Seeing chromosomes

Did you know that chromosomes can be seen under a light microscope, and they get their name from the fact that they are strongly stained by certain colored dyes?

I was reading through our government’s collection of educational materials on genomics. Going through the history of the gene I was startled to learn that people knew about chromosomes by the late 1800s. I had assumed they were a later discovery, requiring more sophisticated instruments than a simple light microscope.

Walther Fleming was the first to report seeing these structures while studying cell division. The dye he was using, aniline, was heavily absorbed by these mysterious granules in the nucleus, which he called chromatin. Though human chromosomes can be quite long (Chromosome 1, the longest, is 85mm long), being a single molecule (!) you can imagine it’s quite invisible even under a microscope when stretched out.

It turns out, however, that during cell division the chromosomes condense and become dense enough that a stain will make them visible under a light microscope. Since the job of the DNA in the chromosomes is to serve instructions to create proteins, it is probably important for the DNA to be “unfurled” so it has a large surface area for ready transcription. When a cell needs to divide, however, the nuclear material needs to be transported and shuffled around, and it is best to have it in a tidy ball to do so.

The study of the structure of chromosomes using microscopes is called cyto-genetics. One could imagine that the physical structure of the chromosome is not important for the understanding of the genetic code. We mostly study DNA as a one dimensional, linear sequence of characters we call the genetic code. It turns out, however, that the physical three dimensional organization of chromosomes can affect which proteins are produced and in what quantity (“gene expression”)

I’m also told that chromosomes have their own domains in the nucleus – they are not just tendrils floating willy nilly in the nucleus.

Yup, that was your small, sticky ball of biological facts from me today …