The structure formed by two sister chromatids held together by a centromere is called a Replicated Chromosome. This specific configuration of genetic material is temporary, yet it represents the cell’s preparation for division and the precise distribution of its DNA. Chromosomes are the organized packages that hold the cell’s genetic blueprint. The replicated chromosome is central to mitosis and meiosis, ensuring that each new daughter cell receives an accurate and complete copy of the genome.
Defining the Replicated Chromosome
The replicated chromosome is a single structure composed of two identical DNA molecules known as sister chromatids. These chromatids are exact copies of the original chromosome sequence. The entire structure typically takes on the recognizable “X” shape when it becomes highly condensed and visible under a microscope during the middle stages of cell division.
This distinctive morphology occurs because the two sister chromatids are held together tightly along their length, with the strongest, most visible connection being at a constricted region called the centromere. The sister chromatids must be genetically identical copies of the same parental chromosome. Each chromatid is essentially one very long, condensed DNA molecule, ready to be separated into a new cell.
The formation of this two-part structure is a mechanism to ensure genetic fidelity during cell division. By keeping the two copies joined, the cell can manage and move the entire genetic package as one unit.
The Role of the Centromere
The centromere is the specialized region of DNA and associated proteins that physically links the two sister chromatids together. It appears as a constriction on the chromosome. The centromere’s primary function is to serve as the assembly site for a complex protein structure called the kinetochore.
The kinetochore is the actual attachment point for the spindle fibers, which are the microscopic cables that pull the chromosomes apart during cell division. By directing the formation of the kinetochore, the centromere ensures that the replicated chromosome can align properly at the cell’s center during metaphase. The precise attachment of spindle fibers to the kinetochores is checked by the cell before the sister chromatids are allowed to separate.
The Life Cycle of a Chromatid
The existence of a sister chromatid is directly tied to the cell cycle’s preparation for division. Before a cell begins to divide, its chromosomes exist in an unreplicated state, consisting of a single DNA molecule within the nucleus. This single-chromatid state is characteristic of the G1 phase of the cell cycle.
The creation of sister chromatids occurs during the S phase (synthesis phase), when the cell duplicates its entire genome through DNA replication. Following replication, each chromosome consists of two identical sister chromatids joined at the centromere, forming the replicated chromosome. This structure persists throughout the remaining stages of interphase and the beginning stages of mitosis, including prophase and metaphase.
The replicated chromosome is dismantled during anaphase, the stage where the centromere connection splits. Once the centromere divides, the sister chromatids immediately become individual, unreplicated chromosomes, sometimes called daughter chromosomes. These newly separated chromosomes are then pulled to opposite poles of the cell, ensuring that each new daughter cell receives a complete set of genetic material.
Understanding Chromosome Number
Counting chromosomes can be confusing because the terminology changes depending on the stage of the cell cycle. The rule for determining the number of chromosomes in a cell is to count the number of centromeres present. Since two sister chromatids share a single centromere, they collectively count as only one chromosome.
For instance, a typical human somatic cell contains 46 chromosomes in the G1 phase, each consisting of one chromatid. After the S phase, the cell still has 46 chromosomes, but each is replicated and composed of two sister chromatids, resulting in 92 total chromatids.
The chromosome number only doubles momentarily when the centromeres split during anaphase, resulting in 92 individual chromosomes that are pulled toward the poles. This temporary doubling is resolved when the cell finally divides into two daughter cells, each restoring the normal count of 46 unreplicated chromosomes.