Agriculture Reference
In-Depth Information
the segregated domains of plasma membrane likely generated the membrane-bound
organelles of eukarya.
One hypothesis is that cells from the prokaryotes joined in a symbiotic relationship
to generate the first eukaryotes about 2.7 bya. Later, lateral transfer of the genome
was surrounded by plasma membrane to become the eukaryotic nucleus. Genomic
evidence has established that eukaryotes acquired mitochondria about 1.8 bya when
a protobacterium became symbiotic. The bacterial genome thus contributed molecu-
lar machinery for ATP synthesis by oxidation phosphorylation. Over many centuries
of evolution, most symbiotic bacterial genes moved into the host cell nucleus.
The acquisition of chloroplasts began when the cyanobacterial symbiant brought
photosynthetic machinery into a primitive algal cell that had mitochondria by lateral
transfer about 1.6 bya. Symbiosis evolved into interdependence when the chloro-
plast genes were assimilated into the nuclear genome. About 2 bya, the algal and
plant branches of eukarya evolved independent strategies for multicellular existence.
Further increases in organism cell number occurred about 1.5 bya (Hedges et al.,
2004). Fossils confirm that animals had evolved multicellular structures by 0.6 bya.
These primitive metazoans (multicell organisms) had mouth, intestine, and sensory
structures. Evolution of genes for intercellular adhesion proteins pre-dated the meta-
zoan animals. The early metazoan animals resemble contemporary animal embryos
in appearance. In this period, animals diverged into the three subdivisions: mol-
lusks, annelid worms, bracheopods, and platyhelminths (~1.3 bya); arthropods and
nematodes (~1 bya); and echinoderms and chordate (~0.5 bya, humans at ~ 0.06 bya)
(Hedges et al., 2004; Pollard et al., 2008, p. 17).
genomIc hIstoRy of dIgestIon
The full scope of nutrient metabolism is discussed in Chapters 5-8. Here, we review
the evolution of the major digestive processes that demark the boundary between
human negative entropy intake (food) and nutrient utilization (metabolism). The
macronutrients are viewed as purveyors of essential food micronutrients. We trace the
genomic history (phylogenies; Huerta-Cepas et al., 2007) of the major human diges-
tion enzymes. As described, digestion was well established in the prokaryotic organ-
isms at 3.5 bya and advanced to a compartmentalized system in early eukarya by 2
bya. In fossil metazoan animals, a gastrointestinal tract was evidenced by 0.6 bya.
P
r of t e i n
D
i g e s t i o in
The central dogma of molecular biology is that DNA is transcribed into RNA, and
this RNA is translated into proteins (Pollard et al., 2008, p. 251). Thus, proteins are
most tightly controlled by the genome. This dogma has significance for the pro-
tein substrates and the digesting enzymes converting food into oligopeptides and
free amino acids for absorption. The synthesis of all proteins is called
translation
because of the conversion of the genetic code into amino acids in the peptide chain
by messenger RNA (mRNA). Small transfer RNA (tRNA) is the purveyor of specific
amino acids in response to successive codons within the mRNA. The codons are
made up of three nucleic acids whose sequence is transcribed from genomic DNA.
