A chromosome is a thread-like structure located inside the nucleus of eukaryotic cells, carrying the organism’s genetic information in the form of DNA. Understanding when a chromosome transitions from having one to two chromatids is central to how cells prepare to divide and accurately pass on their genetic material. This change is a tightly regulated event in the cell cycle.
Chromosome, Chromatid, and Sister Chromatids
A chromosome is essentially a long molecule of DNA tightly packaged around proteins. A chromatid refers to one of the two identical copies of a replicated chromosome. Before a cell divides, each chromosome must be duplicated so that the two resulting daughter cells each receive a complete set of genetic instructions.
When this duplication occurs, the two resulting DNA copies remain physically joined. Each copy is called a chromatid, and because they are exact genetic duplicates of each other, they are specifically referred to as sister chromatids. The entire structure, consisting of the two sister chromatids joined together, is still considered a single, replicated chromosome.
This replicated structure is visually distinct: an unreplicated chromosome resembles a single rod, while the replicated chromosome, with its two sister chromatids, takes on the characteristic “X” shape. The relationship between these structures is dynamic, changing as the cell progresses toward division.
Chromosome Structure Before Replication
The cell’s life cycle begins after division in a period known as the G1 phase, or Gap 1. During this time, the cell grows and performs its normal functions, and the genetic material exists in an unreplicated state. Each chromosome consists of a single, continuous double helix of DNA.
In this G1 phase, the chromosome is composed of only one chromatid. The DNA is not tightly condensed into the visible, rod-like structures seen during cell division. Instead, it is organized as a looser complex of DNA and associated proteins called chromatin, which allows for gene expression.
The chromatin structure during G1 is still highly organized, featuring complex folding patterns and domains that influence gene regulation. Structurally, each chromosome is a single, linear entity, waiting for the signal to prepare for the next division.
The Timing of DNA Synthesis
The transition of a chromosome from one chromatid to two occurs during the Synthesis phase, or S phase, of the cell cycle. The S phase follows the G1 phase and is entirely dedicated to DNA replication, duplicating the entire genome exactly once.
During the S phase, the original DNA double helix unwinds. Each strand serves as a template for the synthesis of a new, complementary strand in a process known as semi-conservative replication. This mechanism ensures that each resulting DNA molecule, or chromatid, is identical to the original one.
Replication results in the creation of a second, identical DNA molecule for every chromosome. These two identical DNA copies are now the sister chromatids, and they remain physically connected to each other, primarily at a region called the centromere. Although the DNA content of the cell has doubled, the number of chromosomes remains the same because the two chromatids are still counted as a single unit until they separate.
Preparing for Cell Division
Once the S phase is complete, the cell enters the G2 phase, where the chromosome already possesses two sister chromatids. This phase is a final preparation period where the cell grows and checks the replicated DNA for any errors before proceeding to division.
As the cell moves into the beginning of mitosis, or Prophase, the chromosomes must condense dramatically. This condensation transforms the long, loose chromatin fibers into the compact, visible “X” shape. Cohesion proteins, notably the cohesin complex, form ring-like structures that physically hold the two sister chromatids together along their entire length.
This two-chromatid structure is necessary to manage the huge amount of DNA during the complex movements of cell division. The sister chromatids, held tightly by cohesin, are now ready to be aligned in the center of the cell and correctly distributed to the future daughter cells. The centromere serves as the attachment point for the spindle fibers.
The Eventual Separation of Chromatids
The two-chromatid structure serves its purpose until the cell reaches the Anaphase stage of mitosis. This stage marks the final step in the life of the sister chromatid structure and the dramatic physical separation of the genetic material. The separation is triggered by the activation of a protease enzyme called separase.
Separase cleaves the cohesin proteins that have been holding the sister chromatids together. Once this molecular glue is dissolved, the sister chromatids are immediately pulled apart by the spindle fibers toward opposite poles of the cell. At this precise moment of separation, each former chromatid is now considered an individual, unreplicated chromosome.
The separation ensures that each new cell receives one complete and identical set of chromosomes. This event effectively returns the chromosomes to their single-chromatid state, and the resulting daughter cells enter the G1 phase, completing the cycle that began with a single-chromatid chromosome and temporarily involved the two-chromatid structure.