Chapter Summary
4.1 Nucleic Acid Structures Reflect Their Function
The unique functions of the nucleic acids—DNA and RNA—are information storage and transfer. DNA is the hereditary material that passes genetic information from one generation to the next, and RNA uses that information to specify the structures of proteins.
Nucleic acids are polymers of nucleotides. A nucleotide consists of a phosphate group, a pentose sugar (ribose in RNA and deoxyribose in DNA), and a nitrogen-
containing base. Review Figure 4.1, Animation 4.1, Activity 4.1 In DNA, the nucleotide bases are adenine (A), guanine (G), cytosine (C), and thymine (T). Uracil (U) replaces thymine in RNA. C, T, and U have single-
ring structures and are pyrimidines. A and G have double- ring structures and are purines. The nucleotides in DNA and RNA are joined by phosphodiester linkages involving the sugar of one nucleotide and the phosphate of the next, forming a nucleic acid polymer. Review Figure 4.2, Activity 4.2
Complementary base pairing due to hydrogen bonds between A and T, A and U, and G and C occurs in nucleic acids. In RNA, the hydrogen bonds result in a folded molecule. In DNA, the hydrogen bonds connect two strands into a double helix. Review Figure 4.3, Focus: Key Figure 4.4
The information content of DNA and RNA resides in their base sequences.
The information in DNA is expressed as RNA in transcription. RNA can then specify the amino acid sequence of a protein in translation. Review Figure 4.5
4.2 The Small Molecules of Life Originated on Primitive Earth
The historical belief that life can originate repeatedly by spontaneous generation was disproven experimentally. Review Figure 4.6, Animation 4.2
A prerequisite for life is the presence of water.
One hypothesis for the origin of life on Earth is chemical evolution: the idea that organic molecules were formed on Earth before life began.
Chemical experiments modeling the prebiotic conditions on Earth support the idea of chemical evolution. Review Figure 4.7, Animation 4.3
Some meteorites that have landed on Earth contain organic molecules, suggesting that life might have originated extraterrestrially.
Experiments with lands on Mars yielded some data consistent with the existence of life on that planet. Review Investigating Life: Can We Find Evidence of Life on Mars?
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4.3 The Large Molecules of Life Originated from Small Molecules
Chemical evolution may have led to the polymerization of small molecules into polymers. This may have occurred on the surfaces of clay particles, in hydrothermal vents, or in hot pools at the edges of oceans.
A catalyst speeds up a chemical reaction. Today most catalysts are proteins, but some RNA molecules can function as both catalysts and information molecules. A catalytic RNA is called a ribozyme.
The existence of ribozymes supports the idea of an “RNA world”—a world before DNA. On early Earth, RNA may have acted as a catalyst for its own replication as well as for the synthesis of proteins. DNA could eventually have evolved from RNA. Review Figure 4.9
In support of the “RNA world” hypothesis, an artificial self-
replicating ribozyme was developed in the laboratory. Review Figure 4.10
4.4 Cells Originated from Their Molecular Building Blocks
A key to the emergence of living cells was the prebiotic generation of compartments enclosed by membranes. Such enclosed compartments permitted the generation and maintenance of internal chemical conditions that were different from those in the exterior environment.
In the laboratory, fatty acids assemble into protocells that have some of the characteristics of cells. Review Figure 4.11
Ancient rocks (3.5 billion years old) have been found with imprints that are probably fossils of early cells.
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