In genetics, ‘sister chromatids’ are pieces of identical DNA that are crucial in the process of cell replication and division. In this lesson, we’ll explore how sister chromatids fit in our genome as well as the role they play in both mitosis and meiosis.
Chromatin, Chromosome, or Chromatid?
Before we get into specific details about sister chromatids, let’s review some of the terms that scientists use to describe genetic material. Because these pieces of genetic material have somewhat similar functions and because so many start with chrom-, things can sometimes get confusing!DNA’s structure is a classic double helix made mostly of sugar, phosphorous, and nitrogenous bases. This double helix is organized, in part, with the help of special proteins called histones. Most of the time, the DNA-histone complex is diffuse, or spread out, in the nucleus of a cell and is called chromatin. However, when a cell is getting ready to divide, chromatin condenses into denser bodies called chromosomes.
When a single chromosome has been replicated in copies, each copy is called a sister chromatid. A special part of the chromosome called the centromere holds the two sister chromatids together. When you see an ‘X’ representing genetic material in an illustration, you are seeing two sister chromatids held together by the centromere.
Kind of like if you wanted to keep two sticks together, you might use a rubber band around the center of both. A centromere is like a built-in rubber band for two sister chromosomes.
Chromatids vs. Homologous Pairs
It’s important to note the difference between sister chromatids and homologous chromosomes.
Sister chromatids are used in cell division, like in cell replacement, whereas homologous chromosomes are used in reproductive division, like making a new person.Sister chromatids are genetically the same. That is, they are identical copies of one another specifically created for cell division. In fact, the term sister chromatid is only used during the parts of cell division when the structures are in that X shape or when the two copies are connected by a centromere. On the other hand, a pair of homologous chromosomes consists of two non-identical copies of a chromosome, one from each parent.
For example, one of your skin cells has a copy of chromosome number one from your mother and a copy of chromosome number one from your father. These are a homologous pair and present in all of your skin cells all of the time. But if you should happen to cut yourself, and skin cells are preparing to divide to heal the wound, then all 46 of the chromosomes in those cells have been replicated, and sister chromatids are present.Let’s see how these differences become important during cell division.
Sister Chromatids in Mitosis
In mitosis, sister chromatids originate during the S phase of the cell cycle. The ‘S’ stands for synthesis – and that is exactly what happens.
The DNA that is synthesized is identical to the original DNA. Therefore, when mitosis begins in a human body cell, its DNA consists of 46 chromosomes and 92 sister chromatids.The sister chromatids stay attached to their identical twin until the anaphase stage of mitosis. At this time, since they are no longer connected by a centromere, they are considered to be chromosomes. By the end of mitosis, two new cells have been created that are identical to the first, with 46 chromosomes in each. Sister chromatids function as a means to get a copy of each chromosome into the new cells.
Sister Chromatids in Meiosis
Sister chromatids play a slightly different role in meiosis, or the process by which reproductive (sperm and egg) cells are made. Like mitosis, meiosis begins with a replication of DNA that creates sister chromatids. Unlike in mitosis, however, the chromosomes organize themselves in a specific way, migrating until they are across from their homologue. This process of matching up homologous chromosomes, also called synapsis, is extremely important in creating genetic variability, or the differences between the DNA that sperm and egg cells carry that ultimately make new organisms genetically unique.During synapsis, genetic material undergoes what is called crossing over.
During crossing over (also called genetic recombination,) chromosomes move between the members of the homologous pair. It is important to note that they do NOT move between sister chromatids.In meiosis, sister chromatids remain attached via their centromere through the entire first cell division (meiosis I.
) It’s not until anaphase II that they separate and become individual chromosomes. Sister chromatids play an important role in meiosis in that they provide a structure for the exchange of genetic information in synapsis and ensure that the correct genetic material gets into the four genetically different cells that result from meiosis.
Sister chromatids are identical pieces of genetic material that are formed during the S (synthesis) phase of the cell cycle. The term sister chromatid only applies when the identical copies are closely associated with one another and held together by a centromere. When they move apart during anaphase of mitosis or anaphase II of meiosis, the genetic material goes from being sister chromatids to individual chromosomes.
Sister chromatids play an important role in both types of cell division, as they help ensure that only one copy of each gene gets into the newly formed cells.