DNA Replication – Definition, Process, Steps, & Labeled Diagram (2024)

DNA replication is the process through which a cell’s DNA forms two exact copies of itself. It occurs in all living organisms as it forms the basis of inheritance in all living organisms. The enzyme carrying out transcription is called DNA polymerase.

The primary mechanism of DNA replication is similar across all organisms. Here, we will focus on DNA replication as it takes place in the bacterium E. coli. However, the procedure is the same in humans and other eukaryotes.

Why is DNA Replication such an Important Process

DNA replication’s primary purpose is to enable living organisms to reproduce. The only way to replace the cells is to copy the cell’s information. It is what DNA replication does. After cell division by mitosis or meiosis, the two daughter cells must contain the same genetic information, or DNA, as the parent cell. Without the accurate copying of the DNA, life would cease to continue as existing organisms would not be able to reproduce.

Role of DNA Polymerase in DNA Replication

DNA polymerase is the main enzyme that carries out DNA replication. It adds nucleotides one at a time to the growing DNA strand. The nucleotides added are arranged such that they are complementary to the template.

Some key features of DNA polymerase are:

  • Needs a template strand to copy
  • Adds nucleotides (ATP, GTP, CTP) to the 3′ end of a DNA strand one at a time
  • Requires a pre-existing chain or short stretch of nucleotides called an RNA primer
  • Removes wrongly added nucleotides by proofreading the activity
  • Adds 1,000 nucleotides per second in E. coli and 50 nucleotides per second in humans

In E. coli, two main DNA polymerases are involved in DNA replication: DNA pol III and DNA pol I.

When and Where does DNA Replication Occur

It happens in the S phase of cell division. DNA must replicate before cells divide so that the two resulting daughter cells contain the same genetic information as the parent cell.

DNA replication occurs in the nucleus of all eukaryotic cells and the nucleoid of prokaryotes.

How is DNA Replicated

Replication happens in three fundamental stages. They are described below in order:

Step 1: Initiation

Unzipping of DNA

DNA replication starts at a particular location on the DNA, called the origin of replication. It is the region where the DNA is unzipped. They have a specific sequence covering about 245 base pairs, mostly A/T base pairs and fewer GT-base pairs. The fewer hydrogen bonds in the AT-rich sequence make the DNA strands separate easily.

Here, the initiator protein, helicase, recognize and then binds to the origin to unwind the DNA. When the DNA unwinds, two Y-shaped structures called replication forks are formed, together making up the replication bubble. The two forks move in opposite directions as the replication proceeds. Single-strand binding (SSB) proteins coat the separated strands of DNA to prevent them from rewinding into a double helix.

Primer Synthesis by Primase

Meanwhile, as the helicase separates the strands, a different enzyme called primase attaches to each strand. It synthesizes a short stretch of nucleotides complementary to the template called an RNA primer. It marks the beginning of the polymerization of the DNA strand.

Step 2: Elongation

After the primer is synthesized, DNA polymerase III adds nucleotides with its 5′-phosphate group to the 3′ end of the last nucleotide of the primer. Synthesis of the growing strand in the 5′-to-3′ direction involves adding nucleotides in a complementary order to the template strand. According to the Watson-Crick DNA model, adenine always pairs with thymine, and cytosine pairs with guanine. This process is known as complementary base pairing.

DNA Polymerase Can Move in One Direction

Two molecules of DNA polymerase III at the replication fork carry out replication. However, DNA polymerases can only make DNA in the 5′ to 3′ direction. It poses a problem. One original strand runs in the 5′ to 3′ direction and the other in the 3′ to 5′ direction. Thus, the two new strands need to be replicated slightly differently.

Synthesis of Leading and Lagging Strand

The strand that runs 5′ to 3′ in the direction of the replication fork is easily replicated continuously as the DNA polymerase moves in the same direction as the replication fork. This strand is called the leading strand.

The replication of the other strand, which runs in the 5′ to 3′ direction away from the fork, is made discontinuously. It happens because as the fork moves forward, the DNA polymerase (which is moving away from the fork) comes off and then reattach to the newly exposed DNA. This strand is called the lagging strand. The small fragments are called Okazaki fragments, named after the discoverer, Japanese molecular biologist Reiji Okazaki, who discovered them. Thus, the leading strand can be extended from one primer alone. In contrast, the lagging strand needs a new primer for each short Okazaki fragment.

Role of Other Proteins

Some other proteins, in addition to helicase, DNA polymerase, and primase, are also involved during the process to ensure that elongation can occur smoothly.

One such protein is the sliding clamp, which holds DNA polymerase III in place as they synthesize DNA. It helps keep the DNA polymerase in the lagging strand from dislocation when it restarts at a new Okazaki fragment.

The other one is the Topoisomerase, which prevents the DNA double helix ahead of the replication fork from tight wounding as the helicase help to open up the DNA. It makes temporary nicks in the helix to release the tension and then reseals the nicks to errors in replication.

Step 3: Termination

Removal of RNA Primers by Exonuclease

Once the continuous and discontinuous strand synthesis is complete, an exonuclease removes all RNA primers in the lagging strand. They are then replaced with DNA through the activity of DNA polymerase I, the other polymerase involved in the replication. The nicks that remain after the primers are replaced get sealed by the enzyme DNA ligase forming a complete strand.

The Role of Tus Protein

Since bacterial chromosomes are circular, replicated ends when the two replication forks meet on the opposite end of the replicated DNA. This step is regulated by a specific termination sequence that, when bound by the Tus protein, enables only one replication fork to pass through while preventing the other. As a result, the replication forks meet within the termination region, causing dissociation of the polymerase and release of the newly synthesized strands.

