DNA replication is probably one of the most amazing tricks DNA can do. Think about it, each cell contains all the DNA it needs to make other cells. The human body begins with a single cell and ends up with trillions. In the process of cell division, all of the information in a cell has to be copied, and it has to be copied perfectly. Thus, DNA is a molecule that can be replicated to make almost perfect copies of itself. The fact that there are almost three billion base pairs of DNA to copy makes this all the more amazing.

For replication, DNA polymerases are used, which are molecules specifically dedicated to copying DNA. It takes several hours of just pure copying to replicate the entire DNA of a single human cell. The cell actually has twice the amount of DNA it needs after this process, and the cell can then divide and parcel this DNA into the daughter cell so that in many cases the daughter cell and the parental cell are genetically identical.

What Is DNA Replication?

DNA replication is the process by which DNA makes a copy of itself during cell division.

An Illustration To Show Replication Of The Leading And Lagging Strands Of DNA.
An Illustration To Show Replication Of The Leading And Lagging Strands Of DNA.
  • The first step in DNA replication is to unravel the double helix structure of the DNA molecule.
  • An enzyme called helicase breaks the hydrogen bonds holding complementary bases of DNA together (A with T, C with G).
  • When two single strands of DNA separate, they form a Y-shaped structure called a replication ‘fork’. In order to make new DNA strands, the two separated strands will act as templates.
  • The leading strand is oriented in the 3′ to 5′ direction (towards the replication fork). A second strand is oriented from 5′ to 3′ away from the replication fork, and this is the lagging strand. The two strands are replicated differently because of their different orientations:

    Leading Strand

    Leading StrandLeading Strand
    A short piece of RNA called a primer (produced by an enzyme called primase) comes along and binds to the end of the leading strand. The primer acts as the starting point for DNA synthesis.Numerous RNA primers are made by the primase enzyme and bind at various points along the lagging strand.
    DNA polymerase binds to the leading strand and then ‘walks’ along it, adding new complementary nucleotide bases (A, C, G and T) to the strand of DNA in the 5’ to 3’ direction.Chunks of DNA, called Okazaki fragments, are then added to the lagging strand also in the 5’ to 3’ direction.
    This sort of replication is called continuous.This type of replication is called discontinuous as the Okazaki fragments will need to be joined up later.
  • When all of the bases are matched (A with T, C with G), an enzyme called exonuclease removes the primer(s). The gaps left by the primers are then filled with complementary nucleotides.
  • In order to ensure that the new DNA sequence is error-free, the new strand is proofread.
  • The sequence of DNA is then sealed up by an enzyme called DNA ligase into two continuous double strands.
  • As a result of DNA replication, two DNA molecules are formed, consisting of one new and one old chain of nucleotides. Because of this, DNA replication is described as semi-conservative, half of the chain comes from the original DNA molecule, and half is completely new.
  • After replication, the new DNA automatically forms a double helix.

DNA Replication Process

How is DNA replicated?

Three major steps are involved in the replication process: opening the double helix and separating the DNA strands, priming the template strand, and assembling the new DNA segment. DNA double helices uncoil at a specific location, called the origin when they are separated. Then, several enzymes and proteins work together to prime the strands for replication.

DNA polymerase organizes the assembly of new DNA strands using a special enzyme. In general, the process described below applies to all cells, but specific variations within the process may occur depending on the organism and cell type.

What Triggers Replication?

Here is a schematic showing the replication of a double-stranded DNA molecule. The left side of the molecule has two strands. A yellow globular structure, which represents the protein helicase, is located in the middle of the molecule at the ends of several nitrogenous bases on the lower strand. The double helix has opened to the right of the helicase protein, and the top strand has separated from the bottom. The right image shows a short segment of the newly replicated double-stranded DNA molecule.

Helicase (Yellow) Unwinds The Double Helix.
Helicase (Yellow) Unwinds The Double Helix.

DNA replication is initiated in two steps. A protein called an initiator unwinds a short stretch of DNA double helix. After that, a protein called helicase attaches to and breaks apart the hydrogen bonds between the bases on the DNA strands, pulling them apart. While the helicase moves along the DNA molecule, it continues to break hydrogen bonds and separate the two polynucleotide chains.

