Aside from the fun of peering at the stuff that codes for the parts that make up us, knowing how a chromosome is arranged inside a cell nucleus might tell us a lot about how our genes work. Understanding the structures of genomes might well be what is needed to make sense of the genetics of complex diseases.
So just what does a chromosome look like?
Are our genomes stuffed into the cell nucleus like loose string jammed into a bag or are they arranged in an organised way? Is the arrangement of genes that are being used in the cell different from those that are not being used? Are genes that are controlled in similar ways near each other? Do different chromosomes interact with eachother?
You’ll have seen images of chromosomes looking like two rods pinched together in the middle, like in the electron micrograph to the right, or perhaps as part of a karyotype (karyogram or idiogram)- showing the collection of chromosomes in your cell stained to revealed a banding pattern shown below.
Karyotypes, like that of a person with Down syndrome below, are typically used to investigate loss of large parts of chromosomes (deletions), or swops of large portions between chromosomes (rearrangements), that can be associated with different diseases or syndromes. Small changes can happen and affect genetics, too, but are too small to be seen this way.
Chromosomes can also change shape, depending on what genes are being used and what stage in the cell’s lifecycle they are in.
Just like us, our cells have a lifecycle, growing and producing offspring (daughter cells). The chromosomes seen in karyotypes and the classic ‘X’ are from cells that are from cells about to divide into two daughter cells. In this stage of a cell’s lifecycle, chromosomes aren’t doing the work of a growing cell, but are tightly packed and aligned up against each other ready to be pulled apart into the two daughter cells. (Also, these cells have been broken apart so that the chromosomes can float free.)
During the ‘growth’ part of the cell cycle, the cell does the chemistry that type of cell gets up to. If you were to look down a microscope into these cel, you couldn’t see how chromosomes are organised, even if you added chemicals that stain DNA or the packaging proteins (histones) that our DNA is wrapped around. What you’d see is an opaque mess that you couldn’t make much sense of.
Staining for specific features can tell scientists if those features are near the edge (periphery) of the nucleus or the middle, or if the features seem to be clustered together inside the nucleus, but you couldn’t really make sense a whole chromosome this way, it’s just too confusing.
This is a classic problem in science: trying to create a picture of something you can’t see or make sense of directly.
What to do then?