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40 Cards in this Set
- Front
- Back
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What are chromosomal mutations?
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Changes in the number or structure of chromosomes
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How common are chromosomal mutations?
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3 groups
a) healthy fertile adults - 0.4% b) adults with fertility issues - 5-13% c) spontaneously aborted embryos - 57-61%. This is likely and underestimate; probably closer to 90% |
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Causes of chromosomal mutations?
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2 general categories
1) spontaneous mistakes in meiosis: non-disjunction, uneven crossing over, crossing over with the wrong chromosome 2) factors inducing double-stranded breaks: ionizing radiation, transposons, failure of mutation in ds repair mechanisms |
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What is aneuploidy?
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The gain or loss of a chromosome
1) monosomy: the loss of 1 (2N -1) -not well tolerated a) unmasking of lethal recessive mutations b) changes in critical gene dosage levels - very rare in humans - more tolerated in other species 2) trisomy: the fain of 1 (2N +1) - more tolerated but still unusually lethal in humans ex: live to adulthood: downsyndrome-21 ex: do not live to adulthood: trisomy13: Patau syndrome trisomy18: Edwards syndrome |
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What is euploidy?
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Variation in the number of haploid sets of chromosomes
2 categories 1) monoploidy - only 1 haploid set 2) polyploidy - extra haploid sets |
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Types of polyploidies?
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1) Autopolyploidy - extra sets from the same species
- causes include: skipping cytokinesis in meiosis I or II, 2 sperm fusing to 1 egg, experimentally induce with chemicals - Generally bigger (agriculture) - Even haploids fertile, odd haploids sterile: for meiosis to occur all must have a partner 2) Allopolyploidy - extra sets from different species - causes include: fusion of gametes from different species (classic example the mule) - almost always sterile - can be fertile if even number haploids |
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Variation in structure - 4 types?
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1) Deletion - segment of a chromosome missing
- consequences: unmasking of harmful recessive traits, gene dosage effects, lead to monosomy if centromere lost - cause cri-du-chat syndrome 2) Duplication - segment of chromosome repeated - usually caused by uneven crossing over (ex. bar mutation in drosophila) - might be evolutionary 3) Inversion - segment of chromosome turned backward a) pericentric - includes centromere b) paracentric - doesn't include centromere - often caused by a mistake in ds break repair - consequences: possible disruption of gene expression, synapsis impaired (trouble lining up, chromosome must form "inversion loop"), crossing over leads to di- (2 centromeres) and a- (no centromere) centric chromatids - common inversion in chromosome 9 in humans (1-1.5% of pop., small increase in miscarriage and decrease in fertility) 4) Translocation - segment moved from one chromosome to another a) nonreciprocal - one direction b) reciprocal - crossing over between two nonhomologous chromosomes - consequences: possible disruption of gene expression (chronic myelogenoud leukemia), impaired synapsis and segregation (can produce gametes with too many or too few copies of segments, can be equiv. to "trisomy" of "monosomy") |
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Why is genetic recombination important for a species?
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Allows for potentially novel combinations of alleles
Many eukaryotes use crossing-over & sexual reproduction while prokaryotes don't (have only 1 chromosome, reproduce asexually) |
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Bacterial recombination?
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Involves physical transfer of DNA from one cell to another
Replacement of one allele with another Can be used to map the bacterial chromosome - use frequency of co-transfer of genes if far apart = occurs rarely if close together = occurs more often |
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3 types of bacterial recombination?
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1) Transformation - uptake of extracellular DNA
2) Conjugation - transfer through direct cell contact 3) Transduction - transfer mediated by a phage |
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Lederberg & Tatum?
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Mixed 2 autotrophic strains of E. coli and got prototrophs, concluded that some form of recombination occurred.
Other scientists proved transfer was undirectional Davis proved it was contact dependent using a u-tube |
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Conjugation: what is transferred?
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A plasmid called the F factor (F+ strains have it, F- don't)
It contains an origin where one strand is nicked and passed and gene encoding proteins that mediate transfer It contains "extra" genes called an F' factor |
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Conjugation: how is plasmid transfered?
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4 steps
1) sex pilus forms 2) strand of F factor nicked 3) one strand passed across 4) strands replicated in both cells |
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Conjugation: how to use map bacterial chromosomes?
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-Requires use of Hfr (high frequency recombination) strains (F plasmid inserted into bacterial chromosome)
-Mix with F- strain and measure time it takes for F- to acquire new alleles -Distance between genes measured by "minutes" -Must use multiple Hfr strains to map entire genome |
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Lederberg & Tatum?
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Mixed 2 autotrophic strains of E. coli and got prototrophs, concluded that some form of recombination occurred.
Other scientists proved transfer was undirectional Davis proved it was contact dependent using a u-tube |
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Conjugation: what is transferred?
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A plasmid called the F factor (F+ strains have it, F- don't)
It contains an origin where one strand is nicked and passed and gene encoding proteins that mediate transfer It contains "extra" genes called an F' factor |
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Transduction: phage life cycle
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2 examples
Virulent phages - after injection, proteins in phage degrade the host chromosomes Temperate phages - viral DNA inserted into host genome called lysogeny, "dormant" virus called a prophage, stressful trigger "reactivates" back to lytic phase |
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Conjugation: how is plasmid transfered?
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4 steps
1) sex pilus forms 2) strand of F factor nicked 3) one strand passed across 4) strands replicated in both cells |
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Conjugation: how to use map bacterial chromosomes?