While Helicase And The Initiator Protein (Not Shown) Separate The Two Polynucleotide Chains, Primase (Red) Assembles A Primer. This Primer Permits The Next Step In The Replication Process.
While Helicase And The Initiator Protein (Not Shown) Separate The Two Polynucleotide Chains, Primase (Red) Assembles A Primer. This Primer Permits The Next Step In The Replication Process.

The replication of a double-stranded DNA molecule is shown schematically. On the right, the double helix has opened, and the top strand has separated from the bottom. Several nitrogenous bases on the lower strand are bound together by a yellow globular structure representing the protein helicase. The enzyme primase, represented by a red globular molecule, is bound to the lower DNA strand to the right of helicase.

Helicase and the initiator protein (not shown) separate the polynucleotide chains, while primase (red) assembles the primer. After this primer is assembled, replication can proceed.

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As the helicase separates the strands, another enzyme called primase briefly attaches to each strand and assembles a foundation for replication to begin. It consists of a short stretch of nucleotides known as a primer.

How Are DNA Strands Replicated?

A schematic shows a region of horizontal single-stranded DNA. A transparent blue globular structure, representing the enzyme DNA polymerase, is bound to a seven-nucleotide-long region on the right-hand side of the DNA strand. The region of DNA bound by DNA polymerase is visible inside the transparent enzyme at a higher magnification.

Beginning At The Primer Sequence, Dna Polymerase (Shown In Blue) Attaches To The Original Dna Strand And Begins Assembling A New, Complementary Strand.
Beginning At The Primer Sequence, DNA Polymerase (Shown In Blue) Attaches To The Original Dna Strand And Begins Assembling A New, Complementary Strand.

There are six nucleotides in this region bound to six complementary nucleotides arranged above and in parallel to the single strand, forming red-green or blue-orange pairs. In the background, about two dozen nucleotides float.

DNA polymerase (shown in blue) attaches to the primer sequence and assembles a new complementarity strand.

The primer is placed on a single, unwound polynucleotide strand, and DNA polymerase attaches nucleotides to the nitrogenous bases exposed. In this way, the polymerase builds a new DNA strand onto the existing one.

Each Nucleotide Has An Affinity For Its Partner. A Pairs With T, And C Pairs With G.
Each Nucleotide Has An Affinity For Its Partner. A Pairs With T, And C Pairs With G.

The schematic shows two rows of nucleotides. An individual nucleotide is represented as an elongated, vertical, colored rectangle (a nitrogenous base) bound to a grey horizontal cylinder (a sugar molecule). Each nitrogenous base forms a pair with its partner, with A and T forming a pair and C and G forming a pair.

As DNA polymerase travels down the unwound DNA strand, it utilizes the pool of free-floating nucleotides around it to build the new strand. The nucleotides in the template strand are paired with their counterparts in the new strand due to their molecular structures; the A and T nucleotides, and the C and G nucleotides, are always paired together. As a result, two complementary DNA strands are created, which is known as complementary base pairing.

A schematic shows a region of DNA, with part of it being single-stranded and most of it being double-stranded. The enzyme DNA polymerase is bound to several nucleotide-long regions along the DNA strand approximately one-quarter of the way from the left. DNA polymerase replicates a single strand to the left of DNA polymerase and a double strand to the right, indicating that it is moving from right to leave as it replicates the DNA strand.

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The sugar-phosphate backbone is shown as a segmented gray cylinder. Each segment of the sugar-phosphate backbone is surrounded by a blue, orange, red, or green vertical rectangle indicating the nitrogenous base. Higher magnification shows the region of DNA bound by DNA polymerase inside the transparent enzyme.

The six nucleotides in this region are bound to six complementary nucleotides arranged above and parallel to the single strand, forming red-green or blue-orange pairs of rungs between the grey cylinders. Several nucleotides are floating in the background.

A New Dna Strand Is Synthesized. This Strand Contains Nucleotides That Are Complementary To Those In The Template Sequence.
A New DNA Strand Is Synthesized. This Strand Contains Nucleotides That Are Complementary To Those In The Template Sequence.

Base pairing ensures that the sequence of nucleotides in the existing template strand will be exactly matched to the complementary sequence in the new template strand, also known as the anti-sequence of the template strand. When the new strand is copied, its complementary strand will contain the same sequence as the original template strand. Due to complementary base pairing, the replication process proceeds as a series of sequence and antisequence copies that preserve the coding of the original DNA.

How Long Does Replication Take?

