Deoxyribonucleic acid (DNA) and ribonucleic acid (RNA) are two examples of nucleic acids. These macromolecules, along with proteins, complex carbohydrates, and lipids, are necessary for organisms. DNA is essential because it contains all of the genetic information for every life form, but what about DNA replication?
In order for cells to divide and multiply, the cells must first replicate the original DNA. As DNA contains the genetic blueprint for each organism, the process of DNA replication involves copying each strand of DNA as flawlessly as possible. Each replicated strand of DNA contains genetic directions for development, organ functions, and growth so that an organism can thrive.
In this article, we will explore DNA at a deeper level to understand its structure and functions. We will then cover the steps of DNA replication and the purpose of the process.
What Is DNA?
DNA is a nucleic acid, one of four types of macromolecules found in living organisms. It is composed of millions of pairs of nucleotides, twisted into a double helix. Learning about the building blocks of DNA will help one understand DNA replication.
DNA Structural Findings
Several groups of people discovered the structure and shape of DNA. Rosalind Franklin and Maurice Wilkins put together the original basis of work on the double helix theory, laying down the foundation for the next group of scientists.
James Watson and Francis Crick combined Franklin and Wilkins’ research with their own findings to solidify the fact that DNA consists of nucleotides twisted around to create a double helix shape.
They also determined that nucleotides contain two distinct patterns of sugar and phosphate groups, connected by pairs of bases. Watson and Crick won the Nobel Prize in 1962 for their work with DNA and nucleotides (source).
Structure of DNA
The word deoxyribonucleic gives clues as to what exactly DNA is. While the –nucleic part of deoxyribonucleic refers to nucleic acid, the suffix –deoxyribo describes deoxyribose, the 5-carbon sugar found in DNA.
The nucleotides that comprise DNA contain deoxyribose, a phosphate group, and one of four bases.
The phosphate group and sugar alternate in the double helix shape. The phosphate group from one nucleotide attaches to the sugar from another nucleotide, and the pattern continues each step of the way in the DNA molecule. This pattern forms the backbone of the double helix.
The double helix has two chains. The chains hold the molecule together as the rungs of the ladder climb up and down the molecule. Each of these rungs is a pair of bases, and the bases include adenine, thymine, cytosine, and guanine.
The bases meet in the middle of each rung and are held together by a hydrogen bond. However, the bases pair together in specific ways — adenine pairs only with thymine, and cytosine pairs only with guanine (source).
Adenine and thymine link together with three hydrogen bonds, while cytosine and guanine combine with two.
The sequence of the base pairs is essential to the genetic code because they come together to form directions or genes for each DNA molecule. The genes can then tell each cell what to do to help an organism develop and survive.
In order to perform the actions necessary for these tasks, the genes help produce proteins to do the work. Genes must access the information stored in the DNA to produce the correct proteins to do each job. Other genes contain instructions for making molecules that are in charge of cellular processes.
The structure of DNA is key to understanding DNA processes, such as replication. The chromosomes DNA forms contain the genetic material for every living thing.
There must be copies of the chromosomes before they can divide and form new cells. These daughter cells will then have a full set of chromosomes once paired with new, complementary bases.
During the DNA replication process, the hydrogen bonds holding the base pairs together break apart, and the double helix separates into single strands.
This process requires an outside energy source to break the bonds before replication, making it an example of an endergonic reaction. For more information on endergonic reactions, see our article, “Exergonic vs. Exothermic: Different Energy Reactions.”
Each DNA molecule contains a full set of bases to make a chromosome. Once the double helix unwinds, every single strand contains half of a full set of bases. In order for each strand to have a complete set, the bases of the single strands need to pair with complementary bases.
The bases of the single strands combine with new bases to create pairs, which are exact replicas of the original DNA. Each new DNA molecule now contains a full set of bases to create a chromosome, copying all of the genetic information from the original molecule.
Scientists refer to DNA replication as being semi-conservative because one strand remains the same in each new molecule.
Steps of Replication
The process of DNA replication requires certain enzymes. These enzymes work in conjunction with the sugar and phosphate groups found in DNA molecules as well as RNA. Knowing the steps of replication can help one understand the roles of the various parts.
Step 1: Initiation
The DNA double helix is “unzipped” as the hydrogen bonds between the base pairs break up. The enzyme DNA helicase moves along the molecule. It breaks the hydrogen bonds between the bases, allowing the double helix to separate, or unwind, into two single strands.
Scientists call the place on the double helix where the process begins the origin of replication.
As the double helix unzips from the point of origin, it creates a Y-shape called the replication fork. The single strands on either side, called the leading strand and lagging strand, are the templates used to create the new strands.
The replication fork is bidirectional, meaning the new strands move both towards and away from the fork. The leading strand replicates moving towards the fork, while the lagging strand replicates moving away from the fork.
The strands also synthesize in various methods. Leading strands add nucleotides one-by-one. A small piece of RNA called a primer attaches to the end of the strand as an anchor. This primer results from the enzyme primase and serves as the starting point for replication.
