA strands.
How does DNA replication differ from DNA duplication? DNA replication is the process of the production of DNA replica by which the daughter DNA is the similar exact copy of the parent DNA. The replication process allows the daughter DNA to retain one strand of the parent DNA, ensuring that the daughter DNA’s sequence is perfectly similar to that of the parent DNA.
How can the daughter DNA have perfectly the same sequence with that of the parent DNA? The process of DNA replication can occur accurately with high level of fidelity because DNA replication is carried out by a series of enzymes that proofread each stage of the replication process. A family of enzymes, called polymerases, are especially scrupulous when it comes to proofreading the replication process. They are always on the alert of any possible anomaly in the replication process, preventing DNA damage from ever occurring during the replication process.
DNA duplication, on the other hand, is the doubling of DNA with the duplicates having different sequence from that of the original DNA. Because they have differing sequence, they also have different function. DNA duplication plays a core role in evolution whereby DNA duplicates and multiplies with different sequences and functions but only the fittest prevails.
DNA Replication as a Semiconservative Replication
The process of replication by which the daughter DNA retains one strand of the parent DNA is referred to as a semiconservative replication. There are two other models of DNA replication process that were ever proposed: conservative and dispersive replications. The three models of DNA replication can be explained in brief as follows.
The daughter DNA inherits one of the two strands of the parent DNA. The retained strand is then used as a template for the formation of its match, producing a full formation of double-helix DNA strands.
The daughter DNA retains both strands of the parent DNA. The retained two strands will then be copied to produce another full double-helix DNA strands.
Two sets of daughter DNA strands will be produced, each of which retain certain segments of the parent DNA strands. The replication process will continue by completing the full sequence of the daughter DNAs based on the segments retained from their parent.
The conservative and dispersive replication models were eventually disproved and proven to be incorrect. The semiconservative replication process is considered the most intuitively appealing because this process involves the separation of the parent DNA strands into two distinct strands, each of which carries all of the information of the parent DNA that makes each strand a reliable template for the formation of full daughter DNA strands. Because of the use of this template and because DNA replication is carried out by a series of highly alert enzymes, the accuracy of the replication process can be guaranteed.
How Does DNA Replication Occur?
DNA replication process starts with the unzipping of the double-helix parent DNA strands. The unzipping process is done by an enzyme called helicase, which breaks down the hydrogen bonds that bind the complementary bases of the DNA strands. The binding of these complementary bases follows a specific pattern, i.e. A with T and C with G. This pattern will be used later when the separated strand becomes a template for the synthesis of new DNA.
This unzipping process doesn’t continue with the synthesis of new DNA strands because the catalyzing enzymes have no capability for such synthesis. Instead, it continues with the extension of one of the existing DNA strand. DNA replication is carried out by a series of enzymes called polymerases. These enzymes are responsible for this extension process. They don’t create new strands, but catalyze the pairing of DNA template with a DNA or RNA primer, which becomes the starting point for the DNA synthesis.
Starting from the primer, the DNA polymerase will bind the two strands by adding new complementary nucleotide bases along the strands. The pattern explained above (A with T and C with G) is used in the addition process. There will then be two new strands of DNA produced: leading strand which is continuous and lagging strand which is discontinuous. Leading strand is the strand that is synthesized along the direction similar to that of the growing replication fork and lagging strand is the strand that is synthesized along the direction opposite to that of the growing replication fork.
After all of the complementary bases are bound, the primer is stripped by an enzyme called exonuclease. The left gaps will then be filled with another set of complementary bases. To make sure that all bases are bound correctly, DNA polymerase will proofread and verify the correctness of the entire formation.
The last stage of DNA replication process is the sealing up of the DNA sequence. This is done by an enzyme called DNA ligase. This explanation should give you a big picture of how DNA replication is carried out by a series of enzymes that guarantee the accuracy of the process.