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Chapter 4
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Tree of Life
The evolutionary relationships between the three domains of life are shown. The root of the
tree is within the bacteria (eubacteria) domain; archea (archibacteria) and eukaryotes
(eukarya) diverged later.
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Bacteria
Representative types of Archea and Eubacteria are indicated together with their
characteristics.
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Archea—methane producing
Pictured is a motile archean that inhabits hot deep sea vents, uses hydrogen gas as a
source of energy, and gives off methane.
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Emergence of life
Based on fossils, chemical evidence, and extrapolation from molecular clocks, the first
appearance and relationships of the major domains of life are indicated against a 4.6billion-year time scale.
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Bacterial cell, generalized
Note the circular DNA, ribosomes, flagellum (for movement), pili (grappling lines to attach
to structures), capsule (jellylike protective coat present in many bacteria), and the plasma
membrane contained within a cell wall.
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Eukaryotic cell, generalized from an animal
Note the plasma membrane containing various organelles, membrane-bound structures
within the cytoplasm, and the nuclear envelope bounding the nucleus, which holds DNA.
Besides the mitochondria (production of energy molecules), the organelles include
compartments and channels involved in manufacture, processing, and transport of
synthesized products (rough endoplasmic reticulum, smooth endoplasmic reticulum, Golgi
apparatus, and lysosomes). Often microvilli, small projections, occur on the cell surface.
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Eukaryotic cell, generalized from a plant
Note the same basic components and organelles as the animal cell plus the addition of a
firm cell wall of cellulose, central vacuole, which sequesters various chemicals, and
chloroplasts that carry out photosynthesis.
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Origin of eukaryotes
Free-living bacteria developed mutually beneficial relationships with a host prokaryotic cell.
Some aerobic bacteria developed into mitochondria and others into chloroplasts eventually
producing the eukaryotic cells of animals and plants.
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Base-pairing
DNA is composed of two specialized strands of nucleic acid wound in a spiral helix. Between
the double strands of DNA, the nitrogenous bases pair specifically and preferentially:
adenine (A) with thymine (T), and guanine (G) with cytosine (C). In large part, this is the
basis for coding of information within DNA, and later transcription to RNA. Each end of a
DNA strand is polarized—one end is the 3’ end, the other the 5’ end. During DNA
duplication, the new strand of DNA lengthens in the 5’  3’ direction.
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Information transfer—DNA to RNA to protein
The organized sequence of bases in DNA is used in transcription to produce a
complementary, chemically matched, mRNA molecule. In turn, three bases each form a
codon, which specifies a particular amino acid. Sequentially, codon by codon, the mRNA is
a cipher program used in translation to produce a connected chain of particular amino
acids, the protein molecule.
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DNA and RNA
a) A single strand of the DNA double helix is shown as it is composed of nucleotides. b) A
strand of RNA is shown as it too is composed of nucleotides.
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Transcription and translation in prokaryotes and eukaryotes
(a) Bacterial genes are transcribed into mRNA, which is translated immediately into a
protein. (b) Eukaryotic genes typically contain long stretches of nucleotides, called introns,
that do not code for proteins. Introns, whose function is poorly known, are removed from
mRNA before mRNA directs the synthesis of the protein.
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The role of various RNA’s in the manufacture of proteins
Different types of RNA play a different role in the synthesis of protein.
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Cell metabolism
Food is broken down during digestion, yielding end the products of carbohydrates, fats, and
proteins. These enter various metabolic pathways where energy is harvested and stored in
ATP or temporarily in NADH. These metabolic pathways include glycolysis, the Krebs cycle,
and the electron transport system, represented diagrammatically in the figure. Various
chemical intermediates in these pathways are indicated (pyruvate, lactate, Acetyl-CoA).
Oxygen (O2) is used up; carbon dioxide (CO2) is given off.
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Photosynthesis
Plant cells include numerous chloroplasts, such as the one shown here in this cut-away
view. In chloroplasts, energy from light is used eventually to produce energy storage
molecules, ATP and NADP+. These fuel the Calvin cycle, which in turn produces PGAL, an
intermediate on its way to becoming a sugar used in cellular respiration and manufacture
of plant tissues. Note that water (H2O) is taken up and oxygen (O2) given off as a
byproduct.
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FIGURE 4.1 Microfossils
Microscopic views of these filamentous microorganisms (Primaevifilum amoenum)
recovered from rocks in Western Australia dated to about 3.5 billion years of age. (Photo
courtesy of J. W. Schopf.)
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FIGURE 4.3 Life in Test Tubes, The Miller-Urey Experiment
Heated water produced water vapor circulating through the closed system of glass chambers. Into the
upper chamber, Miller and Urey placed gases thought present in Earth’s early atmosphere, and applied a
spark. Condensers cooled any gases, causing molecular products to collect in the water. From this water,
samples were taken over the next week and analyzed. Among the organic molecules formed were amino
acids, basic building blocks of protein. Subsequent follow-up trials, by many other biologists, using various
combinations of “primitive atmospheres,” produced even more complex organic compounds.
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FIGURE 4.10 Information Transfer-DNA to RNA to Protein
The organized sequence of bases in DNA is used in transcription to produce a
complementary, chemically matched, mRNA molecule. In turn, three adjacent bases form a
codon, which specifies a particular amino acid. Sequentially, codon by codon, the mRNA is
a cipher program used in translation to produce a connected chain of particular amino
acids, the protein molecule.