Lagging strands only replicate nucleotides in chunks and require a lot more primers. As this is the case, they also need small pieces of complementary DNA to attach to the end of the primers. DNA scientists call these fragments of DNA Okazaki fragments.
Step 2: Elongation
Once the leading and lagging strands have their primers, the next step can begin. Enzymes called DNA polymerases remove the primers from the strands. The polymerase takes the place of the primers. It gets to work pulling complementary bases and attaching them to the existing bases.
Since the leading strands add base pairs one by one, the scientists consider the elongation process continuous. The lagging strands replicate more sporadically, and scientists, therefore, consider these non-continuous.
Another polymerase checks the new DNA strands to ensure there are no issues. If need be, they correct errors and put replacements in place.
DNA ligase then joins the Okazaki pieces on the lagging strand together to form a connected, synthesized strand, complementary base pairs, or nucleotides.
Step 3: Termination
Before the process is complete, the synthesis of the DNA ends, telomeres, must occur. These telomeres consist of repetitive base-pair sequences, which create a cap of protection to keep the molecule safe. Another molecule called telomerase jumpstarts the synthesis.
Once all of the bases have paired up, the original DNA strand coils up with the complementary strand to create the double helix shape. The termination concludes the replication process by producing two daughter DNA molecules from the parent’s original DNA and the new strands (source).
Key Components of Replication
As a review of DNA replication, below are the definitions for the key parts and enzymes needed for the process:
Helicase: the enzyme responsible for unwinding and separating the DNA double helix and breaking the hydrogen bonds between base pairs in the original DNA molecule to start the process of replication.
Origin of replication: the place at which replication starts by helicase.
Replication fork: the Y-shape of the unzipped, original DNA molecule strands.
Leading strand: the single strand of DNA that replicates moving toward the origin and continuously adds bases.
Lagging strand: the single strand of DNA that replicates moving away from the origin and adds bases in chunks.
Primer: a short strand of RNA that attaches to the end of each strand.
Primase: the enzyme responsible for attaching the primer.
Okazaki fragments: short strands of DNA added to the primers at the ends of the lagging strands.
Polymerase: the enzyme responsible for removing the primers from the strands and replacing them with complementary bases.
Telomeres: the ends of the DNA strands that create a cap of protection for the DNA.
Telomerase: the enzyme responsible for synthesizing telomeres.
Significance of DNA Replication
DNA must replicate or make a copy before cell division. The parent cell needs to keep its original DNA while also passing it along to the daughter cells. With each cell division, more and more molecules of DNA form. The original molecule contains all of the DNA needed to make all of the other cells.
When DNA replicates, an exact copy must be present each and every time to make sure there aren’t any problems with cells down the line. If the polymerase responsible for checking the DNA misses an imperfection, there can be exponential effects.
Issues with Replication
Errors in DNA replication occur much more frequently than one might imagine. DNA proofreading and repair mechanisms resolve the majority of the issues quickly.
However, these protective measures can fail. If they do not catch and fix the errors in DNA, the mutations can pass onto the next set of cells and become more serious as they grow through cell division.
Mutations in DNA can cause the formation of mutated proteins. If these mutations cause a change in the function of the proteins, the proteins can then cause serious diseases.
One such serious disease caused by DNA replication issues is cancer. With cancer, cells grow and multiply uncontrollably. Mutations in the DNA cause mutations in the proteins regulating cell division. These mutations accumulate over time and can eventually cause cancer.
Proofreading and Mismatch Repair
Proofreading is one way the DNA polymerases fix errors that occur during DNA replication. As each base attaches to the existing DNA strand, the polymerase checks to ensure it is complimentary.
If the polymerase finds an incorrect base, it removes the base and replaces it with the correct base before continuing the process.
If there is an error with pairing bases that it does not correct, mismatch repair will happen. Once the new DNA forms, a group of proteins, enzymes, and polymerases work together to fix the issue.
In mismatch repair, a group of proteins detect a problem with a base pairing, and they attach to the pair.
Complex proteins and enzymes cut out the base and surrounding DNA to remove the mispairing and its potential effects. The DNA polymerases then repair the DNA by inserting the correct bases, making the strand once again continuous.
Additional Repair Mechanisms
When proofreading and mismatch repair fails, there are other repair mechanisms put in place. These repair mechanisms fix any issues that occurred in DNA replication or synthesis but also those resulting from outside sources. Two such mechanisms are direct reversal and excision repair. This article was written for strategiesforparents.com.
Direct reversal refers to doing just that. Enzymes within the DNA cell can mitigate and erase the damage caused by mutations. Excision repair involves removing and replacing the problem area. The process may remove either the base or an entire section of DNA to fix the damage (source).
Even when following all of the steps correctly, mistakes in replication can still occur. Organisms have mechanisms in place to catch as many issues as possible before permanent mutations occur.
DNA replication is essential for survival and continued development. Without it, cells would not be able to grow and continue to meet the needs of the organism.