Explanation:
Replication is the process by which a double-stranded DNA molecule is copied to produce two identical DNA molecules. DNA replication is one of the most basic processes that occurs within a cell. Each time a cell divides, the two resulting daughter cells must contain exactly the same genetic information, or DNA, as the parent cell. 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 the double helix and separation of the DNA strands, the priming of the template strand, and the assembly of the new DNA segment. During separation, the two strands of the DNA double helix uncoil at a specific location called the origin. Several enzymes and proteins then work together to prepare, or prime, the strands for duplication. Finally, a special enzyme called DNA polymerase organizes the assembly of the new DNA strands. The following description of this three-stage process applies generally to all cells, but specific variations within the process may occur depending on organism and cell type.
What triggers replication?
A schematic shows a double-stranded DNA molecule undergoing the replication process. The left side of the molecule is double-stranded. In the middle of the molecule, a globular yellow structure, representing the protein helicase, is bound to the ends of several nitrogenous bases on the lower strand. To the right of the helicase protein, the double helix has opened and the top strand has separated from the bottom. At right, a short segment of the newly replicated double-stranded DNA molecule is visible.
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.
A schematic shows a double-stranded DNA molecule undergoing the replication process. At right, the double helix has opened and the top strand has separated from the bottom. A globular yellow structure, representing the protein helicase, is bound to the ends of several nitrogenous bases on the lower strand. A red globular molecule, representing the enzyme primase, is bound to the lower DNA strand to the right of helicase.
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.
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 prime
