The cell cycle is an ordered series of events cells undergo as they grow and divide. This process is essential for the growth, development, and maintenance of all living organisms. A primary stage within this cycle is the synthesis phase, or S phase, where a cell duplicates its entire genetic material, deoxyribonucleic acid (DNA). This DNA copying ensures the accurate transmission of genetic information.
Why Cells Replicate DNA
Cells duplicate their DNA before dividing to ensure each new daughter cell receives a complete and identical set of genetic instructions. This replication supports growth and development in multicellular organisms; for example, a human starts as a single cell and requires trillions of divisions. DNA replication also supports the repair and regeneration of damaged tissues, as old or injured cells are continuously replaced. In single-celled organisms, this mechanism is directly linked to reproduction, creating two new, genetically identical individuals.
The DNA Replication Process
DNA replication, the central event of the S phase, involves coordinated steps to produce two identical DNA molecules from one original. This process begins with the unwinding of the DNA double helix. Enzymes called helicases separate the two tightly wound strands by breaking the hydrogen bonds that hold the base pairs together. This unwinding creates a Y-shaped replication fork, providing access to the single DNA strands that will serve as templates.
Once the strands are separated, specialized proteins keep them from rejoining, allowing new strand synthesis to begin. New DNA molecules are constructed using complementary base pairing: adenine (A) pairs with thymine (T), and cytosine (C) pairs with guanine (G). Each exposed original strand acts as a guide, ensuring the new strand is a perfect complement.
DNA polymerase adds new nucleotides to the growing DNA strand, moving along the template according to base-pairing rules. This enzyme requires a short primer to begin synthesis. DNA polymerase also has a proofreading capability, allowing it to detect and correct errors in nucleotide incorporation, which helps maintain genetic code integrity.
While one new strand is synthesized continuously, the other is built in short segments. DNA ligase then joins these segments, forming phosphodiester bonds to create a continuous, intact DNA molecule. This coordinated action of multiple enzymes ensures the entire genome is accurately copied.
The Result of Synthesis
Following the completion of the synthesis phase, the cell’s genetic material has been duplicated. Each original chromosome now consists of two identical copies, called sister chromatids. These sister chromatids are joined together at a central region called the centromere, forming a characteristic X-shape. At this point, the cell effectively contains twice the amount of DNA it had at the beginning of the S phase.
The formation of these sister chromatids is a prerequisite for the subsequent stages of the cell cycle, particularly mitosis or meiosis. This duplication ensures that when the cell eventually divides, each resulting daughter cell will receive a complete and accurate set of chromosomes.