Sister Chromatids: Definition & Concept

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  • 0:00 Chromatin, Chromosome,…
  • 1:26 Chromatids vs.…
  • 2:45 Sister Chromatids in Mitosis
  • 3:37 Sister Chromatids in Meiosis
  • 5:10 Lesson Summary
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Lesson Transcript
Instructor: Jennifer Szymanski

Jen has taught biology and related fields to students from Kindergarten to University. She has a Master's Degree in Physiology.

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.

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