Nucleic Acids and the Origin of Life PDF

4 Nucleic Acids and the Origin of Life © Oxford University Press Chapter 4 Nucleic Acids and the Origin of Life Key Concepts 4.1 Nucleic Acids Are Informational Macromolecules 4.2 The Small Molecules of Life Originated...

4 Nucleic Acids and the Origin of Life © Oxford University Press Chapter 4 Nucleic Acids and the Origin of Life Key Concepts 4.1 Nucleic Acids Are Informational Macromolecules 4.2 The Small Molecules of Life Originated on Primitive Earth 4.3 The Large Molecules of Life Originated from Small Molecules 4.4 Cells Originated from Their Molecular Building Blocks © Oxford University Press Chapter 4 Nucleic Acids and the Origin of Life (IL 1) Investigating LIFE introduction Looking for Life The search for evidence of life on Mars involves the two chemical signatures of life: presence of water and carbon-based chemistry. There is evidence of water on Mars, but so far no life forms have been found. Q&A: Can we find evidence of life on Mars? (See slides 31–33 and 48–49.) © Oxford University Press Concept 4.1 Nucleic Acids Are Informational Macromolecules (1) Nucleic acids: polymers specialized for storage, transmission, and use of genetic information. DNA = deoxyribonucleic acid RNA = ribonucleic acid Monomers are nucleotides: pentose sugar + nitrogenous base + phosphate group. © Oxford University Press Concept 4.1 Nucleic Acids Are Informational Macromolecules (2) DNA has deoxyribose, RNA has ribose. Nucleoside: pentose sugar + nitrogenous base. Two forms of nitrogenous bases: Pyrimidines—single ring Purines—fused double-ring © Oxford University Press Figure 4.1 Nucleotide Chemistry Concept 4.1 Nucleic Acids Are Informational Macromolecules (3) Nucleotides are linked by phosphodiester bonds. Phosphate groups link the 3′ carbon in one sugar to the 5′ carbon in another sugar. Nucleic acids grow in the 5′-to-3′ direction. © Oxford University Press Figure 4.2 Linking Nucleotides Together Concept 4.1 Nucleic Acids Are Informational Macromolecules (4) DNA bases: Purines: Adenine (A) & Guanine (G) Pyrimidines: Cytosine (C) & Thymine (T) Complementary base pairing: Purines pair with pyrimidines by hydrogen bonds. © Oxford University Press Figure 4.3 Base Pairing by hydrogen Bonding between Nucleotides Concept 4.1 Nucleic Acids Are Informational Macromolecules (5) RNA: Single-stranded, but base pairing can occur between different regions of the molecule which results in 3-D structure. RNA has uracil (U) instead of thymine. Complementary base pairing can also take place between RNA and DNA. © Oxford University Press Figure 4.4 RNA Concept 4.1 Nucleic Acids Are Informational Macromolecules (6) DNA: Two strands form a double helix. All DNA molecules have the same structure. Genetic information is carried in the sequence of base pairs. DNA can reproduce itself (replication). © Oxford University Press Figure 4.5 DNA Concept 4.1 Nucleic Acids Are Informational Macromolecules (7) Transcription: DNA sequences are copied into RNA. Translation: RNA sequence specifies the sequence of amino acids in a polypeptide. Transcription + translation = gene expression. © Oxford University Press Figure 4.6 DNA, RNA, and Protein Concept 4.1 Nucleic Acids Are Informational Macromolecules (8) DNA replication and transcription depend on the base-pairing properties of nucleic acids. The entire DNA molecule is replicated. The complete set of DNA in an organism is called its genome. Genes: sequences of DNA that that are transcribed into RNA. © Oxford University Press Figure 4.7 DNA Replication and Transcription Concept 4.1 Nucleic Acids Are Informational Macromolecules (9) DNA carries hereditary information from one generation to the next. Closely related living species have more similar base sequences than do species that are more distantly related. DNA sequencing is now used extensively to trace evolutionary relationships. © Oxford University Press Concept 4.1 Nucleic Acids Are Informational Macromolecules (10) Other roles for nucleotides: ATP—energy transfer in biochemical reactions GTP—energy source in protein synthesis cAMP—essential in many processes, including hormone action Carriers in synthesis and breakdown of carbohydrates and lipids © Oxford University Press Concept 4.2 The Small Molecules of Life Originated on Primitive Earth (1) People once thought that some forms of life arose from decaying matter by spontaneous generation. Francesco Redi first disproved this in 1668. Experiments by Louis Pasteur showed that microorganisms can arise only from other microorganisms. © Oxford University Press Figure 4.8 Disproving Spontaneous Generation: Redi’s Experiment Figure 4.9 Disproving Spontaneous Generation: Pasteur’s Experiment (Experiment) (Part 1) Figure 4.9 Disproving Spontaneous Generation: Pasteur’s Experiment (Experiment) (Part 2) Concept 4.2 The Small Molecules of Life Originated on Primitive Earth (2) But these experiments did not prove that spontaneous generation had never occurred. How did life first originate? There are two main theories. © Oxford University Press Concept 4.2 The Small Molecules of Life Originated on Primitive Earth (3) 1. Chemical evolution: conditions on primitive Earth led to formation of simple molecules such as amino acids, which led to formation of life forms. Miller and Urey (1950s) experimented with reconstructing those primitive conditions using gases thought to be in the early atmosphere: H2, NH3, CH4, H2O. © Oxford University Press Figure 4.10 