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.
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.
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.
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.
To make a karyotype, take an image of your stained chromosomes and cut them out. Then line them up in pairs based on the banding pattern, in order from the longest to the shortest set of chromosomes. The longest set of chromosomes is Chromosome 1, the second longest is Chromosome 2, and so on. You line them up in pairs because humans have two copies of each chromosome - one from Mom and one from Dad.
In humans, we have Chromosome 1 through Chromosome 22. What about the 23rd set? Well, the 23rd set is the set of sex chromosomes. If you're female, you will have two X chromosomes. If you're male, you will have an X and a Y chromosome.
Why Stain Chromosomes?
One context in which staining chromosomes is useful is that we can see abnormalities in the chromosome. We can find Down syndrome, for instance, in which there is one extra copy of Chromosome 21. We can also see abnormalities in chromosome structure; for instance, chromosomes that form a ring shape instead of straight lines.
Another context in which staining chromosomes is useful is that we can compare the similarities in banding patterns between samples. For instance, you share many genes with a chimpanzee. You can see this when you look at a chimpanzee chromosome's banding pattern. The banding is nearly the same as yours! This allows us to visualize the evolutionary relationship between humans and chimpanzees.
A chromosome is a unit of tightly-wrapped DNA. We use chromosome banding to dye chromosomes so that we can better see and understand their features. There are several types of chromosome banding. Some common types of chromosome banding are Q-banding, which yields a fluorescent pattern; C-banding, which only stains the centromeres (remember that centromeres are little constricted portions of chromosomes); and R-banding, which stains non-centrometric regions. However, G-banding is the most common and causes chromosomes to yield a characteristic striped pattern. The G in G-banding comes from Giemsa stain, which was developed in the early 20th century by German scientist Gustav Giemsa. It is a mixture of several stains including methylene blue and azure, which yield a compound called an eosin compound.
Scientists use karyotyping, a way to organize and characterize chromosomes, to view similarities in chromosomes and to find possible genetic abnormalities such as Down's syndrome. In karyotyping, pairs of chromosomes are lined up from longest to shortest. Humans have 23 pairs of chromosomes. They are labeled Chromosome 1 through Chromosome 22, and the sex chromosomes, X and Y.