Why does DNA duplicate?

DNA is the genetic material that defines each cell. Before a cell is duplicated and divided into new daughter cells through mitosis or meiosis, biomolecules, and organelles must be copied to be distributed among the cells.

DNA, which is found within the nucleus, must be repeated to ensure that each new cell receives the correct number of chromosomes. The process of DNA duplication is called DNA replication.

Replication follows several steps involving various proteins called replication enzymes and RNA. In eukaryotic cells, such as animal cells and plant cells, DNA replication occurs in the S phase of interphase during the cell cycle. The process of DNA replication is essential for cell growth, repair, and reproduction in organisms.

Mechanisms for DNA Replication

1. Semiconservative Replication: Semiconservatively, i.e. the double helix band of old DNA molecules will open with the help of enzymes. Then in each old DNA band, a new DNA band is formed. (50% parental DNA & 50% Daughter DNA)

2. Conservative Replication: Conservatively, the molecules of the old DNA remain or do not open, then in addition to each old DNA molecule a new DNA molecule is formed. (100% similar to parental DNA)

3. Dispersive Replication :
Dispersive, i.e. DNA molecule is broken into several parts, then in each piece new DNA is formed.

DNA replication will not take place without enzymes that catalyze different steps in the process. Enzymes that participate in the eukaryotic DNA replication process include:

1. DNA helicase – unwraps 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.
2. DNA primase – a type of RNA polymerase that generates RNA primers. Primers are short RNA molecules that serve as templates for the starting point of DNA replication.
3. DNA polymerase – Synthesizes new DNA molecules by adding nucleotides to DNA strands that lead and fall.
4. Topoisomerase or DNA Gyrase – Extract and rewind DNA strands to prevent the DNA from becoming entangled or supercoiled.
5. Exonucleases – a group of enzymes that remove nucleotide bases from the end of a DNA chain.
6. DNA ligase – link DNA fragments together by forming phosphodiester bonds between nucleotides.

Process of replication

Before DNA can be replicated, the double-stranded molecule must be “unlocked” into two single strands. DNA has four bases called adenine (A), thymine (T), cytosine (C), and guanine (G) that form pairs between the two strands. Adenine alone pairs with thymine and cytosine bind only with guanine. To relax DNA, these interactions between base pairs must be broken. It is carried out by an enzyme known as DNA helicase. DNA helicase disrupts the hydrogen bond between base pairs to separate the strands into a Y-form known as the replication fork. This area will be the template for replication to get started.

DNA is directional in both strands, indicated by 5 ‘and 3’ ends. This notation indicates which side group is the DNA backbone. The 5 ‘end attached a phosphate (P) group, while the 3’ end attached a hydroxyl (OH) group. This direction is important for replication as it only progresses in the 5 ‘to 3’ direction. However, the replication fork is bidirectional; one strand is oriented in the 3 ‘to 5’ direction (front strand), while the other 5 ‘to 3’ (posterior strand) is oriented. The two sides are thus replicated with two different processes to accommodate the direction difference.

Initiation of Replication
Step 1: Primer Binding
The front string is the simplest to repeat. Once the DNA strands are separated, a short piece of RNA binds a primer to the 3 ‘end of the strand. The primer always binds as the starting point for replication. Primers are generated by the enzyme DNA primase.

Extension/Elongation of Replication
Step 2: Extension
Enzymes known as DNA polymerases are responsible for creating the new strand through a process called elongation. There are five different known types of DNA polymerase in bacteria and human cells. In bacteria such as E. coli, polymerase III is the major replication enzyme, while polymerases I, II, IV, and V are responsible for error control and repair. DNA polymerase III binds to the strand at the site of the primer and begins with the addition of new base pairs that are complementary to the strand during replication. In eukaryotic cells, polymerase alpha, delta, and epsilon are the primary polymerases involved in DNA replication. As replication moves in the 5 ‘to 3’ direction on the leading strand, the newly formed strand is continuous.

The underlying strand begins replication by binding with multiple primers. Each primer is only a few bases apart. DNA polymerase then adds pieces of DNA, Okazaki fragments, to the strand between primers. This process of replication is discontinued as the newly created fragments are incompatible.

Termination of replication
Step 3: Termination
Once both the continuous and discontinuous strands are formed, an enzyme called exonuclease removes all RNA primers from the original strands. These primers are then replaced with appropriate bases. Another exonuclease “proofreads” the newly formed DNA to check for, remove and replace any defects. Another enzyme called DNA ligase links Okazaki fragments together to form a single unified strand. The ends of the linear DNA present a problem since 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 end of the DNA. Once completed, the parent strand and the complementary DNA strand roll into the known double helix form. In the end, replication produces two DNA molecules, each with one strand of the older molecule and one new strand.

DNA Replication Summary
DNA replication is the production of identical DNA  from a single double-stranded DNA molecule. Each molecule consists of a strand of the original molecule and a newly formed strand. Before replication, the DNA uncoils and strands separate separately. A replication fork is formed that serves as a template for replication. Primers bind to the DNA and DNA polymerase adds a new nucleotide sequence in the 5 ‘to 3’ direction. This addition is continuous in the front string and fragmented in the back strand. Once elongation of the DNA strands is complete, the strands are checked for defects, repairs are made, and telomere sequences are added at the ends of the DNA.

Read : RNA : Structure, types & Functions

Frequently Asked Questions

1. What is DNA
Answer: DNA is a molecule in which the genetic code of any human being and almost all animals is present. DNA is also present in animals, plants, protists, archaea, and bacteria. DNA is in every cell of every organism and this determines what proteins the cells will make. Enzymes are produced in maximum quantity in proteins made by the cell. This is the reason why many symptoms of parents occur in children, such as skin color, hairstyle, and eye color.

2. Who Discovered DNA
Answer: DNA was discovered in 1950 AD by American biologist James Watson (James Watson) and English physicist Francis Crick (Francis Crick). It was first discovered in 1869 AD by German biochemist Frederich Miescher.

But for many years researchers did not realize the importance of this molecule at all. But this was the case only until 1953, until James Watson, Francis Crick, Morris Wilkins, and Rosland Franklin discovered the structure of DNA. This structure was in the form of a double helix which he felt could carry biological information.

Molecular structure (molecular structure) of nucleic acids Watson Crick and Wilkins were awarded the 1962 Nobel Prize in the field of medicine for their discoveries about its importance for the transfer of information between organisms. Franklin was not included in the award, although his work was separate from research.

By the way, a lot of differences are found among the people, who have discovered it. We have also tried to give you information about who in reality gave information about it for the first time and then who made the search for it mainly usable.

3. What are the Function of DNA 
Answer :

1. DNA stores the information needed to make and control a cell.
2. The transfer of information from mother to daughter, which we also know as gene transfer, is through the DNA replication process.
3. DNA replication occurs when a cell makes a duplicate copy of its DNA and the cell is folded, resulting in the correct distribution of a DNA copy in each cell.
4. DNA can also be chemically degraded and used in this form of nucleosides and nucleotides for the cell. Unlike other macromolecules, DNA does not play a structural role in cells.