Replication in Eukaryotes

Prokaryotic and eukaryotic replications are dissimilar in many ways, although they follow the same pattern.

Similarities

  • Require primers for initiation
  • DNA elongation occurs in the 5′ to 3′ direction
  • Replication is continuous in the leading strand and discontinuous in the lagging strand.
  • However, there are also some differences between them.

Differences

  • Eukaryotic replication has many replication origins (often thousands). Unlike prokaryotes, the origin is not conserved and varies according to the species.
  • Eukaryotic replication utilizes five different DNA polymerases – α, β, γ, δ, and ε.
  • The DNA in eukaryotes is wound around proteins called histones, and thus, unwinding and rewinding happen in a different way
  • In bacteria with circular DNA, nucleotides quickly replace the excised primer, leaving no gap in the newly synthesized DNA. However, linear eukaryotic DNA has a small gap left at the end of the DNA due to the lack of a 3′-OH group for nucleotides to bind. A unique enzyme called telomerase prevents the DNA molecule from getting shorter in every round of DNA replication
  • Compared to prokaryotes, DNA polymerase adds nucleotides at a much slower rate of 50 nucleotides per second

Is DNA Replication Semi-conservative

The result of DNA replication is two new DNA molecules. Each DNA consists of one newly synthesized strand while the other is obtained from the parent molecule. Thus, DNA replication is semi-conservative.

Article was last reviewed on Friday, February 17, 2023

DNA Replication – Definition, Process, Steps, & Labeled Diagram (2024)

FAQs

What are the steps and process of DNA replication? ›

How is DNA replicated? Replication occurs in three major steps: the opening of the double helix and separation of the DNA strands, the priming of the template strand, and the assembly of the new DNA segment. During separation, the two strands of the DNA double helix uncoil at a specific location called the origin.

What are the steps for DNA replication quizlet? ›

  1. Step: 1. Helicase unzips the DNA molecule.
  2. Step: 2. Primase attaches RNA primers to the DNA strand to begin replication.
  3. Step: 3. DNA polymerase attaches nucleotides to the DNA strand.
  4. Step: 4. Ligase fills in the missing phosphate backbone.
  5. Step: 5. The end product is 2 identical pieces of DNA.

What is an example of DNA replication? ›

Replication is the process by which a cell copies its DNA prior to division. In humans, for example, each parent cell must copy its entire six billion base pairs of DNA before undergoing mitosis.

How are the DNA strands labeled during replication? ›

DNA is made differently on the two strands at a replication fork. One new strand, the leading strand, runs 5' to 3' towards the fork and is made continuously. The other, the lagging strand, runs 5' to 3' away from the fork and is made in small pieces called Okazaki fragments.

What are the steps of DNA replication translation? ›

DNA serves as the molecular basis of heredity through replication, expression, and translation processes. Replication creates identical DNA strands, while transcription converts DNA into messenger RNA (mRNA). Translation then decodes mRNA into amino acids, forming proteins essential for life functions.

What are the steps in the DNA processing process? ›

The DNA testing process is comprised of four main steps, including extraction, quantitation, amplification, and capillary electrophoresis.

Which list gives the steps of DNA replication in the correct order? ›

Expert-Verified Answer. DNI replication occurs in three steps: 1) Initiation that involves the strands separation and replication fork formation. 2) Elongation that allows the new strains to grow. 3) Termination gives a final to the replication process.

What are the steps of DNA replication in mitosis? ›

During mitosis, the cell breaks apart into two identical cells. Each of the two new cells has the same DNA and number of chromosomes as the parent cell. In other words, the main function of mitosis is nuclear division which relies on the process of DNA replication.

What are the steps of data replication? ›

Step 1: The first step of replicating data is identifying your desired data source and the destination system, where you'll store the replica. Step 2: Once you've decided on your data source and destination, you need to copy the desired database tables and records from your source database.

What is the definition of DNA replication? ›

Definition. 00:00. DNA replication is the process by which the genome's DNA is copied in cells. Before a cell divides, it must first copy (or replicate) its entire genome so that each resulting daughter cell ends up with its own complete genome.

What is the first step that must occur in DNA replication? ›

The very first step in DNA replication is unzipping the double helix of the DNA molecule, the unwinding of DNA occurs by enzyme helicase and gyrase. After unwinding the DNA forms a replication fork and both the strands act as a template for the formation of new strands.

What is DNA replication in kid terms? ›

The process of copying a DNA molecule to produce two identical DNA molecules is called DNA replication. It has to happen before a cell can divide.

What are the steps in DNA replication? ›

Each time a cell divides, its DNA is carefully copied, creating a new DNA molecule that is passed to the new daughter cell. This is the process of DNA replication. DNA replication involves three main steps: initiation, elongation and termination.

Which of the following best summarizes the process of DNA replication? ›

The process of DNA replication can be summarized as follows: DNA unwinds at the origin of replication. New bases are added to the complementary parental strands. One new strand is made continuously, while the other strand is made in pieces.

What is the method of DNA replication? ›

Researchers commonly replicate DNA in vitro using the polymerase chain reaction (PCR). PCR uses a pair of primers to span a target region in template DNA, and then polymerizes partner strands in each direction from these primers using a thermostable DNA polymerase.

What are the steps of DNA transcription? ›

Transcription is performed by enzymes called RNA polymerases, which link nucleotides to form an RNA strand (using a DNA strand as a template). Transcription has three stages: initiation, elongation, and termination.

What are the steps for DNA replication of the lagging strand into the correct order? ›

Final answer: The steps of the DNA replication process in order are: helicase, single-stranded binding proteins, topoisomerase, primase, DNA polymerase III, elongation of leading strands, elongation of lagging strands, DNA polymerase I, and DNA ligase.

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