Lecture 36 Genomes
Campbell; Chapter 19. 6
Ed.; 356-361 7
We are now in the era of the genome. Scientists have been conducting genome projects
(meaning that the nucleotide sequence of an entire genome is being determined) on several species.
In addition to the human genome project, the genomes of important model organisms, including
(fruit fly), yeasts, and
have been determined in the last few years.
Genomes of important crops (like rice) and pathogens, including bacteria and protozoa, are also on
the list. By learning more about our genes and their protein products, we can devise new strategies
for fighting diseases that result from defective genes. Genomes of crops are being sequenced with
the goal of developing improved strains, while pathogen genomes will hopefully aid in developing
improved therapies for the diseases caused by these organisms.
Gene numbers and genome sizes
The following table lists the numbers of genes, and also the size
(in million base pairs or Mbp) of the genomes of several species.
# of genes
Genome size (Mbp)
C. elegans (worm)
This table illustrates the maximum gene number generally found in prokaryotes (i.e., 4,288 genes in
E. coli) and also the range of gene numbers found in eukaryotes (i.e., from around 6,000 genes in
unicellular organisms to around 25,000-30,000 in higher eukaryotes.) Several surprising points
emerge from this table. First, even though budding yeast (a eukaryote) seems much more complex
than E. coli, since it has organelles like our own, it doesn’t have that many more genes. Second,
very complex organisms (like us) have only 7 times as many genes as E. coli. Third, plants, which
may seem at first glance to be much simpler than animals, have almost as many genes. A final
important point is the lack of correlation between gene number and genome size in different
organisms. Comparing E. coli and humans illustrates this. We may have only 7 times more genes
than E. coli, but our genome is 5,800 times as large! Simpler multi-celled eukaryotes (
) are intermediate between E. coli and mammals in their gene number:genome size
Explanation for the wide variation in gene number:genome size ratio
Some organisms have a lot of DNA that doesn’t code for RNA or protein, while others have very
little. At one extreme, more than 90% of the E. coli genome codes for RNA and protein. Much of
the remaining DNA is made of promoter sequences and regulatory sequences like operators.
Humans are at the other extreme: only 1.5% of our DNA codes for useful RNA and protein! Most