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

222 Chapter 4: DNA, Chromosomes, and Genomes
Figure 4–67 Synteny between human
and mouse chromosomes. In this
diagram, the human chromosome set
is shown above, with each part of each
chromosome colored according to the
mouse chromosome with which it is
syntenic. The color coding used for each
mouse chromosome is shown below.
Heterochromatic highly repetitive regions
(such as centromeres) that are difficult to
sequence cannot be mapped in this way;
these are colored black. (Adapted from
E.E. Eichler and D. Sankoff, Science
301:793–797, 2003. With permission
from AAAS.)
1 2 3 4 5 6 7 8 9 10 11 121314 15 16 17 18 19 20 2122 X
mouse
chromosome
index 1 2 3 4 5 6 7 8 9 10 111213 14 15 16 1718 19 X
Good evidence for the loss of DNA sequences in small blocks during evolution
can be obtained from a detailed comparison of regions of synteny in the human
and mouse genomes. The comparative shrinkage of the mouse genome can be
clearly seen from such comparisons, MBoC6 m4.601/4.66 with the net loss of sequences scattered
throughout the long stretches of DNA that are otherwise homologous (Figure
4–68).
DNA is added to genomes both by the spontaneous duplication of chromosomal
segments that are typically tens of thousands of nucleotide pairs long
(as will be discussed shortly) and by insertion of new copies of active transposons.
Most transposition events are duplicative, because the original copy of the
transposon stays where it was when a copy inserts at the new site; see, for example,
Figure 5–63. Comparison of the DNA sequences derived from transposons
in the human and the mouse readily reveals some of the sequence additions
(Figure 4–69).
It remains a mystery why all mammals have maintained genome sizes of
roughly 3 billion nucleotide pairs that contain nearly identical sets of genes,
even though only approximately 150 million nucleotide pairs appear to be under
sequence-specific functional constraints.
The Size of a Vertebrate Genome Reflects the Relative Rates of
DNA Addition and DNA Loss in a Lineage
In more distantly related vertebrates, genome size can vary considerably, apparently
without a drastic effect on the organism or its number of genes. Thus, the
chicken genome, at one billion nucleotide pairs, is only about one-third the size
200,000 bases
human chromosome 14
mouse chromosome 12
Figure 4–68 Comparison of a syntenic
portion of mouse and human genomes.
About 90% of the two genomes can be
aligned in this way. Note that while there
is an identical order of the matched index
sequences (red marks), there has been a
net loss of DNA in the mouse lineage that
is interspersed throughout the entire region.
This type of net loss is typical for all such
regions, and it accounts for the fact that the
mouse genome contains 14% less DNA than
does the human genome. (Adapted from
Mouse Genome Sequencing Consortium,
Nature 420:520–562, 2002. With permission
from Macmillan Publishers Ltd.)