2: Origins of life
- Page ID
- 49478
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- 2.1: The Double Helix of DNA
- This structure of DNA was worked out by Francis Crick and James D. Watson in 1953. It revealed how DNA - the molecule that Avery had shown was the physical substance of the genes. It could be replicated and so passed on from generation to generation. For this epochal work, they shared a Nobel Prize in 1962.
- 2.2: Base Pairing in DNA and RNA
- The rules of base pairing tell us that if we can "read" the sequence of nucleotides on one strand of DNA, we can immediately deduce the complementary sequence on the other strand. The rules of base pairing explain the phenomenon that whatever the amount of adenine (A) in the DNA of an organism, the amount of thymine (T) is the same (called Chargaff's rule). Similarly, whatever the amount of guanine (G), the amount of cytosine (C) is the same.
- 2.3: The Cell Membrane
- The plasma membrane is referred to as the fluid mosaic model and is composed of a bilayer of phospholipids, with their hydrophobic, fatty acid tails in contact with each other. The landscape of the membrane is studded with proteins, some of which span the membrane. Some of these proteins serve to transport materials into or out of the cell. Carbohydrates are attached to some of the proteins and lipids on the outward-facing surface of the membrane. These function to identify other cells.
- 2.4: Comparing Prokaryotic and Eukaryotic Cells
- Cells fall into one of two broad categories: prokaryotic and eukaryotic. The predominantly single-celled organisms of the domains Bacteria and Archaea are classified as prokaryotes (pro- = before; -karyon- = nucleus). Animal cells, plant cells, fungi, and protists are eukaryotes (eu- = true).
- 2.5: The Origin of Life
- To account for the origin of life on our earth requires solving several problems: How the organic molecules that define life, e.g. amino acids, nucleotides, were created. How these were assembled into macromolecules, e.g. proteins and nucleic acids, - a process requiring catalysts. How these were able to reproduce themselves. How these were assembled into a system delimited from its surroundings (i.e., a cell). A number of theories address each of these problems.
- 2.7: Endosymbiosis
- The endosymbiosis theory postulates that the mitochondria of eukaryotes evolved from an aerobic bacterium (probably related to the rickettsias) living within an archaeal host cell and the chloroplasts of red algae, green algae, and plants evolved from an endosymbiotic cyanobacterium living within a mitochondria-containing eukaryotic host cell.