4 Nucleic Acids and the Origin of Life

4 Nucleic Acids and the Origin of Life

4 Nucleic Acids and the Origin of Life 4. 1 What Are the Chemical Structures and Functions of Nucleic Acids? 4. 2 How and Where Did the Small Molecules of Life Originate? 4. 3 How Did the Large Molecules of Life Originate? 4. 4 How Did the First Cells Originate?

4 Nucleic Acids and the Origin of Life About 7, 000 cheetahs survive in the world today. The genomes (DNA) of all cheetahs are extremely similar, suggesting that they all derive from a few individuals that survived an event that almost wiped out their species. Opening Question: Can DNA analysis be used in the conservation and expansion of the cheetah population?

4. 1 What Are the Chemical Structures and Functions of Nucleic Acids? Nucleic acids are polymers specialized for the storage, transmission, and use of genetic information. DNA = deoxyribonucleic acid RNA = ribonucleic acid

4. 1 What Are the Chemical Structures and Functions of Nucleic Acids? Nucleotides are the monomers that make up nucleic acids. Nucleotides consist of a pentose sugar, a phosphate group, and a nitrogencontaining base. A nucleoside consists only of a pentose sugar and a nitrogenous base.

Figure 4. 1 Nucleotides Have Three Components

Figure 3. 16 Monosaccharides Are Simple Sugars

4. 1 What Are the Chemical Structures and Functions of Nucleic Acids? RNA contains the sugar ribose. DNA contains deoxyribose.

4. 1 What Are the Chemical Structures and Functions of Nucleic Acids? Nucleotides are linked together in condensation reactions to form phosphodiester linkages. The phosphate groups link carbon 3′ in one sugar to carbon 5′ in another sugar. Nucleic acids are said to grow in the 5′to-3′ direction.

Figure 4. 2 Linking Nucleotides Together

4. 1 What Are the Chemical Structures and Functions of Nucleic Acids? Oligonucleotides (about 20 monomers): RNA “primers” to start DNA duplication, RNA that regulates gene expression, etc. Polynucleotides, or nucleic acids (DNA and RNA): can be very long—up to millions of monomers.

4. 1 What Are the Chemical Structures and Functions of Nucleic Acids? DNA bases: • Adenine (A) • Cytosine (C) • Guanine (G) • Thymine (T) RNA has uracil (U) instead of thymine.

Table 4. 1

4. 1 What Are the Chemical Structures and Functions of Nucleic Acids? Complementary base pairing: purines pair with pyrimidines by hydrogen bonds.

4. 1 What Are the Chemical Structures and Functions of Nucleic Acids? RNA is single-stranded, but base pairing occurs between different regions of the molecule. Base pairing determines the threedimensional shape of some RNA molecules. Complementary base pairing can also take place between RNA and DNA.

Figure 4. 3 RNA

4. 1 What Are the Chemical Structures and Functions of Nucleic Acids? The two strands of a DNA molecule form a double helix. All DNA molecules have the same structure; diversity lies in the sequence of base pairs. DNA is an informational molecule: information is encoded in the sequences of bases.

Figure 4. 4 DNA

4. 1 What Are the Chemical Structures and Functions of Nucleic Acids? DNA transmits information in two ways: • DNA can reproduce itself (replication). • DNA sequences can be copied into RNA (transcription). The RNA can specify a sequence of amino acids in a polypeptide (translation).

4. 1 What Are the Chemical Structures and Functions of Nucleic Acids? Transcription plus translation = expression

4. 1 What Are the Chemical Structures and Functions of Nucleic Acids? DNA replication and transcription depend on base pairing. DNA replication involves the entire molecule, but only relatively small sections of the DNA are transcribed into RNA.

4. 1 What Are the Chemical Structures and Functions of Nucleic Acids? The complete set of DNA in a living organism is called its genome. Not all the information is needed at all times; sequences of DNA that encode specific proteins are called genes.

Figure 4. 5 DNA Replication and Transcription

4. 1 What Are the Chemical Structures and Functions of Nucleic Acids? DNA carries hereditary information between generations. Determining the sequence of bases helps reveal evolutionary relationships. The closest living relative of humans is the chimpanzee.

4. 1 What Are the Chemical Structures and Functions of Nucleic Acids? Other roles for nucleotides: • ATP—energy transducer in biochemical reactions • GTP—energy source in protein synthesis • c. AMP—essential to the action of hormones and transmission of information in the nervous system

4. 2 How and Where Did the Small Molecules of Life Originate? During the European Renaissance (14 th to 17 th centuries), most people thought that at least some forms of life arose repeatedly from inanimate or decaying matter by spontaneous generation.

4. 2 How and Where Did the Small Molecules of Life Originate? Francesco Redi first disproved spontaneous generation in 1668.

4. 2 How and Where Did the Small Molecules of Life Originate? Experiments by Louis Pasteur showed that microorganisms can arise only from other microorganisms.

Figure 4. 6 Disproving the Spontaneous Generation of Life (Part 1)

Figure 4. 6 Disproving the Spontaneous Generation of Life (Part 2)

4. 2 How and Where Did the Small Molecules of Life Originate? But these experiments did not prove that spontaneous generation had never occurred. Eons ago, conditions on Earth and in the atmosphere were vastly different. About 4 billion years ago, chemical conditions, including the presence of water, became just right for life.

4. 2 How and Where Did the Small Molecules of Life Originate? Two of theories on the origin of life: 1. Life came from outside Earth. • In 1969, fragments of a meteorite were found to contain molecules unique to life, including purines, pyrimidines, sugars, and ten amino acids. • Evidence from other meteorites suggest that living organisms could possibly have reached Earth within a meteorite.

