Replication isthe process by which a double-stranded DNA molecule is copied to produce twoidentical DNA molecules. DNA replication is one of the most basic processesthat occurs within a cell. Each time a cell divides, the two resulting daughtercells must contain exactly the same genetic information, or DNA, as the parentcell. To accomplish this, each strand of existing DNA acts as a template for replication.
How is DNA replicated?
Replication occurs in three major steps: the opening of thedouble helix and separation of the DNA strands, the priming of the templatestrand, and the assembly of the new DNA segment. During separation, the twostrands of the DNA double helix uncoil at a specific location called the origin. Several enzymes and proteinsthen work together to prepare, or prime,the strands for duplication. Finally, a special enzyme called DNA polymerase organizes the assemblyof the new DNA strands. The following description of this three-stage processapplies generally to all cells, but specific variations within the process mayoccur depending on organism and cell type.
What triggers replication?
Figure 1:Helicase (yellow) unwinds the double helix.
The initiation of DNA replication occurs in two steps. First, a so-calledinitiator protein unwinds a short stretch of the DNA double helix. Then, aprotein known as helicase attachesto 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 DNAmolecule, it continues breaking these hydrogen bonds and separating the twopolynucleotide chains (Figure 1).
Figure 2: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.
Figure Detail
Meanwhile, as the helicase separates the strands, anotherenzyme called primase brieflyattaches to each strand and assembles a foundation at which replication canbegin. This foundation is a short stretch of nucleotides called a primer (Figure 2).
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.
After the primer is in place on a single, unwound polynucleotide strand, DNA polymerase wraps itself around that strand, and it attaches new nucleotides to the exposed nitrogenous bases. In this way, the polymerase assembles a new DNA strand on top of the existing one (Figure 3).
Figure 4:Each nucleotide has an affinity for its partner. A pairs with T, and C pairs with G.
Figure Detail
As DNA polymerase makes its way down the unwoundDNA strand, it relies upon the pool of free-floating nucleotides surroundingthe existing strand to build the new strand. The nucleotides that make up thenew strand are paired with partner nucleotides in the template strand; becauseof their molecular structures, A and T nucleotides always pair with oneanother, and C and G nucleotides always pair with one another. This phenomenonis known as complementary base pairing(Figure 4), and it results in the production of two complementary strands ofDNA.
Figure 5: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 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.
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?
In the prokaryotic bacterium E. coli, replication canoccur at a rate of 1,000 nucleotides per second. In comparison,eukaryotic human DNA replicates at a rate of 50 nucleotides per second. In bothcases, replication occurs so quickly because multiple polymerases cansynthesize two new strands at the same time by using each unwound strand fromthe original DNA double helix as a template. One of these original strands iscalled the leading strand, whereas the other is called the lagging strand. Theleading strand is synthesized continuously, as shown in Figure 5. In contrast,the lagging strand is synthesized in small, separate fragments that areeventually joined together to form a complete, newly copied strand.
