Chromosome Banding: Definition & Techniques

Chromosome Banding: Definition & Techniques
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  • 0:03 What is Chromosome Banding?
  • 1:35 Giemsa Staining
  • 2:30 Karyotypes
  • 3:50 Why Stain Chromosomes?
  • 4:35 Lesson Summary
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Lesson Transcript
Instructor: Stephanie Gorski

Steph has a PhD in Entomology and teaches college biology and ecology.

Here we will discuss what chromosome banding is. We will also look at several different types of chromosome banding, techniques for chromosome banding, and why one would want to perform chromosome banding.

What is Chromosome Banding

You may talk about your genes from time to time - 'Oh, I have the gene for that.' But how do you see your genes? A gene is a functional unit of DNA, and your DNA is organized onto chromosomes. Chromosome banding is a little like tie-dying your chromosomes.

A chromosome is a unit of tightly-packed DNA. DNA has to wrap tightly around itself, because you have quite a lot of it. In fact, if you unrolled all the DNA in a single one of your cells, it would be about three meters long. Humans have 46 chromosomes - 23 from Mom and 23 from Dad.

In chromosome banding, we treat chromosomes with chemicals to stain them and learn about a chromosome by how it stains. There are several different types of stains we can use.

Banded chromosomes
Banded chromosomes

There are several types of chromosome banding. Here, we will list a few of the most common types.

  • G-banding uses a stain called Giemsa stain. G-banding gives you a series of light and dark stripes along the length of the chromosome. We will discuss G-banding in the most detail, because you will likely see G-banding if you take a genetics class.
  • Q-banding uses a stain called quinacrine. Q-banding yields a fluorescent pattern. It is similar in pattern to G-banding, but glows yellow.
  • C-banding only stains the centromeres. Centromeres are little constricted portions of chromosomes. That's where sister chromatids (two copies of the same chromosome) will attach to each other when the cell is getting ready to divide.
  • R-banding is the opposite of C-banding. R-banding stains non-centromeric regions.

Giemsa Stain

G-banding is useful because the patterns of stripes on the chromosomes are unique enough that you should be able to confidently identify each chromosome.

Giemsa staining was named after the German scientist Gustav Giemsa, who worked in the early part of the 20th century. Giemsa's immediate goal was to find a stain that would work on Plasmodium, the parasite that causes malaria. Giemsa stain, however, was quickly found to have many uses. Dr. Giemsa lamented the fact that he would be known for his staining procedure rather than for his work on tropical diseases.

Giemsa stain is a mixture of a stain called methylene blue and one called azure, which form a type of stain called an eosin compound. Researchers will typically wash a sample in Giemsa stain for around seven minutes. You would typically stain chromosomes during the early parts of the cell cycle (prophase or metaphase), because the chromosomes are partially but not fully condensed.

Karyotypes

A karyotype is a profile of a person's chromosomes, organized by size. Scientists will use a karyotype to identify any abnormalities that may lead to a genetic disorder. For instance, people who have Down syndrome carry an extra copy of Chromosome 21. Having an extra chromosome makes it hard for cells to properly regulate how much protein to make. Down syndrome is a developmental disorder that is characterized by intellectual disability and distinctive facial features such as a flat face, abnormal ears, large tongue, and upward-slanting eyes. People with Down syndrome are prone to medical complications including respiratory problems, heart defects, hearing loss, and leukemia.

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