Figure 4. 7 The Murchison Meteorite

4. 2 How and Where Did the Small Molecules of Life Originate? 2. Life arose on Earth through chemical evolution. • Chemical evolution: conditions on primitive Earth led to formation of simple molecules (prebiotic synthesis); these molecules led to formation of life forms. • Scientists have experimented with reconstructing those primitive conditions.

4. 2 How and Where Did the Small Molecules of Life Originate? Miller and Urey (1950 s) set up an experiment with gases thought to have been present in Earth’s early atmosphere. An electric spark simulated lightning as a source of energy to drive chemical reactions. After several days, organic molecules had formed, including amino acids.

Figure 4. 8 Miller and Urey Synthesized Prebiotic Molecules in an Experimental Atmosphere (Part 1)

Figure 4. 8 Miller and Urey Synthesized Prebiotic Molecules in an Experimental Atmosphere (Part 2)

Working with Data 4. 1: Could Biological Molecules Have Been Formed from Chemicals Present in Earth’s Early Atmosphere? In the 1950 s Miller and Urey experiments, the sources of energy impinging on Earth were:

Working with Data 4. 1: Could Biological Molecules Have Been Formed from Chemicals Present in Earth’s Early Atmosphere? Question 1: Of the total energy from the sun, only a small fraction is in the ultraviolet range, less than 250 nm. What proportion of total solar energy is the energy with wavelengths below 250 nm?

Working with Data 4. 1: Could Biological Molecules Have Been Formed from Chemicals Present in Earth’s Early Atmosphere? Question 2: The molecules CH 4, H 2 O, NH 3, and CO 2 absorb light at wavelengths of less than 200 nm. What fraction of total solar radiation is in this range?

Working with Data 4. 1: Could Biological Molecules Have Been Formed from Chemicals Present in Earth’s Early Atmosphere? Question 3: Miller and Urey used electric discharges as their energy source. What other sources of energy could be used in similar experiments?

4. 2 How and Where Did the Small Molecules of Life Originate? In another experiment, Miller filled tubes with NH 3, HCN, and water and kept them sealed at – 78°C for 27 years. When opened, they contained amino acids and nucleotide bases. Cold water within ice on ancient Earth or other planets may have allowed prebiotic synthesis of organic molecules.

4. 2 How and Where Did the Small Molecules of Life Originate? The Miller and Urey experiments sparked decades of research. Ideas about Earth’s original atmosphere have changed: volcanoes may have added CO 2, N 2, H 2 S, and SO 2 to the atmosphere. Adding these gases to the experimental atmosphere results in formation of more small organic molecules.

4. 3 How Did the Large Molecules of Life Originate? Conditions in which polymers might have been first synthesized: • Solid mineral surfaces—silicates within clay may have been catalysts • Hydrothermal vents—metals as catalysts • Hot pools at ocean edges— concentrated monomers favored polymerization (the “primordial soup”)

4. 3 How Did the Large Molecules of Life Originate? In living organisms, the many biochemical reactions require catalysts—molecules that speed up the reactions. A key to the origin of life is the appearance of catalysts—proteins called enzymes.

4. 3 How Did the Large Molecules of Life Originate? Proteins are synthesized from information contained in nucleic acids. So which came first, nucleic acids or protein catalysts?

4. 3 How Did the Large Molecules of Life Originate? RNA may have been the first catalyst. The 3 -D shape and other properties of some RNA molecules (ribozymes) are similar to enzymes. RNA could have acted as a catalyst for its own replication and for synthesis of proteins. DNA could eventually have evolved from RNA.

Figure 4. 9 The “RNA World” Hypothesis

4. 3 How Did the Large Molecules of Life Originate? Several lines of evidence support this “RNA world” hypothesis: • Peptide linkages are catalyzed by ribozymes today. • In retroviruses, an enzyme called reverse transcriptase catalyzes the synthesis of DNA from RNA.

Figure 4. 10 An Early Catalyst for Life?

4. 3 How Did the Large Molecules of Life Originate? • Short, naturally occurring RNA molecules catalyze polymerization of nucleotides in experimental settings. • An artificial ribozyme has been developed that can catalyze assembly of short RNAs into a longer molecule that is an exact copy of itself.

4. 4 How Did the First Cells Originate? The chemical reactions of metabolism and replication could not occur in a dilute aqueous environment. The compounds involved must have been concentrated in a compartment. Today, living cells are separated from their environment by a membrane.

4. 4 How Did the First Cells Originate? In water, fatty acids will form a lipid bilayer around a compartment. These protocells allow small molecules such as sugars and nucleotides to pass through. If short nucleic acid strands capable of self-replication are placed inside protocells, nucleotides can enter and be incorporated into polynucleotide chains.

Figure 4. 11 Protocells

4. 4 How Did the First Cells Originate? Protocells may be a reasonable model for the evolution of cells: • They are organized systems of parts with substances interacting, in some cases catalytically. • They have an interior that is distinct from the exterior environment. • They can self-replicate.

4. 4 How Did the First Cells Originate? In the 1990 s, evidence of cells in rocks 3. 5 billion years old was found in Australia. The cells were probably cyanobacteria (blue-green bacteria) that could perform photosynthesis. Photosynthesis uses CO 2, and leaves a specific ratio of carbon isotopes (13 C: 12 C), which were found in the fossils.

Figure 4. 12 The Earliest Cells?

4. 4 How Did the First Cells Originate? It is plausible that it took about 500 million to a billion years from the formation of the Earth until the appearance of the first cells.

Figure 4. 13 The Origin of Life

4 Answer to Opening Question DNA sequencing allows conservation biologists to mate pairs of cheetahs with the greatest differences in DNA. The offspring will thus have the greatest possible diversity of DNA. Genetic homogeneity causes male cheetahs to have low sperm counts. Artificial insemination is used to overcome this problem.