Multicellular Systems - Molecular Theory of the Living Cell - page 513

(Sect. 3.1 ) . To use a familiar metaphor, embryos are akin to audio music and the
genomes are akin to sheet music . Because the quality of audio music depends on
many more factors (such as the artistic skills of the musician, the kinds of
instruments employed, the environmental factor, etc.) than the quality of a sheet
music, the information content of audio music is much greater than the information
content of a sheet music. One characteristic of sheet music is that it can be handed
down from one generation to another without any change. As evident in Fig. 15.1 ,
both the genome and the volume of the Drosophila embryo remain more or less
constant during most of the embryogenesis, but the algorithmic information content
of the embryo (and hence the IDE) obviously increases as the Drosophila embryo-
genesis progresses. The genomic information density of a species tends to increase
over generations (i.e., on the diachronic timescale, not on the synchronic timescale;
see Sect. 14.4 and Fig. 14.3 for the definitions of “synchronic ” and “diachronic ”
timescales), leading to Statement 15.3:
The information density of the genome increases with diachronic time; the information
density of embryos increases with synchronic time. (15.3)

Statement 15.3 is consistent with the Law of Maximum Complexity, Statement
14.15, described in Sect. 14.3 .
The transcription factors coded for by segmentation genes also regulate the
Law of Maximum Complexity . These genes are located in Drosophila chromosome
3, and the order of the genes on the chromosome determines the order in which they
are expressed along the anterior-posterior axis of the embryo. The Antennapedia
group of homeotic selector genes includes labial, antennapedia, sex combs
reduced, deformed, and proboscipedia. Labial and deformed genes are expressed
in head segments (where their protein products activate the genes defining head
features), and sex-combs-reduced and antennapedia genes encode the proteins that
specify the properties of thoracic segments. In Drosophila , antennae and legs are
created by the same program except a single transcription factor. When this
transcription factor is mutated, the fly grows legs on its head in place of antennae,
a phenomenon known as the antennapedia mutation ( http://zygote.swathmore.edu/
droso4.html ) .
The antennapedia homeodomain is a sequence-specific transcription factor
from Drosophila encoded by the Antennapedia ( antp ) gene. The antennapedia
homeodomain (Antp) is a member of a regulatory system that gives cells specific
positions on the anterior-posterior axis of the organism. Antp aids in the control of
cell development in the mesothorax segment in Drosophila . The homeobox domain
(or homeodomain) binds DNA through a helix-turn-helix structural motif.
Homeobox genes (about 180 base pairs) were discovered in 1983, and the
proteins they encode, the homeodomain proteins (~60 amino acid residues long)
have been found to play important roles in the developmental processes of many
multicellular organisms. They have been shown to play crucial roles in embryo-
genesis and have a wide phylogenetic distribution. Hundreds of homeobox genes
have been described in baker's yeast, plants, and all animal phyla (B

€ urglin 1996).