Table 1 summarizes selected characteristics of organisms from each of the three domains of life.

Terms in this set (18)

Over the years, the classification system that scientists use has changed several times. Currently, most scientists use a system based on DNA analysis that accurately reflects evolutionary history and the relationships among organisms. This system is called the three-domain system In it, all life is organized into three domains: Bacteria, Archaea, and Eukarya.
Figure 9.1 shows the organization of the current three-domain system of classification. (The term Monera is no longer used in this system. Instead, prokaryotes are spread across two different domains, Archaea and Bacteria.)
Table 9.1 shows a comparison of Bacteria, Archaea, and Eukaryotes. Notice, for some characteristics, the archaea resemble eukaryotes more than they resemble prokaryotes.

■ All are single-celled prokaryotes with no internal membranes (no nucleus, mitochondria, or chloroplasts).
■ Some are anaerobes; some are aerobes.
■ Bacteria play a vital role in the ecosystem as decomposers that recycle dead organic matter.
■ Many are pathogens, causing disease.
■ Bacteria play a vital role in genetic engineering. The bacteria from the human intestine, Escherichia coli, are transformed to manufacture human insulin.
■ Some bacteria carry out conjugation, a primitive form of sexual reproduction where individuals exchange genetic material.
■ They have a thick, rigid cell wall containing a substance known as peptidoglycan.
■ Some carry out photosynthesis, but others do not.
■ No introns (noncoding regions within the DNA).
■ Corresponds roughly to the old grouping Eubacteria and includes blue-green algae, bacteria like E. coli that live in the human intestine, those that cause disease like Clostridium botulinum and Streptococcus, and those necessary in the nitrogen cycle, like nitrogen-fixing bacteria and nitrifying bacteria.
■ Viruses are placed here because we do not know where else to place them.

Membrane-enclosed organelles: Absent
Peptidoglycan in cell wall: Present
RNA Polymerase: One type
Introns (noncoding regions of genes): Absent
Antibiotic sensitivity to streptomycin, chloramphenicol: Inhibited

Eukarya is a domain that includes four of the original kingdoms, protista, fungi, plants, and animals.
■ All organisms have a nucleus and internal organelles
■ No peptidoglycan in cells
■ Includes the four remaining kingdoms: Protista, Fungi, Plantae, and Animalia (see Table 9.2)
Today, taxonomy as a separate discipline is being replaced by the study of systematics, which includes taxonomy but considers biological diversity in an evolutionary context. Systematics focuses on tracing the ancestry of organisms. This is particularly important in light of current advances in DNA techniques that allow scientists to compare two species at the molecular level.
REMEMBER
The term "monera" is no longer used as the name of a kingdom.

Membrane-enclosed organelles: Present
Peptidoglycan in cell wall: Absent
RNA Polymerase: Several kinds
Introns (noncoding regions of genes): Present
Antibiotic sensitivity to streptomycin, chloramphenicol: Not inhibited

All are heterotrophic, multicellular eukaryotes
Are grouped in 35 phyla; but this book discusses 9 main phyla: Porifera, Cnidaria, Platyhelminthes, Nematoda, Annelida, Mollusca, Arthropoda, Echinodermata, and Chordata
Most animals reproduce sexually with a dominant diploid stage
In most species, a small, flagellated sperm fertilizes a larger, nonmotile egg
This category is monophyletic, meaning all animal lineages can be traced back to one common ancestor
Classified by anatomical features (homologous structures), DNA data, and embryonic development

Germ layers are the main layers that form various tissues and organs of the body. They are formed early in embryonic development as a result of gastrulation. Complex animals are triploblastic. They consist of the ectoderm, endoderm, and mesoderm.
■ The ectoderm, or outermost layer, becomes the skin and nervous system, including the nerve cord and brain.
■ The endoderm, the innermost layer, becomes the viscera (guts) or the digestive system.
■ The mesoderm, middle layer, becomes the blood and bones.
Primitive animals, like the Porifera and Cnidarians, have only two cell layers and are called diploblastic. Their bodies consist of ectoderm, endoderm, and mesoglea (middle glue), which connects the two layers together.

Figure 9.4 shows a phylogenetic tree for the animals listed in the table of the term above. It includes ingroups, the organisms of interest—lamprey, bass, frog, turtle, and leopard—and as a point of reference, the outgroup, lancelet. The outgroup is the group that diverged before the lineage evolved, in this case, vertebrates. When two lineages diverge, the split is depicted as a node. In this example, the nodes are development of a vertebral column, hinged jaws, four walking legs, amnion, and the development of hair. All animals share characteristics with their ancestors and also differ from them. In this tree, all the animals (except the lancelets) share a vertebral column. That trait is known as a shared ancestral trait or character. In contrast, each animal in one clade or lineage has a trait that is not shared with their ancestors. That new trait is known as a shared derived trait or character. For example, hair in the leopard (and in all mammals) is a derived trait.

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