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BIOL 107 (4)
Chapter 2

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University of Saskatchewan
BIOL 107
Kara Somerville

 Structural and Developmental Homology o Organisms show curious similarities in structure and development unrelated to function. These similarities o The underlying design of many vertebrae forelimbs is similar even though their function and appearance are different o Such similarity makes sense if all vertebrae descended from a common ancestor, from which they inherited the fundamental design of their limbs. o What causes these similarities?  Darwin argued that descent from a common ancestor is the most logical explanation  Molecular Homology o Advances in molecular genetics have revealed other similarities among organisms. Prominent among these is the genetic code. o With a few minor exceptions, all organisms studied to date use the same nucleotide triplets to specify the same amino acids to be incorporated into proteins.  However, an enormous number of alternative codes is theoretically possible, some of which provide a given species advantages.  If using alternative codes is advantageous, then why do virtually all organisms use the same one?  Darwin’s suggestion again answered this question. All organisms inherited their genetic code from a common ancestor. o One example can be found on chromosome 17 in the human genome. Read elaboration on page 57. o Another example concerns another kind of genetic quirk that might be considered as a flaw: processed pseudogenes  These are nonfunctional copies of normal genes that originate when processed mRNAs are accidentally reverse transcribed to DNA, then inserted back into the genome at a new location. They are readily distinguished from their mother genes because they lack both introns and promoters.  Since processed pseudogenes have no function, they tend to accumulate mutations. The older a processed pseudogene, the more mutations it will have accumulated.  Using Darwin’s thinking, if species are related from a common ancestor, then older processed pseudogenes should be shared by a greater variety of species.  These pseudogenes are found in common with a great range of primates of different life times and prove as evidence for the notion of common ancestry.  See Fig. 2.24 on page 58.  The Modern Concept of Homology o Many biologist, now, define homology as similarity due to the inheritance of traits from a common ancestor.  Read Box 2.2. on page 59.  Relationships among Species o Read page 60 o The exis
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