Lect7_selection

Lect7_selection - Base additions or deletions may have a...

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1 Inheritance and Selection: The Essence Lecture #7 • Frameshift. All amino acids after mistake changed • Usually results in extended missense with eventual nonsense • Frameshifts have a large affect on phenotype AUG•U A U•UAU AUG• UU U•AU Met Tyr Tyr Met ??? Base additions or deletions may have a large impact on proteins Frameshift (Analogy) – THEREDCATATETHEFATRAT – Start codon = THE -- can also be found within a protein (sentence!) (AUG means methionine as well as start) – Reading frame determined by ribosome = 3 bases read at a time (non-overlapping) – ORF = open reading frame (part of a protein, a sequence of “sense” codons) • Frameshift: Adding or subtracting bases (in groups of 1 or 2) – THE-RED-CAT-ATE-THE-FAT-RAT THE-REC-ATA-TET-HEF-ATR-AT All subsequent 3-letter words (codons and therefore amino acids) are changed D THE-RE_-CAT-ATE-THE-FAT-RAT • No Frameshift from Adding or subtracting bases in groups of 3 • THE-RED-CAT-ATE-THE-FAT-RAT A 3-letter words (codon and thus amino acid) are added or removed Big a THE-BIG-RED-CAT-ATE-THE-FAT-RAT Each of the following is a modification of the sentence THECATATETHERAT . A. THERATATETHECAT B. THETACATETHERAT C. THECATARETHERAT D. THECATATTHERAT E. CATATETHERAT Which of the above is analogous to a frameshift mutation?
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2 • A new mutation is most likely to be – A. Harmful (deleterious) – B. Neutral – C. Beneficial – D. B and C are both very likely • Try to convince your neighbor that your answer is correct! • A new mutation arises is most likely to be – A. Harmful (deleterious) – B. Neutral – C. Beneficial – D. B and C are both very likely Any mutations that arise despite repair and proofreading mechanisms are likely to be deleterious. Why? It’s easier to break something than to improve it. Today’s 3 Goals • 1. Understand “Muller’s Ratchet”, a model showing the consequences of replication without selection • 2. Understand how selection defeats the ‘ratchet’ • 3. Understand what biologists mean by natural selection A model: Muller’s Ratchet • As generations pass, the DNA is copied millions of times • Mistakes inevitably happen • Since most mutations are harmful, the cell lineage becomes less able to survive and reproduce over time • The ‘ratchet’ turns one way – mutations accumulate, and reduce fitness over time
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3 = mutation Bacterial Cell with circular chromosome Reproduction = mutation = mutation 1 Error 2 Errors 3 Errors = mutation 1 Error 2 Errors 3 Errors What has happened to fitness over time? A. It has very likely been reduced B. It has very likely stayed the same C. It has very likely improved A model: Muller’s Ratchet • Since mistakes are inevitable, mutations will accumulate over time in any population of organisms – If all the offspring survive, fitness (ability to survive and reproduce) will decline over time – A mutational meltdown will take place where offspring
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Lect7_selection - Base additions or deletions may have a...

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