Sister chromatids are identical copies of a single chromosome, formed during the process of DNA replication. These two identical strands are joined together at a specific point called the centromere. When a chromosome is in this duplicated state, it often appears as an “X” shape. This structure is temporary and serves as a way to manage and transport the duplicated genetic information before a cell divides.
Formation of Sister Chromatids
The creation of sister chromatids is a fundamental part of a cell’s life cycle, occurring during a period known as interphase. Specifically, they are synthesized during the Synthesis phase, or S phase, which happens before the cell is ready to undergo division. During the S phase, the cell undertakes the complex task of DNA replication, where it makes an exact copy of its entire genome.
Think of the cell’s original set of chromosomes as a master blueprint of instructions. Before the cell can divide to create two new functional cells, it must first make a perfect photocopy of this entire blueprint. This “photocopy” is the second chromatid. The original and the copy—the two sister chromatids—remain attached, ready to be distributed. This ensures that when the cell splits, each new daughter cell receives a complete and identical set of genetic instructions.
Role in Cell Division
The function of sister chromatids is to facilitate the accurate distribution of genetic material during cell division. In both mitosis (the division of body cells) and meiosis (the creation of sex cells), once the sister chromatids are formed, they condense and become tightly coiled, making them visible under a microscope. During a stage called metaphase, these duplicated chromosomes, each consisting of two sister chromatids, align at the cell’s equator.
This alignment is a prelude to the main event of separation, which occurs during a stage called anaphase. A complex of proteins, including a substance called cohesin that has been holding the sister chromatids together, is broken down. Once the connection is severed, spindle fibers, which are part of the cell’s structural skeleton, pull the sister chromatids apart toward opposite poles of the cell. At this moment of separation, each chromatid is now considered a chromosome.
In mitosis, this process results in two daughter cells that are genetically identical to the parent cell, both containing a full set of chromosomes (diploid). In the context of producing gametes through meiosis, a similar separation of sister chromatids occurs during the second meiotic division (meiosis II). This step is what allows for the creation of haploid cells, which have half the number of chromosomes as the original cell.
Sister Chromatids vs. Homologous Chromosomes
A common point of confusion is the difference between sister chromatids and homologous chromosomes. Homologous chromosomes are a pair of chromosomes found in diploid organisms, with one chromosome inherited from the maternal parent and the other from the paternal parent. They carry genes for the same traits, such as eye color or blood type, but they are not genetically identical because they can carry different versions (alleles) of those genes.
In contrast, sister chromatids are the result of DNA replication. They are two exact, identical copies of a single chromosome, created just before cell division. Therefore, their genetic information is identical, barring any rare mutations that might occur during the replication process. The distinction lies in their origin: homologous chromosomes come from two different parents, while sister chromatids are duplicate copies made by the cell itself.
Their behavior during cell division also differs, particularly in meiosis. During the first meiotic division (meiosis I), it is the homologous chromosome pairs that separate from each other, moving to different daughter cells. The sister chromatids, however, remain attached and move together as a single unit. It is only in the second meiotic division (meiosis II) that the sister chromatids separate, in a process similar to what happens in mitosis.
Consequences of Separation Errors
Errors during the precise separation of sister chromatids can have significant consequences. The failure of sister chromatids to separate correctly is known as nondisjunction. When this happens, it disrupts the equal distribution of chromosomes into the daughter cells. One daughter cell may end up with an extra chromosome, while the other will be missing one.
This imbalance in chromosome number is a condition called aneuploidy. Aneuploidy can lead to various medical conditions if it occurs during the formation of gametes. One of the most well-known examples of aneuploidy resulting from nondisjunction is Trisomy 21, commonly known as Down syndrome. This condition occurs when an individual inherits three copies of chromosome 21 instead of the usual two. The presence of this extra genetic material leads to the characteristic physical and developmental traits associated with the syndrome.