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Pseudogene Lesson B
|Students compare the DNA sequence data for a portion of the rat GULO gene to the corresponding sequence in the inactive GULO gene ("pseudogene") in humans, chimpanzees, orangutans, and crab-eating macaques by identifying the shared sequences in their alignment. They compare the pseudogene sequences and note a shared deletion. In addition, students do an alignment for the first 25 codons of the functional human beta globin gene and its pseduogene in humans, gorillas, and chimpanzees, then compare the pseudogenes and again note a shared deletion, as well as two other shared significant differences from the functional human sequence. Such shared deletions provide strong evidence for shared common ancestry (descent with modification), a natural process of macroevolution.|
|Many features of modern organisms reflect the structure of their ancestors in ways that are not adaptive.|
|1. Inactive DNA sequences that resemble the sequences of active genes are termed pseudogenes and can arise in one of three ways. |
2. Descendants of a mutation-carrying individual may carry that same mutation.
3. Although single-base substitutions may undergo reverse mutation at a later time, deletions are unlikely to revert.
4. Independently occurring deletions are unlikely to occur at exactly the same site.
5. Shared deletions are strong evidence for shared common ancestry.
(Click here to get PDF files for downloading)
|Student Handouts (Background and Worksheet) |
Genetic Code chart
green colored pencils or highlighters
yellow colored pencils or highlighters
|STUDENT HANDOUTS||See Materials (above)|
1. This lesson assumes a basic understanding of enzyme structure/function and of gene expression (protein synthesis). It is intended to follow the first lesson (A) in this suite: "Why Do We Need Vitamin C In Our Diet?"
Click here for an animated PowerPoint presentation that nicely introduces the concepts in this lesson, including the One-Gene-One-Enzyme idea, how the GULO gene and pseudogenes compare, and how the inability for primates to make vitamin C points to common ancestry. This link will take you to the script for the PPP and how to request the PPP itself.
1. Students may work alone, or preferably in pairs.
7. Using the beta globin data (Part B-2), have students use a green colored pencil or highlighter to mark positions at which all four sequences are identical and a yellow colored pencil or highlighter to mark the shared differences among the pseudogenes.
|1. Check student worksheets to determine whether they were able to color-code the sequences correctly. |
2. Ask students to explain why a shared deletion is strong evidence for common ancestry.
3. Ask students to propose a biologically reasonable explanation for why the human GULO pseudogene shares a deletion with the GULO pseudogenes of certain primates and why the human psi beta pseudogene shares a deletion with the psi beta pseudogenes of certain primates.
1. This lesson was meant to follow lesson A: "Why Do We Need Vitamin C in Our Diet?"
2. The data from which these partial sequences were taken consists of the complete coding sequence corresponding to rat exon 10 from the rat, human, chimpanzee, orangutan, and crab-eating macaque and from the complete coding sequences for the beta globin gene and pseudogenes. This data is available through Biology Workbench for computer manipulation. (See lesson C: "Exploring Primate Pseudogenes With Biology Workbench").
3. The comparison of different species in the two examples reflects the gaps in our knowledge of gene sequences across species. An extension of the activity would be to have students research currently accepted phylogenetic trees for a "favorite" or suggested species and predict, on the basis of the inferred relationships, other species in which the same shared deletions will someday be found.
4. Do the "Polar Bear / Giant Panda Ancestry" lesson, by Caroline Maier, in which DNA sequence data are used for building phylogenetic trees. This lesson will soon be added to this ENSI site. It can be found in The American Biology Teacher journal of November/December 2001 (63(9):642-646).
5. SPECIAL NOTE: Click here to explore many of the different lines of evidence pointing to speciation and macroevolution.
Chang, L.Y.; Slightom, J.:
Isolation and nucleotide sequence analysis of the beta-type globin pseudogene from human, gorilla, and chimpanzee.
J Mol Biol 180:767-84, 1984. PubMed ID: 6098690
Ohta, Y; Nishikimi, M.:
Random nucleotide substitutions in primate nonfunctional gene for L-gulono-gamma-lactone oxidase, the missing enzyme in L-ascorbic acid biosynthesis.
Biochim Biophys Acta 1472:408-11, 1999. PubMed ID: 10572964
Saitou, N; Nei, M.:
The number of nucleotides required to determine the branching order of three species, with special reference to the human-chimpanzee-gorilla divergence.
J Mol Evol 24:189-204, 1986. PubMed ID: 3104615
GENOME EVOLUTION by Ricki Lewis
Feature article in the online newsletter The Scientist for 1/27/03
Provides lucid description of the many ways genes are duplicated in nature, a primary mechanism of evolution, leading to gene families and pseudogenes.
Patterns seem to suggest major duplications initiated vertebrate origins at the onset of the Cambrian, with subsequent pulses of duplication leading to major jumps in vertebrate evolution.
Some of the ideas in this lesson may have been adapted from earlier, unacknowledged sources without our knowledge. If the reader believes this to be the case, please let us know, and appropriate corrections will be made. Thanks.
1. Original author requests anonymity.
2. Edited/Revised for ENSI website by L. Flammer: