Anthony B. Wight
An interesting way to draw together various ideas in the history of evolution would be an examination of the debate currently stirring in popular science circles concerning the origin of modern humans. Billed by
Discover
magazine as “the big battle between bones and genes,”
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this is a case study of the quest for an answer to the same question by two very different branches of science—paleoanthropology and molecular biology—each operating with its own assumptions, techniques, and rules. DNA exists in human cells chiefly in the nucleus. Outside the cell, however, nuclei are rod-like mitochondria within which there is also a form of DNA. This “mitochondrial DNA” is unique in that it apparently is inherited only from the mother. (Nuclear DNA is combined genetic material from both mother and father.) The only differences between mitochondrial DNA of a child and that of its mother, grandmother, or great-grandmother are the result of random mutations, according to evolutionary theory. Vincent Sarich and Allan Wilson, two biochemists at Berkeley, have hypothesized that mutations occur across millennia at a steady rate, establishing a kind of “molecular evolutionary clock.”
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By studying genetic differences and calculating backwards in time with their clock, Sarich and Wilson establish dates of divergence among species and within the hominid family, the emergence of modern humans.
On the other hand, paleoanthropologists use field evidence—bones, tool fragments, fossils—and geologic or radiocarbon dating techniques (assuming another form of steady rate clock) to establish emergence dates. Significant differences in the timing of human evolution result from the two techniques and hence the debate. Once students have gained a solid understanding of cell structure and DNA they may be prepared to delve further into this debate.
Of most fascination is the possibility that future studies of the mitochondrial DNA may lead scientists back along the trail of human evolution toward an ancestral female “Eve,” breathing new life into the mythologies considered in Section 1 of this unit.
III. Chemical Structure of Life—The Human Genome
Objectives: Students should be able
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1. To explain the relationships among atoms, molecules, elements, and compounds.
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2. To explain and give examples of inorganic and organic compounds.
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3. To name the six most common elements in human bodies
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4. To relate the characteristics and functions of the four classes of macromolecules and give examples of each.
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5. To explain how enzymes catalyze chemical reactions.
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6. To recognize the importance of nucleic acids to inheritance.
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7. To describe the structure of a nucleotide.
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8. To describe the structure and replication of DNA.
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9. To explain what a gene is.
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10. To define the human genome and explain its significance.
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11. To explain the significance of mutation in DNA.
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12. To describe the human genome mapping project.
Strategies:
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1. Reading, lecture, note-taking.
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2. Cooperative learning in small groups to develop diagrams and concept maps.
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3. Molecular model building.
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4. Construction of nucleotide and DNA models.
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5. Microscope slides and projector slides of human chromosomes.
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6. Guest lecturers from Bios Corporation and Yale School of Medicine, Human Genetics Department on study of genomic DNA and inherited disorders.
Discussion: Molecular biology is not trivial aspect of biological systems. It is at the heart of the matter. Almost all aspects of life are engineered at the molecular level and without understanding molecules we can only have a sketchy understanding of life itself.
— Francis Crick
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In teaching a molecular view of life to students in high school it becomes quickly apparent that, however uncertain they feel about basic tenets of evolution theory, they seem universally to accept an atomic theory of matter from previous science study. I find it useful, therefore, to provide a quick review of atoms—elements, molecules—compounds, following a basic text in biology or chemistry. It helps to have a periodic chart and to point out to students that of the some 112 known elements, 92 are naturally occurring (and quite likely have been present since the beginning of earth). Of these 92, only about 18 are usually found in living things. And among these 18, the six most essential to human life are oxygen, carbon, hydrogen, nitrogen, calcium, and phosphorous.
Elemental atoms bond together in various combinations to form molecular compounds.
Inorganic
compounds such as water (H2O) and carbon dioxide (CO2) exist regardless of living organisms.
Organic
compounds are produced by living organisms and/or by synthetic means, but in all cases this class of compounds contains carbon as an essential bonding element, usually combined with hydrogen and oxygen. Generally, organic compounds are more structurally complex than the inorganics.
Carbon can combine in long chains that form the backbone of large, complex macromolecules in which the carbon atom backbone is called the carbon skeleton—appearing in straight chain or ringed form to which other atoms attach themselves.
Four groups of macromolecules are present in living systems:
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1.
carbohydrates
(e.g., sugars, starch, cellulose)
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2.
lipids
(e.g., fats and oils)
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3.
proteins
(several thousand types exist made by the linking of amino acids with peptide bonds; the type, number, and sequence of amino acids distinguishes one protein from another. Specialized proteins, called “enzymes,” serve as catalysts to lower the energy required for reactions to proceed within the living cell.)
4.
nucleic acids
(passed from one generation to another, it is nucleic acid which stores information determining the genetic characteristics of cells and organisms; nucleic acids are the macromolecules that dictate the amino acid sequence of proteins which then control all basic life processes.)
Nucleic acids are made of units, called
nucleotides
, joined into long chains. Each nucleotide contains three parts:
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1. sugar (either deoxyribose or ribose)
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2. a phosphate group (PO3)
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3. a nitrogen base—either a single or double carbon ring with nitrogen and hydrogen. See diagram 3.1 for the four bases found in DNA (Adenine = A. Thymine = T. Cytosine = C. Guanine = G).
(figure available in print form)
Figure 3.1 The four nitrogen bases in DNA
Ribonucleic acids (RNA) contain ribose sugar in their nucleotides while deoxyribose acids (DNA) have deoxyribose sugar. DNA molecules have two long chains of nucleotides intertwined to form a double helix. (See figure 3.2.) The backbones of the spiral are made of the sugar-phosphates while the rungs are chemically-bonded nitrogen bases. Note that the bases always have very specific pairing:
A to T
and
G to C
. (This is the structure which Watson and Crick deduced in 1953.)
(figure available in print form)
Figure 3.2 Double helix structure of DNA
(based upon diagrams in
Mapping our Genes — The Genome Projects
. OTA-BA-373, U.S. Congress, Office of Technology Assessment)
The structure of DNA quickly led to an understanding of how this crucial molecule replicates itself in the course of cell division. During cell division, the DNA double helix unwinds (or “unzips”), the weak bonds between base pairs break, and the DNA strands separate. Free nucleotides are then matched up with their complementary bases on each of the separated chains, and two new complementary, identical, double helix chains are made. Figure 3.3 illustrates the beginning of this process. Students will benefit greatly from a laboratory exercise in small groups constructing and replicating their own models of DNA.
(An extension of the exercise will allow students to construct RNA using their DNA molecules as a guide and then to show how the RNA moves out of the cell nucleus to provide the code for construction of proteins at the ribosomes.)
(figure available in print form)
Figure 3.3 Replication of DNA by “unzipping” of the molecule and attachment of free nucleotides to each strand.
(based upon diagrams in
Mapping our Genes — The Genome Projects
. OTA-BA-373, U.S. Congress, Office of Technology Assessment)