Before a cell divides, it must make a complete copy of its genetic material. This duplication process ensures that each new daughter cell will have the necessary instructions to function correctly. When a chromosome is copied, it results in the formation of two identical strands that are temporarily joined together.
These paired, identical copies of a single chromosome are known as sister chromatids. As long as these two copies are connected, they are considered a single duplicated chromosome. This paired structure is a temporary state for the genetic material, allowing for its organized management and distribution during cell division.
Formation During DNA Replication
The creation of sister chromatids occurs during the S phase (synthesis phase) of the cell cycle. During this time, the cell duplicates its entire genome in preparation for division. The process begins with the DNA double helix unwinding its two strands. Each single strand then serves as a template for the construction of a new, complementary strand.
Cellular machinery synthesizes new DNA, resulting in two identical double-helix molecules where only one existed before. The original chromosome and its newly made counterpart are the sister chromatids. From the moment of its creation, the new chromatid is linked to its original template and maintained by a specialized group of proteins.
This connection is mediated by a protein complex called cohesin. Cohesin acts as a molecular glue, forming a ring-like structure that holds the two sister chromatids together along their entire length. This process happens during DNA replication, ensuring that as a segment of DNA is copied, it is immediately tethered to its original, keeping the pair secure.
Function in Organizing Genetic Material
The pairing of sister chromatids serves an important organizational purpose. A human cell contains approximately two meters of DNA, which must be managed without tangling during cell division. This paired structure allows the cell to package its genetic material into a much more compact and manageable form.
During prophase, an early stage of cell division, the long strands of chromatin undergo a transformation. They condense and coil upon themselves, a process called supercoiling. This makes the paired sister chromatids so compact that they become visible under a light microscope as the familiar X-shaped structure.
The most constricted point of the “X” is a region of DNA called the centromere, where the two sister chromatids are held together most tightly. On the surface of each chromatid’s centromere is a protein structure known as the kinetochore. The kinetochore functions as the attachment point for microtubules, the fibers that orchestrate chromosome movement.
Separation During Cell Division
The separation of sister chromatids occurs during anaphase. After the duplicated chromosomes have aligned at the cell’s equator, a biochemical signal initiates the next step. This signal is generated by the anaphase-promoting complex (APC).
The activation of the APC unleashes an enzyme known as separase. For most of the cell cycle, separase is kept inactive by an inhibitory protein called securin. The APC targets securin for destruction, liberating separase to perform its function. Active separase then cleaves the cohesin complexes that hold the sister chromatids together.
With the cohesin broken, the mitotic spindle fibers attached to the kinetochore on each chromatid begin to shorten. This pulls the now-independent chromatids toward opposite ends of the cell. The moment they are pulled apart, they are no longer referred to as sister chromatids; each is now considered an individual chromosome. This separation ensures one full set of chromosomes is delivered to each forming daughter cell.
Errors in Separation and Genetic Integrity
The process of separating sister chromatids is highly regulated, but it is not infallible. An error during this stage, where the sister chromatids fail to separate correctly, is termed nondisjunction. When this occurs, the mitotic spindle pulls both sister chromatids to one pole of the cell, leaving the other pole with a missing chromosome.
The outcome of nondisjunction is aneuploidy, where cells contain an incorrect number of chromosomes. One daughter cell will be trisomic for that chromosome (having three copies), while the other will be monosomic (one copy). Most instances of aneuploidy in humans are not compatible with life, leading to the failure of an embryo to develop.
A well-known example of aneuploidy is Down syndrome, or Trisomy 21. This condition occurs when nondisjunction affects chromosome 21 during the formation of an egg or sperm cell. If this gamete is involved in fertilization, the resulting individual will have three copies of chromosome 21 in every cell. Frequent errors in chromosome separation are also a hallmark of many cancers, as this chromosomal instability contributes to the genetic chaos that fuels tumor growth.