Explore sister chromatids. Learn the definition of sister chromatids and understand the differences between homologous chromosomes and sister chromatids.

What are Sister Chromatids?

The replication of genetic material represents a central process of cell division in living organisms. In humans, there are two different types of cell division—mitosis and meiosis.

Mitosis is cell division that involves the somatic cells of the body. Somatic cells refer to the cells that make up the different structures of the body. The daughter cells produced during mitosis represent exact copies of the parent cell and contain the same genetic material. Moreover, the cells produced during mitosis are diploid, meaning that each cell has two sets of chromosomes. One set is derived from each parent.

By contrast, meiosis refers to the cell division process occurring in gametes. Gametes are the cells such as eggs and sperm that are involved in sexual reproduction. Unlike the cells produced during mitosis, the products of meiosis exhibit genetic variability derived from the recombination of genes from both parents. Furthermore, the eggs and sperm made during meiosis are haploid. Haploid is the term used to describe a single set of unpaired chromosomes.

In order for cell division to occur smoothly in living organisms, DNA is organized into discrete units referred to as chromosomes. Each strand of DNA is comprised of a long sequence of amino acids that codes for different proteins. To fit within the small space of a cell, DNA is first made more compact in size through structures known as histones. Histones are special proteins that act as the spool upon which DNA can wind, much like sewing thread.

When DNA wraps around a histone, it forms a new structure referred to as a nucleosome. Each nucleosome is arranged in a linear sequence like the beads on a necklace. These nucleosomes become further compacted to form a dense complex of DNA known as chromatin. Chromatin represents the genetic material that has been arranged and condensed to form a chromosome. Each chromosome consists of all the genetic material of an organism.

When chromosomes are replicated, they form chromatids. A chromatid refers to a chromosome that has undergone replication in preparation for cell division. What are sister chromatids? Sister chromatids are formed when two identical chromatids are joined together at the centromere. The centromere is a constricted region where two sister chromatids are in contact with each other.

In this lesson, the role, importance, and definition of sister chromatids will be further explored, particularly with regard to cell division.

Relationship Between Chromosome and Chromatid

The chromosome and chromatid are closely interrelated to one another. A chromosome consists of all the genetic information coding for the different traits in a living organism. When a cell is growing and is not undergoing mitosis or meiosis, chromosomes exist in a partially decondensed state in which they are not as tightly bound together by proteins. However, once mitosis or meiosis is initiated, the chromosomes must become better organized in order to facilitate the replication of genetic material. This is accomplished through the winding of DNA into tight bundles comprised of histones and nucleosomes. The nucleosomes undergo additional compaction to form a chromosome.

When a chromosome is replicated during mitosis or meiosis, two and four chromatids are formed respectively. The chromatids are identical in terms of their genetic information and are joined by a central region known as the centromere. The number of chromatids found in an organism is dependent upon the total number of chromosomes found in specific species. For example, fruit flies have a total of eight chromosomes. During mitosis, when these chromosomes are replicated, 16 sister chromatids would be present. By contrast, humans have 46 chromosomes. Therefore, 92 sister chromatids are formed during mitosis and chromosomal replication.

Homologous Chromosomes vs. Sister Chromatids

Sister chromatids are often confused with homologous chromosomes. However, there are several important distinctions when looking at homologous chromosomes vs sister chromatids.

First of all, sister chromatids refer to paired chromatids formed when chromosomes replicate. The sister chromatids are joined to each other via the centromere and only become separated when cell division occurs. Homologous chromosomes, by contrast, are pairs of genetically distinct chromosomes. While each pair codes for similar genetic traits, they are genetically non-identical. One chromosome comes from one parent, while the second chromosome comes from the second parent.

For example, when a cat receives a copy of chromosome number two from its mother and its father, these copies, together, are called a homologous pair. Humans, by contrast, have 23 pairs of homologous chromosomes, for a total of 46 chromosomes.

While sister chromatids are important for both mitosis and meiosis to occur, homologous chromosomes are specifically useful for cell division during meiosis. In meiosis, the homologous chromosomes facilitate crossing over, in which segments of a chromosome are exchanged between chromatids that are in physical contact with each other, a process known as synapsis.

During synapsis, homologous chromosomes pair up with each other to form a tetrad, in which four chromatids are present. Non-sister chromatids, which are genetically different from each other, then exchange segments of DNA to facilitate genetic variability during crossing over.

Importance of Sister Chromatids

Sister chromatids play several important roles during cell division. In the following subsections, the importance of sister chromatids in mitosis and meiosis will be further discussed.

Sister Chromatids in Mitosis

Mitosis represents the process of somatic cell division in humans. The daughter cells produced by mitosis are genetically identical to the parent cell.

During interphase, in which the cell is growing and preparing for cell division, the chromosomes undergo replication to form sister chromatids that are identical to each other. The sister chromatids are joined by a special structure known as the centromere. In prophase, the second stage of mitosis, spindle fibers form and attach to the centromeres of each sister chromatid pair. By metaphase, the next phase in the process, the spindle fibers have lined up the sister chromatids along the metaphase plate, an imaginary line drawn between the two opposite poles of the cell.

The sister chromatids are then separated from each other during anaphase, with each chromatid from a pair pulled to opposite sides of the cell. By telophase and cytokinesis, when two new cells have been formed, the sister chromatids are no longer present, and instead, the complete set of chromosomes is found. As suggested by this sequence, the sister chromatids are formed during interphase and remain attached to each other until anaphase. At this point, the sister chromatids will become separated from one another.

Sister Chromatids in Meiosis

Meiosis represents a similar but much longer process that eventually results in the creation of four haploid cells, each containing half the genetic material of the parent cell.

As with mitosis, sister chromatids are formed during interphase. However, because meiosis involves two rounds, the sister chromatids are not separated until the second half of this process. During the first phase of meiosis, the sister chromatids remain associated with each other in the form of homologous chromosomes. Two sister chromatids come from the mother and two from the father. When the homologous chromosomes are in physical contact with each other, synapsis has occurred. Then the process of crossing over causes a chromatid from the mother to exchange a segment of genetic information with the corresponding chromatid of the father. This helps to create genetic variations in living organisms.

When do sister chromatids separate? During the second phase of meiosis, starting with prophase II, spindle fibers again attach to the centromeres of each pair of sister chromatids. In metaphase II, the spindle fibers line up the sister chromatids along the metaphase plate. Then during anaphase II, the spindle fibers pull the sister chromatids apart.

At this point, the sister chromatids, having been altered by crossing over, are now genetically distinct from each other. By the end of meiosis, four haploid daughter cells have been created, each containing one of the four chromatids formed during the first phase of meiosis.

Lesson Summary

Chromosomes contain all of the genetic material associated with a specific organism. When a chromosome is replicated during mitosis and meiosis, chromatids are formed. A chromatid represents a chromosome that has undergone replication. Sister chromatids refer to pairs of chromatids that are genetically identical to each other. The number of sister chromatids formed in a cell is dependent upon the species, with fruit flies having 8 chromosomes capable of forming 16 sister chromatids.

An individual inherits one chromosome from his/her mother and one from his/her father. This pair of chromosomes is referred to as a homologous pair, (or homologous chromosomes). Homologous pairs undergo crossing over, a form of genetic recombination that only occurs during meiosis, when they are in close association with each other, a process known as synapse. During mitosis, sister chromatids are formed during interphase and remain attached to each other through metaphase. In meiosis, sister chromatids are present through both rounds of meiosis until reaching anaphase II, when the sister chromatids are separated.