Helicase is bound to the ends of several nitrogenous bases on the lower strand. 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. How do scientists visualize DNA? Figure This karyotype depicts all 23 pairs of chromosomes in a human cell, including the sex-determining X and Y chromosomes that together make up the twenty-third set lower right.
Watch this video for a closer look at the relationship between chromosomes and the DNA double helix. What are karyotypes used for?
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Scitable Chat. Register Sign In. Visual Browse Close. The two strands are held together by bonds between the bases, adenine forming a base pair with thymine, and cytosine forming a base pair with guanine. A double helix has become the icon for many, many kinds of discussions about where science has been and where it's going. This really is an amazing structure. You can't stare at the double helix for very long without having a sense of awe about the elegance of this information molecule DNA, with its double helical form basically being the way in which all living forms are connected to each other, because they all use this same structure for conveying that information.
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