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-Requires use of Hfr (high frequency recombination) strains (F plasmid inserted into bacterial chromosome)
-Mix with F- strain and measure time it takes for F- to acquire new alleles -Distance between genes measured by "minutes" -Must use multiple Hfr strains to map entire genome |
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Transduction: phage life cycle
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2 examples
Virulent phages - after injection, proteins in phage degrade the host chromosomes Temperate phages - viral DNA inserted into host genome called lysogeny, "dormant" virus called a prophage, stressful trigger "reactivates" back to lytic phase |
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Lederberg & Tatum?
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Mixed 2 autotrophic strains of E. coli and got prototrophs, concluded that some form of recombination occurred.
Other scientists proved transfer was undirectional Davis proved it was contact dependent using a u-tube |
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Conjugation: what is transferred?
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A plasmid called the F factor (F+ strains have it, F- don't)
It contains an origin where one strand is nicked and passed and gene encoding proteins that mediate transfer It contains "extra" genes called an F' factor |
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Conjugation: how is plasmid transfered?
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4 steps
1) sex pilus forms 2) strand of F factor nicked 3) one strand passed across 4) strands replicated in both cells |
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Conjugation: how to use map bacterial chromosomes?
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-Requires use of Hfr (high frequency recombination) strains (F plasmid inserted into bacterial chromosome)
-Mix with F- strain and measure time it takes for F- to acquire new alleles -Distance between genes measured by "minutes" -Must use multiple Hfr strains to map entire genome |
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Transduction: phage life cycle
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2 examples
Virulent phages - after injection, proteins in phage degrade the host chromosomes Temperate phages - viral DNA inserted into host genome called lysogeny, "dormant" virus called a prophage, stressful trigger "reactivates" back to lytic phase |
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Lederberg & Linder
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Repeated experiments with salmonella, mixed auxotrophs and got prototrophs, repeated Davis u-tube experiment and got prototrophs with filter, concluded a "filterable agent" mediating recombination
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2 types of transduction?
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1) General - a piece of degraded host DNA gets loaded into the phage
2) Specialized - can only be done by temperate phages, the prophage gets excised imperfectly carrying some of the host genome with it |
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How to determine co-transduction frequency to map genes?
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When 2 genes transduced in 1 event
Typical experiment: 1) Incubate phage with prototrophic bacteria 2) Generate "lysate" containing new phage 3) Mix lysate with auxotrophic bacteria 4) Plate on selective media and count # of colonies |
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What are population genetics?
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The investigation of the genetic variation found in a population
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What is a population?
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A group of interbreeding individuals of the same species, found in the same geographical area, and share the same gene pool
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Describe variation in a gene pool at one locus?
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1) Genotypic frequency - "selected" by natural selection
f(genotype) = # with that genotype divided by the total # of individuals 2) Allelic frequency - what is passed on to the next generation f(allele) = (2 X # of homozygotes) plus (# of heterozygotes) divided by (2 X total # of individuals) |
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Hardy-Weinberg History?
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Early 1900s
Darwin's ideas well established and Mendel's rediscovered. 1908 Hardy and Weinberg - mathematical approach described: relationship between allelic and genotypic frequencies and how change over time happened in the abbsence of influence |
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Hardy-Weinberg Law
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If a population:
1) Is infinitely large 2) Is random mating 3) Has no mutation 4) Has no migration 5) Is under no pressure from natural selection Then: 1) Allelic frequencies won't change over time p=f(A), q=f(a), p+q=1 2) Genotypic frequencies will remain in the proportions p2, 2pq, q2 and p2+2pq+q2=1 |
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Consequences of Hardy-Weinberg Law?
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Shows that variability will be maintained and dominant traits will not take over.
Allows for the prediction of all frequencies from one piece of data |
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Factors that influence allelic and genotypic frequencies?
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Mutation
Migration Genetic Drift Non-random mating Natural selection |
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Mutation?
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Changes one allele to another
- from old allele to new one - from A to a and a to A (expressed as a rate; mutation found in a # of gametes) A slow acting force if acting alone (dramatic if combining with natural selection) May keep harmful recessive alleles at a low frequency (called "mutation-selection balance hypothesis) |
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Migration?
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Gene flow between populations due to the movement of individuals
- A homogenizing force between 2 populations - Impact depends on: proportion of immigrants and the difference in frequencies between populations - A significant force in the human population (e.g. blood type) |
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Genetic Drift?
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Random fluctuation of allelic frequency between generation
- Due to "sample error" - Influence of chance in: who is breeding and which gametes are used - Most significant in a small population Can cause and allele to become fixed or lost in a population - Most significant in 2 circumstances 1) Sharp reduction in pop. size - The "bottleneck effect" 2) Small group migrates to a new location and forms a new population - The "founder effect" - Albinism in early America - Y chromosome diversity in Finland |
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Non-random mating?
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When individuals with similar or dissimilar phenotypes mate preferentially
- If similar - positive assortative mating - It dissimilar - negative assortative mating Affects genotypic but not allelic frequency |
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Natural Selection?
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Proposed in mid 1800s by Darwin and Wallace
4 key elements: 1) Variation in traits exist 2) These traits are inheritable 3) More offspring produced than can survive, must compete for limited resources 4) Individuals with advantageous traits will survive longer and have more offspring |
