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Animation: DNA Replication: A Closer Look

by Pearson
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Let's now look at the process of DNA replication more closely, starting at an origin of replication. The DNA opens up there to form a small bubble. Molecules of an enzyme called helicase attach to the DNA at the ends of the bubble and continue the unwinding of the double helix. We'll focus on one end of the bubble, on the Y-shaped region called a replication fork. The two strands would naturally tend to rewind but are held apart by molecules of a single-strand binding protein. The synthesis of a new strand begins when an enzyme called primase attaches and synthesizes a short RNA strand that is complementary to one of the DNA strands. This piece of RNA is a primer. DNA polymerase then adds DNA nucleotides to the 3' end of the primer. It continues to lengthen the new DNA strand by adding nucleotides complementary to the template strand. Notice that DNA synthesis always proceeds in a 5' to 3' direction. The strand just made here is called the leading strand. For clarity, we'll keep the new double-stranded DNA untwisted. The other new strand is called the lagging strand. Unlike the leading strand, the lagging strand cannot be made continuously because DNA polymerase can only add nucleotides at the 3 prime end. Instead, the lagging strand must be made of fragments that are linked together. Despite their names, the leading and lagging strands are actually made at the same time. For clarity, here we will just show the lagging strand being pieced together. We'll start with a brief overview of fragments being formed and linked. First, an RNA primer, shown here in red, forms the beginning of each fragment. The rest of each fragment is then synthesized from DNA nucleotides in a 5 prime to 3 prime direction. The resulting segments are called Okazaki fragments. Next, each RNA primer is replaced with DNA. Then the gaps are closed to form the lagging strand. Now let's look closely at how enzymes work together to assemble the lagging strand. First, the enzyme primase removes the single-strand binding proteins, shown here in blue, and makes an RNA primer to begin an Okazaki fragment. The enzyme DNA polymerase then adds the complementary DNA nucleotides to synthesize the rest of the fragment. The assembly process continues, as primase makes new RNA primers and DNA polymerase adds DNA nucleotides to create more Okazaki fragments. After the fragments are made, another kind of DNA polymerase replaces each RNA primer with DNA. Next, the enzyme DNA ligase links the Okazaki fragments to form the lagging strand. Here you can see the simultaneous formation of the leading and lagging strands. A DNA polymerase assembles a continuous leading strand, while primase, other DNA polymerases, and ligase all work together to make the lagging strand. The enzyme helicase continues to untwist the double helix, exposing more template strand DNA for replication.
Let's now look at the process of DNA replication more closely, starting at an origin of replication. The DNA opens up there to form a small bubble. Molecules of an enzyme called helicase attach to the DNA at the ends of the bubble and continue the unwinding of the double helix. We'll focus on one end of the bubble, on the Y-shaped region called a replication fork. The two strands would naturally tend to rewind but are held apart by molecules of a single-strand binding protein. The synthesis of a new strand begins when an enzyme called primase attaches and synthesizes a short RNA strand that is complementary to one of the DNA strands. This piece of RNA is a primer. DNA polymerase then adds DNA nucleotides to the 3' end of the primer. It continues to lengthen the new DNA strand by adding nucleotides complementary to the template strand. Notice that DNA synthesis always proceeds in a 5' to 3' direction. The strand just made here is called the leading strand. For clarity, we'll keep the new double-stranded DNA untwisted. The other new strand is called the lagging strand. Unlike the leading strand, the lagging strand cannot be made continuously because DNA polymerase can only add nucleotides at the 3 prime end. Instead, the lagging strand must be made of fragments that are linked together. Despite their names, the leading and lagging strands are actually made at the same time. For clarity, here we will just show the lagging strand being pieced together. We'll start with a brief overview of fragments being formed and linked. First, an RNA primer, shown here in red, forms the beginning of each fragment. The rest of each fragment is then synthesized from DNA nucleotides in a 5 prime to 3 prime direction. The resulting segments are called Okazaki fragments. Next, each RNA primer is replaced with DNA. Then the gaps are closed to form the lagging strand. Now let's look closely at how enzymes work together to assemble the lagging strand. First, the enzyme primase removes the single-strand binding proteins, shown here in blue, and makes an RNA primer to begin an Okazaki fragment. The enzyme DNA polymerase then adds the complementary DNA nucleotides to synthesize the rest of the fragment. The assembly process continues, as primase makes new RNA primers and DNA polymerase adds DNA nucleotides to create more Okazaki fragments. After the fragments are made, another kind of DNA polymerase replaces each RNA primer with DNA. Next, the enzyme DNA ligase links the Okazaki fragments to form the lagging strand. Here you can see the simultaneous formation of the leading and lagging strands. A DNA polymerase assembles a continuous leading strand, while primase, other DNA polymerases, and ligase all work together to make the lagging strand. The enzyme helicase continues to untwist the double helix, exposing more template strand DNA for replication.