In the prokaryotic bacterium E. coli, replication occurs at a rate of 1,000 nucleotides per second. Compared to eukaryotic DNA, human DNA replicates at a rate of 50 nucleotides per second. Because multiple polymerases can synthesize two new strands at the same time by using each unwound strand from the original DNA double helix as a template, replication occurs so rapidly in both cases.

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One of these original strands is called the leading strand, while the other is called the lagging strand. As shown in Figure, the leading strand is synthesized continuously. On the other hand, the lagging strand is synthesized in small, separate fragments that are eventually joined together to form a complete, new strand.

When Does DNA Replication Occur?

Every time you create a new cell, your DNA needs to copy itself. Each new cell must have the same DNA as the rest of your cells. Otherwise, it might malfunction. That’s why DNA replication, the process by which DNA is copied, must be very accurate.

DNA must replicate (copy) itself in order for each resulting cell after mitosis and cell division to contain the same DNA as the parent cell. These cells, the mother cell, and the two new daughter cells are all genetically identical.

The replication of DNA takes place during the S phase (the synthesis phase) of the cell cycle, before mitosis and cell division. The rules of base pairing are crucial to DNA replication. DNA replication occurs when two identical DNA molecules are formed.

DNA Replication Quizlet

DNA Replication the process of making identical copies of DNA before cell division
Replication Origin The specific sequence of DNA where DNA synthesis begins.
Semiconservative replication Each half of an original DNA molecule serves as a templete for a new strand, and the two new DNA molecules each have one old and one new strand.
DNA helicase An enzyme that unwinds the double helix of DNA and separates the DNA strands in preparation for DNA replication.
DNA Polymerase The enzyme involved in DNA replication that joins individual nucleotides to produce a DNA molecule
RNA Primer Sequence of RNA nucleotides bound to a region of single-stranded DNA to initiate DNA replication.
leading strand the strand of DNA that is continuously synthesized into the replication fork.
lagging strand The strand that is synthesized away from the replication fork , in fragments using sections called Okazaki fragments.
Okazaki fragments Small fragments of DNA produced on the lagging strand during DNA replication, joined later by DNA ligase to form a complete strand.
DNA ligase the enzyme which connects the individual okazaki fragments on the lagging strand by forming covalent bonds

Important Enzymes in DNA Replication

Multiple replication origins are present in eukaryotes. DNA polymerase adds nucleotides one by one to the growing chain after a primer initiates synthesis. A leading strand is synthesized continuously, while a lagging strand is synthesized in short stretches called Okazaki fragments.

The RNA primers are replaced by DNA nucleotides; the DNA remains in one continuous strand by joining the DNA fragments with DNA ligase. The following table summarizes the major enzymes discussed in this reading, listed roughly by their activity during replication.

  • Topoisomerase: Relaxes the super-coiled DNA
  • DNA helicase: Unwinds the double helix at the replication fork
  • Primase: Provides the starting point for DNA polymerase to begin the synthesis of the new strand
  • DNA polymerase: Synthesizes the new DNA strand; also proofreads and corrects some errors
  • DNA ligase: Re-joins the two DNA strands into a double helix and joins Okazaki fragments of the lagging strand


What is the DNA replication process?

In DNA replication, a double-stranded DNA molecule is copied to produce two identical DNA molecules. Once the DNA in a cell is replicated, the cell can divide into two cells, each containing a copy of the original DNA.

What are the 7 steps of DNA replication?

The series of events that occur during prokaryotic DNA replication have been explained below.

  • Initiation.
  • Primer Synthesis.
  • Leading Strand Synthesis.
  • Lagging Strand Synthesis.
  • Primer Removal.
  • Ligation.
  • Termination.

Where does DNA replication occur?

Prokaryotes replicate their DNA in the cytoplasm, while eukaryotes replicate it in the nucleus. DNA replication is the same regardless of where it occurs. DNA’s structure lends itself to replication. Each side of the double helix runs in an opposite direction (anti-parallel).

Why does DNA replication occur?

Cells must replicate their DNA before they can divide. This ensures that each daughter cell gets a copy of the genome, and therefore, successful inheritance of genetic traits. DNA replication is an essential process and the basic mechanism is conserved in all organisms.

How do you remember DNA replication?

Breaking the word O-ka-za-ki in to fragments allows for the remembering of what the fragments are called. DNA polymerase I then replace the RNA primers with DNA.