The genetic blueprint for an organism is stored and organized within the cell nucleus in structures known as chromosomes. These thread-like entities are composed of deoxyribonucleic acid (DNA) tightly wound around proteins, forming a complex called chromatin. The physical form of a chromosome is not static, instead changing shape and density depending on the stage of the cell’s life cycle. The transition from a single DNA strand to a duplicated structure is a requirement for successful cell division.
Understanding the Unreplicated Chromosome
Before a cell prepares to divide, the chromosome exists in its unreplicated state, which is structurally a single, long molecule of double-stranded DNA. This single DNA molecule is intricately packed with proteins, primarily histones, which help condense the massive length of the genetic material so it fits inside the nucleus. During the G1 phase of the cell cycle, this DNA is typically less condensed, allowing the genetic information to be accessed for processes like transcription and repair.
This unreplicated form is sometimes referred to as a single chromatid, though it is considered a full chromosome at this stage. Once the cell receives the signal to divide, this single-stranded chromosome must first be duplicated to ensure that each new daughter cell receives a complete and identical genome.
DNA Replication: Creating a Copy
The signal for cell division initiates the synthesis phase (S phase), during which the cell’s entire DNA content is precisely copied, a process known as DNA replication. This duplication is necessary to provide a full and identical set of genetic instructions to the two resulting cells. The original double-stranded DNA molecule serves as a template to create a new, complementary strand for each half of the helix.
The replication process results in an exact duplicate of the original DNA sequence, effectively doubling the genetic material associated with each chromosome. This is achieved through the coordinated action of numerous enzymes, and the overall process is semi-conservative, meaning each new DNA molecule consists of one old strand and one newly synthesized strand. Although the DNA content has doubled, the cell’s chromosome number is considered unchanged at this point, as the two copies remain physically joined together.
The Answer: Sister Chromatids
The structure that results from DNA replication is a single chromosome composed of two identical DNA molecules joined together. Each of these identical halves is called a sister chromatid. This duplicated chromosome, formed during the S phase, is often visually represented as the classic X-shape seen in diagrams of cell division.
Each sister chromatid contains an identical sequence of genes, ensuring that when they finally separate, the resulting cells receive the same genetic information. Even though the total amount of DNA has doubled, the structure is still counted as one chromosome as long as the two chromatids are attached. This naming convention prevents a temporary doubling of the chromosome count during preparation for cell division. The integrity of this duplicated structure is supported by cohesin, a protein complex that holds the two sister chromatids together along their entire length after replication.
The Centromere and Final Separation
The two sister chromatids are most visibly and tightly joined at a specialized DNA region called the centromere. The centromere serves as the primary attachment point and is crucial for the correct distribution of the genetic material. Specific proteins form a structure on the centromere known as the kinetochore, which acts as the docking site for the spindle fibers during cell division.
During the anaphase stage of mitosis, the cohesin proteins holding the sister chromatids together are cleaved, allowing the two identical copies to separate. Once separated, each chromatid is now considered a full, individual chromosome and moves toward opposite ends of the dividing cell. This separation ensures that each new daughter cell receives an exact, complete set of chromosomes, maintaining genetic stability.