14.3 The First Life Forms

1. 14.3 The First Life Forms Chapter 14 Origin of Life

2. The Origin of Heredity • Scientists still are investigating competing hypotheses dealing with the possible transitions from simple organic molecules to cells (cell life). • ? How could molecules be organized into self-replicating systems? • … in other words, to explain the origin of heredity…

3. The Origin of Heredity… • Chapter 10 explained how hereditary information affects the phenotype of cells. • Hereditary information in DNA is transcribed into mRNA (an RNA message). • RNA message is translated into a protein • DNA template for RNA; RNA template for specific proteins

4. The Origin of Heredity… • DNA doesn’t make proteins directly • ? • RNA structures are unique for the jobs involved in making proteins. • mRNA; tRNA; rRNA • DNA structure is less unique for this function • Double helix - replication

5. The Origin of Heredity… • RNA plays an important role in the production of proteins • RNA shapes require hydrogen bonds between amino acids • It has been speculated that some RNA molecules might behave like proteins and catalyze chemical reactions.

6. The Roles of RNA • Thomas Cech (1947-) 1980s • Type of RNA found in some unicellular eukaryotes is able to act as a chemical catalyst (similar to an enzyme) • Ribozyme = RNA molecule that acts as a catalyst to promote a specific chemical reaction • Later studies indicated that ribozymes could act as catalysts for their own replication. • Self-replicating systems of RNA molecules have been created in the laboratory. • This supports the hypothesis that life could have started with self-replicating molecules of RNA.

7. The RNA World • RNA vital for • DNA replication • Protein synthesis • + other basic biochemistry • Maybe the chemistry and genetics of early cells was based on RNA

8. The RNA World • First case of heredity and competition??? • Self-replication might involve competing with other RNA molecules for a limited number of available nucleotides. • An RNA molecule with a slight advantage, maybe in form, might be capable of out-competing other RNA molecules without the advantage. • Over time, the more efficient RNA molecules would survive, and the world of RNA would change – slowly.

9. The RNA World Several competing hypotheses: • Certain kinds of minerals formed a template on which organic molecules lined up to form polymers. • Self-replicating RNA began to evolve inside cell-like structures such as microspheres or coacervates. • The self-replicating RNA could have provided the hereditary information that the cell-like structures lack. • If the RNA molecules were able to direct the assembly (the manufacture, making) of the structures that carried them, a cell-like system would be formed.

10. The First Cells • No direct evidence of the first cells • Scientists make inferences…the process of arriving at some conclusion based on some degree of probability relative to the premises. Scientists believe that little or no oxygen existed on early earth. The oldest fossils that are thought to be cells are the size and shape of some living prokaryotes. The first cells might have developed in an environment filled with organic molecules for food. The first cells were probably anaerobic, heterotrophic prokaryotes.

11. The First Cells • A growing population of heterotrophs that depended on spontaneously formed organic molecules for food eventually would have removed these molecules from the environment. • Now, autotrophs would have begun to have an advantage. • The first autotrophs probably did not depend on photosynthesis, as they do now.

12. Chemosynthesis • Archaea = related group of unicellular organisms • Harsh environmental conditions • Methanosarcinabarkeri • Anaerobic conditions • Chemosynthesis • CO2 serves as carbon source • Assembly of organic molecules • Energy comes from the oxidation of various inorganic substances, such as sulfur.

13. Photosynthesis & Aerobic Respiration • Some forms of life had become photosynthetic ~ 3 billion years ago. • Geological evidence • Chemical traces of photosynthetic activity • Cyanobacteria • Photosynthetic, unicellular prokaryotes • Similar to oldest fossils found

14. Photosynthesis and Aerobic Respiration • Oxygen – byproduct of photosynthesis • Harmful to many unicellular organisms • Could destroy their coenzymes • These enzymes essential to cell function • In some organisms, O2 bonded to other compounds. • Prevented it from doing harm • This was one of the first steps in aerobic respiration. • An early functionofa.r. may have been to preserve important organic compounds.

15. Photosynthesis & Aerobic Respiration • ~ 1 billion + years for oxygen gas levels to reach current levels • O2 eventually reached the upper levels of the atmosphere • Bombarded by sunlight • Some wavelengths of sunlight can split O2 • This forms highly reactive single O atoms. • O + O2 = O3 (ozone) • Poisonous to plant and animal life • In upper atmosphere, absorbs much UV radiation from sun (necessary because…)

16. The First Eukaryotes • Eukaryotic cells are larger than prokaryotic cells. • DNA organized into chromosomes within a nucleus • Cytoskeleton and membrane-bound organelles • These features unique to eukaryotes are thought to have evolved from prokaryotes. • i.e., E.R. and a nuclear membrane formed from the infolding of the plasma membrane of a prokaryotic cell. • Lynn Margulis – researcher • Proposed further evolved from a mutually beneficial relationship between primitive eukaryotes and the prokaryote it engulfed. • The theory of endosymbiosis • Figure 14.11, page 290 (add slide)

17. The First Eukaryotes • Aerobic prokaryotes that were engulfed by eukaryotic cells evolved into the mitochondria. • Performs aerobic respiration • Engulfed photosynthetic cyanobacteria evolved into chloroplasts. • Performs photosynthesis in modern plant and algae cells • The eukaryote provided a beneficial environment for the prokaryote. • Likewise, the prokaryote provided a source of energy for the eukaryote.

18. The First Eukaryotes • Strong evidence supports this theory. • Chloroplasts and mitochondria replicate independently from the replication of the cell that contains them. • Chloroplasts and mitochondria contain some of their own genetic material. • Which is more similar to that of prokaryotes than it is to the rest of the cell • Finally, the DNA of these organelles is found in a circular arrangement, which is characteristic of prokaryotic cells.