Chapter 6
Chromosomes: The Big Picture
IN THIS CHAPTER
How DNA is packaged into chromosomes
Understanding chromosome structure
How sex is determined in humans and other animals
Nearly every cell of the human body has a complete set of chromosomes. Chromosomes, which means “colored bodies,” were named as such because these structures were found to absorb certain dyes when cells undergoing mitosis were studied. Chromosomes were identified long before we knew that DNA was the genetic material and genes were the individual “units of heredity.” Chromosomes were first observed in plant cells in the 1840s by the Swiss botanist Karl Wilhelm von Nägeli.
Now we know that all organisms have chromosomes. We also know that the location and number of chromosomes vary depending on the species. In prokaryotes like bacteria, there is a single ring-shaped chromosome inside the cell (there is no cell nucleus). In humans, there are 46 total chromosomes (23 pairs) in the nucleus of each cell. The number of chromosomes is generally consistent within a species, but it can vary significantly between types of organisms, ranging from 1 to more than 200!
Chromosome structure allows all an organism’s DNA ...
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