Sister chromatids are central to mitosis, the cell division responsible for growth and repair. Mitosis ensures a parent cell accurately distributes its replicated genetic material into two new, genetically identical daughter cells. Sister chromatids represent the duplicated state of the cell’s chromosomes, and their eventual separation is the main event of this division.
What Sister Chromatids Are and How They Form
A sister chromatid is one of the two identical copies of a single, replicated chromosome. Before a cell can enter mitosis, its genetic material must be duplicated in the Synthesis (S) phase. This replication produces two exact copies of the DNA molecule. These copies remain physically linked at a constricted region called the centromere, forming the characteristic X-shape. The centromere acts as the physical connection point, maintained by a protein complex known as cohesin, which prevents premature separation.
Alignment and Preparation for Division
Sister chromatids become visible during prophase, the first stage of mitosis. The long, thread-like DNA (chromatin) condenses and coils tightly, making the replicated chromosomes compact enough for orderly movement.
The centromere region develops a specialized protein structure called the kinetochore. The kinetochore is the attachment point for the microtubules of the mitotic spindle, the cellular machinery responsible for moving the chromosomes.
During metaphase, the spindle fibers capture the chromosomes and align every sister chromatid pair precisely along the cell’s central axis, known as the metaphase plate. This alignment confirms that each sister chromatid is properly tethered to microtubules extending from opposite poles of the cell, setting the stage for their eventual separation.
The Moment of Separation and New Identity
The process culminates in anaphase, where the sister chromatids finally separate from each other. This separation is triggered by the breakdown of the cohesin proteins that hold the centromeres together. An enzyme called separase cleaves the cohesin, releasing the two chromatids from their tight connection.
Once the physical link at the centromere is broken, each former sister chromatid is instantly considered a full, independent chromosome. These newly independent chromosomes are then rapidly pulled toward opposite poles of the dividing cell by the shortening of the spindle microtubules attached to their kinetochores. The movement of the separated chromosomes toward the poles ensures that the genetic material is evenly and accurately distributed. The successful completion of anaphase guarantees that the two resulting daughter cells will each possess a complete and identical set of chromosomes.