Biol168_10F_Lecture 8_11Oct2010

Biol168_10F_Lecture 8_11Oct2010 - Dr. Morris Maduro, UC...

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Dr. Morris Maduro, UC Riverside Biology 168 – 10F – Lecture 8, page 1 Lecture #8: Drosophila : Homeotic Genes continued Textbook: 2nd edition: p. 117; 182-188; 3rd edition: 78-85; 156. Genes of the Bithorax Complex (BX-C) are part of the homeotic cluster (HOM-C) So we see that there are mutations that define coding regions and those that define regulatory regions. In fact, there are regulatory mutations that affect each of the segments. The differing mutations clearly implicated the BX-C in specification of these segments, but the complementation pattern was complex. Lewis initially proposed that there was a unique gene for each segment that was mutated in the BX-C. A molecular analysis of the BX-C revealed that there are only three transcripts made from the region. The arrangement of the genes and enhancers in the BX-C is similar in principle to the stripe-specific enhancers for the eve gene. The mutations that resulted in phenotypes of each segment corresponded to segment-specific regulatory regions. For example, mutations defining iab-4 result in a transformation of A4 into A3, while iab-6 results in transformation of A6 into A5. The mutation bithorax ( bx ) results in a transformation of the anterior half of the haltere into an anterior wing, while postbithorax ( pbx ) results in transformation of the posterior half of the haltere into a posterior wing. A double mutant for bx and pbx results in a complete transformation of the haltere into a wing. Mutation of the coding regions results in greater effects than individual regulatory mutations. Ubx-: T3, A1 Æ T2 abdA-: A2-A4 Æ A1 AbdB-: A5-A8 Æ A4 Each of the regulatory regions corresponds to a domain that is required to activate a correct pattern of specification in a given segment. As you might surmise, each is subject to regulation by different gap and pair-rule patterns. How do Ubx, abdA and AbdB gene products actually specify the segments? These genes all encode transcription factors, suggesting they activate other genes downstream, but this is still an unanswered question. It is believed that, similar to domains of pair-rule and gap gene expression, each segment identity is specified by a unique balance of these genes. For example, Cbx is an insertion of pbx sequences into the bx region – causes a T3-like expression pattern in T2, resulting in transformation of
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Biol168_10F_Lecture 8_11Oct2010 - Dr. Morris Maduro, UC...

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