Genomes: Gene Number, Density & Size

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  • 00:00 Definition of Reference Groups
  • 00:27 Genome Size
  • 1:42 The Number of Genes
  • 2:57 Gene Density
  • 3:55 Lesson Summary
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Lesson Transcript
Instructor: Artem Cheprasov

Artem has a doctor of veterinary medicine degree.

This lesson goes over the concepts of genome size, gene density, and gene number, and compares these three concepts to the three domains: Bacteria, Eukarya, and Archaea.

Definition of Reference Groups

People vary in number, like how many people there are in one country or another. We also vary in density; cities are more densely populated than rural communities. And, of course, people vary in size. Some of us are vertically challenged, and others have to duck under a doorway. Sort of similarly, genomes can vary too in size, number, and density. Let's see what this actually means.

Genome Size

When we classify living organisms, we typically organize everything into three domains. They are: bacteria, archaea, and eucarya. Bacteria include, well, bacteria. Archaea involves stuff like microorganisms living in really crazy environments like volcanic springs. Eukarya includes you and me as well as other animals, fungi, and plants. One of the ways we can understand variations in genome size is to compare these domains.

In general, most bacterial genomes have somewhere around 1 to 6 million base pairs. For example, E. coli, a bacterium, has 4.6 million base pairs (which we can abbreviate as 4.6 Mb). The genome sizes for archaea are pretty similar to those of bacteria, but compared to both of these domains many eukaryotic genomes are larger. A fungus like Saccharomyces cerevisiae has about 12 Mb. Why so little? Well, it is only made of one cell. But if we look at multi-cellular organisms like plants and animals, then their genomes are at least 100 Mb. Humans have 3,000 Mb.

The Number of Genes

The number of genes that a particular organism has also varies between the three domains, as you can only imagine. In general, bacteria and archaea have anywhere from 1,500 to 7,500 genes. For eukaryotes, this number varies from 5,000 genes for unicellular fungi to at least 40,000 genes for some multicellular organisms. What's really interesting to note is that the number of genes humans have, estimated to be about 21,000 or so, is actually similar to the number of genes a nearly microscopic nematode (a roundworm) has.

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