There are two copies of each chromosome in every cell of an organism. One set of chromosomes comes from the mother and one comes from the father. The chromosomes shown to the right have been spread out on a slide to help scientists called cytogeneticists to tell them apart.

Cytogeneticists use three things to tell chromosomes apart:

1. characteristic banding patterns (caused by staining the chromosomes with dyes)
2. position of the centromeres (see also below)
3. chromosome size

Look at the graphic of the two chromosomes to the left. They are the same length and have a similar centromere position, but the banding pattern is different. Do you think they are the same?

Banding patterns you see in the image of the real chromosomes above are NOT genes. They are just areas of the chromosome that are dyed more deeply than other areas. Print the picture of the chromosomes, cut the chromosomes out, and arrange them in their pairs, using staining patterns, centromere position, and size as your guide. Can you match the pairs? Hang in there! This is tough!

Why are the chromosomes in the picture above shaped like X’s? (Click on the picture for a closeup.) These chromosomes were taken from a cell that was getting ready to divide. Before cell division, chromosomes are duplicated so that each of the resulting two cells can have a complete set of chromosomes. In the chromosomes that look like X’s, the cross of the X joins the two duplicate chromosomes together. This point is called the centromere. Joining duplicate chromosomes helps organize the cell division process. Cells have several safety mechanisms like this to make sure that every cell receives one copy of each chromosome. Scientists like to look at the chromosomes just prior to cell division because they are easiest to see at this stage. At other times of the cell cycle, chromosomes look like a plate of spaghetti noodles.