Deoxyribonucleic acid (DNA) holds the genetic instructions for the development and function of all known living organisms. DNA replication involves making an exact copy of this genetic material so that when a cell divides, each new daughter cell receives a full set of instructions. This duplication takes place during the Synthesis phase, commonly abbreviated as the S phase, of the cell cycle.
The Cell Cycle: A Blueprint for Growth
The life of a cell is governed by the cell cycle, a sequence of events that results in the division of a single parent cell into two new daughter cells. This cycle is broadly divided into two main stages: Interphase, the period of cell growth and DNA preparation, and the Mitotic (M) phase, the actual process of cell and nuclear division.
Interphase is segmented into three distinct phases. The G1 phase involves cell growth, the production of new organelles, and the synthesis of proteins needed for subsequent stages. After G1, the cell enters the S phase, where the genetic material is duplicated. The final stage is G2, a second gap phase where the cell continues to grow and synthesizes proteins needed for the upcoming M phase.
The S Phase: When DNA Replication Occurs
The S phase derives its name from “synthesis,” indicating that this is when the cell synthesizes a complete copy of its DNA. Before this phase begins, each chromosome consists of a single strand of DNA. The result of the S phase is that the amount of DNA within the cell doubles, preparing the cell to distribute a full genome to each daughter cell.
During the Synthesis phase, chromosomes are duplicated, but the two copies remain physically attached. Each replicated chromosome consists of two identical structures called sister chromatids, linked together at a central region known as the centromere. This duplication ensures that when the cell separates the chromosomes during the M phase, each resulting cell receives a genetically identical set.
The Mechanics of DNA Duplication
The method by which DNA is copied is known as semi-conservative replication. This means that each new double-stranded DNA molecule produced consists of one original, “parental” strand and one newly synthesized, “daughter” strand. The original strand serves as a template to guide the assembly of the new, complementary strand.
Replication begins when the enzyme helicase unwinds and separates the two strands of the DNA double helix, breaking the hydrogen bonds between the base pairs and creating the replication fork. Following this unwinding, DNA polymerase moves along the exposed template strands. This enzyme is responsible for synthesizing the new DNA by selecting and adding complementary nucleotides to the growing chain.
The antiparallel nature of the DNA strands and the polymerase’s ability to only synthesize in one direction necessitate different strategies for copying the two template strands. The leading strand is synthesized continuously in the direction of the replication fork movement. The lagging strand must be synthesized in short, disconnected segments that are later joined together by the enzyme DNA ligase.
Quality Control: Cell Cycle Checkpoints
The cell cycle is regulated by control mechanisms called checkpoints, which monitor the cell’s internal and external conditions. These checkpoints halt the progression of the cycle until all necessary conditions are met, protecting the integrity of the genome.
The G1/S checkpoint, sometimes called the restriction point, occurs just before the S phase begins. It checks for sufficient cell size, adequate nutrient reserves, and assesses the DNA for any existing damage before committing to replication.
A second checkpoint, the G2/M checkpoint, is positioned at the end of the G2 phase, right before the cell enters mitosis. Its function is to ensure that DNA replication in the S phase has been completed accurately. If any errors or unreplicated segments are detected, the cell cycle is paused, allowing time for damage repair. Failure of these checkpoints to stop a compromised cell can lead to the transmission of mutations, contributing to the uncontrolled division seen in cancer.