In: Facts In the Cell. This is carried out by an enzyme called helicase which breaks the hydrogen bonds holding the complementary bases of DNA together A with T, C with G. The two separated strands will act as templates for making the new strands of DNA. As a result of their different orientations, the two strands are replicated differently: An illustration to show replication of the leading and lagging strands of DNA. Related Content:. What is a genome?
What is DNA? What is a cell? How helpful was this page? What's the main reason for your rating? Which of these best describes your occupation? What is the first part of your school's postcode? How has the site influenced you or others? Beside it, four nitrogenous bases, each attached to a sugar molecule, have been annealed to complementary nitrogenous bases on the bottom strand.
About three dozen individual nucleotides float in the background. Meanwhile, as the helicase separates the strands, another enzyme called primase briefly attaches to each strand and assembles a foundation at which replication can begin. This foundation is a short stretch of nucleotides called a primer Figure 2. As DNA polymerase makes its way down the unwound DNA strand, it relies upon the pool of free-floating nucleotides surrounding the existing strand to build the new strand.
The nucleotides that make up the new strand are paired with partner nucleotides in the template strand; because of their molecular structures, A and T nucleotides always pair with one another, and C and G nucleotides always pair with one another.
This phenomenon is known as complementary base pairing Figure 4 , and it results in the production of two complementary strands of DNA. Base pairing ensures that the sequence of nucleotides in the existing template strand is exactly matched to a complementary sequence in the new strand, also known as the anti-sequence of the template strand. Later, when the new strand is itself copied, its complementary strand will contain the same sequence as the original template strand.
Thus, as a result of complementary base pairing, the replication process proceeds as a series of sequence and anti-sequence copying that preserves the coding of the original DNA.
In the prokaryotic bacterium E. In comparison, eukaryotic human DNA replicates at a rate of 50 nucleotides per second. In both cases, replication occurs so quickly 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. One of these original strands is called the leading strand, whereas the other is called the lagging strand. The leading strand is synthesized continuously, as shown in Figure 5.
In contrast, the lagging strand is synthesized in small, separate fragments that are eventually joined together to form a complete, newly copied strand. This page appears in the following eBook. Aa Aa Aa. How is DNA replicated? What triggers replication? Figure 1: Helicase yellow unwinds the double helix. The initiation of DNA replication occurs in two steps. First, a so-called initiator protein unwinds a short stretch of the DNA double helix.
Then, a protein known as helicase attaches to and breaks apart the hydrogen bonds between the bases on the DNA strands, thereby pulling apart the two strands. As the helicase moves along the DNA molecule, it continues breaking these hydrogen bonds and separating the two polynucleotide chains Figure 1.
How are DNA strands replicated? Figure 3: Beginning at the primer sequence, DNA polymerase shown in blue attaches to the original DNA strand and begins assembling a new, complementary strand. Figure 4: Each nucleotide has an affinity for its partner.
A pairs with T, and C pairs with G. The color of the rectangle represents the chemical identity of the nitrogenous base. A grey horizontal cylinder is attached to one end of the rectangle in each nucleotide and represents a sugar molecule. The nucleotides are arranged in two rows and the nitrogenous bases point toward each other.
A set of four nucleotides are in both the upper and lower rows. From left to right, the nucleotides in the top row are adenine green , cytosine orange , thymine red , and guanine blue. From left to right, the complementary nucleotides in the bottom row are: thymine red , guanine blue , adenine green , and cytosine orange.
Figure 5: A new DNA strand is synthesized. This strand contains nucleotides that are complementary to those in the template sequence. How long does replication take? More on replication.
How does DNA polymerase work? What does the molecular structure of a nucleotide look like? What does the lagging strand look like? Watch this video for a summary of DNA replication in eukaryotes. Key Questions What if an error happens during replication?
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