Molecular Biology of the Cell by Bruce Alberts, Alexander Johnson, Julian Lewis, David Morgan, Martin Raff, Keith Roberts, Peter Walter by by Bruce Alberts, Alexander Johnson, Julian Lewis, David Morg

18 Chapter 1: Cells and Genomes
764 genes in families with
4–19 gene members
273 genes in families
with 3 gene members
283 genes in families with
38–77 gene members
2126 genes with
no family relationship
Figure 1–20 Families of evolutionarily
related genes in the genome of Bacillus
subtilis. The largest gene family in this
bacterium consists of 77 genes coding for
varieties of ABC transporters—a class of
membrane transport proteins found in all
three domains of the living world. (Adapted
from F. Kunst et al., Nature 390:249–256,
1997. With permission from Macmillan
Publishers Ltd.)
568 genes in families
with 2 gene members
are likely to have diverged in their function—are called paralogs. Genes that are
related by descent in either way are called homologs, a general term used to cover
both types of relationship (Figure 1–21).
Genes Can Be Transferred Between Organisms, Both in the
MBoC6 m1.24/1.20
Laboratory and in Nature
Prokaryotes provide good examples of the horizontal transfer of genes from one
species of cell to another. The most obvious tell-tale signs are sequences recognizable
as being derived from viruses, those infecting bacteria being called bacteriophages
(Figure 1–22). Viruses are small packets of genetic material that have
evolved as parasites on the reproductive and biosynthetic machinery of host cells.
Although not themselves living cells, they often serve as vectors for gene transfer.
A virus will replicate in one cell, emerge from it with a protective wrapping, and
then enter and infect another cell, which may be of the same or a different species.
Often, the infected cell will be killed by the massive proliferation of virus particles
inside it; but sometimes, the viral DNA, instead of directly generating these particles,
may persist in its host for many cell generations as a relatively innocuous
passenger, either as a separate intracellular fragment of DNA, known as a plasmid,
or as a sequence inserted into the cell’s regular genome. In their travels, viruses
can accidentally pick up fragments of DNA from the genome of one host cell and
ferry them into another cell. Such transfers of genetic material are very common
in prokaryotes.
Horizontal transfers of genes between eukaryotic cells of different species
are very rare, and they do not seem to have played a significant part in eukaryote
evolution (although massive transfers from bacterial to eukaryotic genomes have
occurred in the evolution of mitochondria and chloroplasts, as we discuss below).
ancestral organism
ancestral organism
gene G
gene G
SPECIATION TO GIVE TWO
SEPARATE SPECIES
GENE DUPLICATION
AND DIVERGENCE
species A
species B
later organism
gene G 1
gene G A gene G B
gene G 2
genes G A and G B are orthologs
genes G 1 and G 2 are paralogs
(A)
(B)
Figure 1–21 Paralogous genes and
orthologous genes: two types of
gene homology based on different
evolutionary pathways. (A) Orthologs.
(B) Paralogs.