Intro/Background
description
Transcript of Intro/Background
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Generating Diversity: how genes and genomes evolve
Erin “They call me Dr. Worm” Friedman29 September 2005
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Intro/Background
• Why do we care about generating diversity?• What exactly is mutation?• How does evolution act on mutations?• How does evolution work on a molecular
scale?• It’s not just about frog mating calls…
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Genetic Change Mutation
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Point Mutations• Nucleotide change, addition, deletion
• Silent Mutation – same AA (synonymous)• Sense Mutation – different AA (nonsynonymous)• Nonsense Mutation – stop codon (translation termination)
Plotkin et. al., Nature 2004
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Generating Point Mutations
• Replication Errors (Polymerase isn’t perfect)– Human rate 1 in 1010
• Chemical mutagens or radiation• Repair failure after such DNA damage
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Sickle Cell Anemia
• Caused by a point mutation in beta chain of hemoglobin (GAGGTG)
• Changes glutamic acid to valine– What kind of mutation is this?
• Autosomal Recessive Disorder• Cell morphology
http://users.rcn.com/jkimball.ma.ultranet/BiologyPages/M/Mutations.html
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Sickle Cell Evolution
• Sickle cell anemia vs. sickle cell trait (het)
• Low oxygen Acidity sickling
• Link between sickle cell trait and malaria
• Positive selection for sickle cell trait in some regions with high malaria incidence
http://www.nhlbi.nih.gov/health/dci/Diseases/Sca/SCA_WhatIs.html
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Gene duplication / deletion
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Globin Gene Evolution
• Especially important in larger, multicellular organisms– Diffusion doesn’t cut it
• Multiple duplication events• Primitive animals have one globin chain;
higher vertebrates have two• Beta-globin duplicated / mutated again
(fetal, adult) and each product duplicated…
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Gene duplication / deletion
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Duplication Divergence
• New gene copy is free to mutate• Not all duplications lead to functional new
genes (pseudogenes)• Impact of gene deletions?
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Exon Duplication
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Fig. 9-10
Exon Shuffling• Recombination between non-
homologous genes• A few thousand exons could
explain protein variability today
• Combinations of different exon elements
• (symbols = different protein domains)
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Exon shuffling
• Can bacteria use exon duplication / shuffling to form a functional gene?
• Why would big introns be beneficial for generating diversity in this manner?
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Transposable Elements
• Parasitic DNA sequences
• Can disrupt function, alter regulation, or make new genes by bringing along gene segments with it
• Inverted repeats• Transposase binding
domains
http://engels.genetics.wisc.edu/Pelements/fig1.html
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Transposons
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Transposable Elements in regulatory regions
Common human example of gene inactivation from transposon insertion: Factor VIII hemophilia
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Transposons as a tool
• Transposable elements can be used to study particular genes
• Knock out genes and look for a phenotype (reverse genetics)
• Drosophila P-Element (transposon)– Can carry different genes, map insertion by
phenotype
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Horizontal Transfer: organisms exchanging genes
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Conjugation animation
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Consequences of Horizontal Transfer
• Gene duplication• Rapid evolution• Bacterial antibiotic resistance
– Some strains of TB are resistant to 9 antibiotics– “Drug resistance may have contributed to the 58
percent rise in infectious disease deaths among Americans between 1980 and 1992.” (Mayo Clinic)
– Where in genome would resistance genes live?
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What Now?
• We can use this information to reconstruct an evolutionary tree
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Analyzing Genomes• Homologous genes have common ancestry,
similar nt sequences– Finding homologues with BLAST
• Different genome segments evolve differently– Highly conserved / essential genes = constrained
• Purifying selection removes dysfunctional individuals
• Positive selection preserves beneficial mutations• Genetic Drift: random, unconstrained mutation• How we measure mutation rate
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Evolutionary descent
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Why reconstruct ancestor sequences?
• Can be used to study evolution rates– Why can’t you just compare 2 genes? – Measuring rates with dN/dS, PAML
• Evolution rate is a good screen for looking for candidate genes (compare gene to ancestor, not to homologous gene):– Some genes likely evolve rapidly (e.g. those involved
in infection, defense)– Highly conserved, essential genes likely evolve slowly
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Conserved synteny
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Rediers et. al., Microbiology 2004
Sample genome analysis
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PST v PSB
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Synteny in Pseudomonas
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PST v PP
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PST v PA
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“Junk” DNA
Exons more highly conserved than introns: different evolutionary constraints in different parts of genome
What kind of selection is acting on the exons?
What phenomenon is taking place in the intron?
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Introns (noncoding DNA) are non-essential
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Conserved sequences: comparing distant genomes
Small subunit of rRNA