Modeling the Formation of Biological Molecules from Chemicals Present in Earth’s Early Atmosphere (Experiment) Concept 4.2 The Small Molecules of Life Originated on Primitive Earth (4) The Miller and Urey experiments sparked decades of research. Volcanoes may also have added CO2, N2, H2S, and SO2 to the atmosphere. Adding these gases to the experimental atmosphere results in formation of many more small organic molecules. © Oxford University Press Concept 4.2 The Small Molecules of Life Originated on Primitive Earth (5) 2. Life came from outside Earth Some meteorites contain molecules such as purines, pyrimidines, and amino acids, suggesting that living organisms might have reached Earth within a meteorite. Some scientists question whether organisms could survive in a meteorite. © Oxford University Press Figure 4.11 The Murchison Meteorite Concept 4.2 The Small Molecules of Life Originated on Primitive Earth (6) Investigating LIFE: Can We Find Evidence of Life on Mars? Hypothesis: Martian soil can be tested by a probe on Mars to show chemical changes consistent with life. Method: Expose soils collected by Mars Viking landers to nutrients labeled with radioisotopes. After 4 days, check for radioactive gases, such as CO2, that might be given off by live organisms during metabolism. © Oxford University Press Investigating Life: Can We Find Evidence of Life on Mars? Experiment (1) Investigating Life: Can We Find Evidence of Life on Mars? Experiment (2) Concept 4.3 The Large Molecules of Life Originated from Small Molecules (1) The next step for the chemical evolution theory is a plausible explanation of how polymers formed. Several model systems are used to simulate possible conditions including powdered clays, hydrothermal vents, and hot pools. © Oxford University Press Concept 4.3 The Large Molecules of Life Originated from Small Molecules (2) A key to the origin of life is the appearance of catalysts—today the catalysts are proteins called enzymes. Proteins are synthesized from information in nucleic acids. So which came first, nucleic acids or protein catalysts? © Oxford University Press Concept 4.3 The Large Molecules of Life Originated from Small Molecules (3) 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. © Oxford University Press Figure 4.12 The “RNA World” Hypothesis Concept 4.3 The Large Molecules of Life Originated from Small Molecules (4) Evidence supporting the “RNA world” hypothesis: Ribose can be formed in prebiotic chemical synthesis experiments. Peptide linkages are catalyzed by ribozymes today. In retroviruses, reverse transcriptase catalyzes synthesis of DNA from RNA. © Oxford University Press Concept 4.3 The Large Molecules of Life Originated from Small Molecules (5) 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. © Oxford University Press Concept 4.4 Cells Originated from Their Molecular Building Blocks (1) The compounds involved in the origin of life must have been concentrated in a compartment. Today, living cells are separated from their environment by a membrane. This allows cells to maintain a chemical composition that is different from the external environment. © Oxford University Press Concept 4.4 Cells Originated from Their Molecular Building Blocks (2) Experiments suggest how cells may have formed. In water, fatty acids will form a lipid bilayer around a liquid 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 pass through the bilayer and be incorporated into polynucleotide chains. © Oxford University Press Figure 4.13 Protocells Concept 4.4 Cells Originated from Their Molecular Building Blocks (3) Protocells may be a reasonable model for the evolution of cells: They are organized systems of parts with substances interacting, in some cases catalytically. The interior is distinct from the exterior environment. Capable of limited replication. © Oxford University Press Concept 4.4 Cells Originated from Their Molecular Building Blocks (4) In the 1990s, evidence of cells in 3.5 billion year- old rocks was found in Australia. The cells appeared to be cyanobacteria (blue- green bacteria) that could perform photosynthesis. Photosynthesis uses CO2, and leaves a specific ratio of carbon isotopes (13C:12C). © Oxford University Press Figure 4.14 The Earliest Cells? Concept 4.4 Cells Originated from Their Molecular Building Blocks (5) Carbon isotopes indicative of photosynthesis were found in the fossils, and also showed that several other types of bacteria were present as well. It probably took about 500 million to 1 billion years from the formation of the Earth until the appearance of the first cells. © Oxford University Press Figure 4.15 The Origin of Life Chapter 4 Nucleic Acids and the Origin of Life (IL 2) Investigating LIFE conclusion (1 of 2) Q&A: Can we find evidence of life on Mars? Finding evidence for current or past life on Mars is a primary goal of NASA. Curiosity rover is currently on Mars; in 2018 soil analyses detected complex organic compounds. More rovers are planned by the European Space Agency and Russia. © Oxford University Press Chapter 4 Nucleic Acids and the Origin of Life (IL 3) Investigating LIFE conclusion (2 of 2) Q&A: Can we find evidence of life on Mars? A Mars Sample Return mission will bring Martian soil samples to Earth for experimentation. Future missions are also planned to investigate three moons: Jupiter’s Europa and Saturn’s Enceladus and Titan. Another mission will bring a soil sample back from the asteroid Bennu. © Oxford University Press