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Chapter 14 Chromosomal Rearrangements and Changes in Chromosome Number Reshape Eukaryotic Genomes.

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Presentation on theme: "Chapter 14 Chromosomal Rearrangements and Changes in Chromosome Number Reshape Eukaryotic Genomes."— Presentation transcript:

1 Chapter 14 Chromosomal Rearrangements and Changes in Chromosome Number Reshape Eukaryotic Genomes

2 Outline of Chapter 14 Rearrangements of DNA sequences within and between chromosomes Rearrangements of DNA sequences within and between chromosomes Deletions Deletions Duplications Duplications Inversions Inversions Translocations Translocations Movements of transposable elements Movements of transposable elements Changes in chromosome number Changes in chromosome number Aneuploidy: monosomy and trisomy Aneuploidy: monosomy and trisomy Monoploidy Monoploidy Polyploidy Polyploidy

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4 Deletions remove genetic material from genome Fig. 13.2

5 Phenotypic consequences of heterozygosity Homozygosity for deletion is often but not always lethal Homozygosity for deletion is often but not always lethal Heterozygosity for deletion is often detrimental Heterozygosity for deletion is often detrimental Fig. 13.3

6 Deletion heterozygotes affect mapping distances Recombination between homologues can only occur if both carry copies of the gene Recombination between homologues can only occur if both carry copies of the gene Deletion loop formed if heterozygous for deletion Deletion loop formed if heterozygous for deletion Identification of deletion location on chromosome Identification of deletion location on chromosome Genes within can not be separated by recombination Genes within can not be separated by recombination Fig a

7 Deletion loops in polytene chromosomes Fig b

8 Deletions in heterozygotes can uncover genes Pseudodominance shows a deletion has removed a particular gene Pseudodominance shows a deletion has removed a particular gene Fig. 13.5

9 Deletions can be used to locate genes Deletions to assign genes to bands on Drosophila polytene chromosomes Deletions to assign genes to bands on Drosophila polytene chromosomes Complementation tests crossing deletion mutants with mutant genes of interests Deletion heterozygote reveals chromosomal location of mutant gene Deletion heterozygote reveals chromosomal location of mutant gene Fig. 13.6

10 Deletions to locate genes at the molecular level Labeled probe hybridizes to wild-type chromosome but not to deletion chromosome Labeled probe hybridizes to wild-type chromosome but not to deletion chromosome Fig a

11 Molecular mapping of deletion breakpoints by Southern blotting Fig b, c

12 Duplications add material to the genome Fig a,b

13 Duplication loops form when chromosomes pair in duplication heterozygotes In prophase I, the duplication loop can assume different configurations that maximize the pairing of related regions In prophase I, the duplication loop can assume different configurations that maximize the pairing of related regions Fig c

14 Duplications can affect phenotype Novel phenotypes Novel phenotypes More gene copies More gene copies Genes next to duplication displaced to new environment altering expression Genes next to duplication displaced to new environment altering expression Fig. 13.9

15 Unequal crossing over between duplications increases or decreases gene copy number Fig

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17 Summary of duplication and deletion effects on phenotpye Alter number of genes on a chromosome and may affect phenotype of heterozygote Alter number of genes on a chromosome and may affect phenotype of heterozygote Heterozygosity create one or three gene copies and create imbalance in gene product altering phenotypes (some lethal) Heterozygosity create one or three gene copies and create imbalance in gene product altering phenotypes (some lethal) Genes may be placed in new location that modifies its expression Genes may be placed in new location that modifies its expression Deletions and duplications drive evolution by generating families of tandemly repeated genes Deletions and duplications drive evolution by generating families of tandemly repeated genes

18 Inversions reorganize the DNA sequence of a chromosome Produced by half rotation of chromosomal regions after double-stranded break Produced by half rotation of chromosomal regions after double-stranded break Also rare crossover between related genes in opposite orientation or transposition Also rare crossover between related genes in opposite orientation or transposition Fig a,b

19 An inversion can affect phenotype if it disrupts a gene Fig c

20 Inversion heterozygotes reduce the number of recombinant progeny Inversion loop in heterozygote forms tightest possible alignment of homologous regions Inversion loop in heterozygote forms tightest possible alignment of homologous regions Fig

21 Gametes produced from pericentric and paracentric inversions are imbalanced Fig

22 Pericentric inversion Paracentric inversion (cont’d) (cont’d) Fig cont’d

23 Inversions suppress recombination Balancer chromosomes carry both a dominant marker D and inversions (brackets) that prevent recombination with experimental chromosome. Balancer chromosomes carry both a dominant marker D and inversions (brackets) that prevent recombination with experimental chromosome. Heterozygous parent will transmit balancer or experimental chromosome. Heterozygous parent will transmit balancer or experimental chromosome. Dominant mutation has an easily distinguished phenotpye (e.g., curly wing) Dominant mutation has an easily distinguished phenotpye (e.g., curly wing)

24 Translocations attach on part of a chromosome to another Translocation – part of one chromosome becomes attached to nonhomologous chromosome Translocation – part of one chromosome becomes attached to nonhomologous chromosome Reciprocal translocation – two different parts of chromosomes switch places Reciprocal translocation – two different parts of chromosomes switch places Fig a

25 Robertsonian translocations can reshape genomes Reciprocal exchange between acrocentric chromosomes generate large metacentric chromosome and small chromosome Reciprocal exchange between acrocentric chromosomes generate large metacentric chromosome and small chromosome Tiny chromosome may be lost from organism Tiny chromosome may be lost from organism Fig

26 Leukemia patients have too many blood cells Fig

27 Heterozygosity for translocations diminishes fertility and results in pseudolinkage Fig a.b

28 Three possible segregation patterns in a translocation heterozygote from the cruciform configuration Pseudolinkage – because only alternate segregation patterns produce viable progeny, genes near breakpoints act as if linked Fig c

29 Semisterility results from translocation heterozygotes Semisterility results from translocation heterozygotes < 50% of gametes arise from alternate segregation and are viable < 50% of gametes arise from alternate segregation and are viable Fig d

30 Translocation Down syndrome translocation of chromosome 21 is small and thus produces viable gamete, but with phenotypic consequence Fig

31 Transposable elements move from place to place in the genome 1930s Marcus Rhoades and 1950s Barbara McClintock – transposable elements in corn 1930s Marcus Rhoades and 1950s Barbara McClintock – transposable elements in corn 1983 McClintock received Nobel Prize 1983 McClintock received Nobel Prize Found in all organisms Found in all organisms Any segment of DNA that evolves ability to move from one place to another in genome Any segment of DNA that evolves ability to move from one place to another in genome Selfish DNA carrying only information to self-perpetuate Selfish DNA carrying only information to self-perpetuate Most 50 – 10,000 bp Most 50 – 10,000 bp May be present hundreds of time in a genome May be present hundreds of time in a genome LINES, long interspersed element in mammals LINES, long interspersed element in mammals ~ 20,000 copies in human genome up to 6.4kb in length ~ 20,000 copies in human genome up to 6.4kb in length SINES, short interspersed elements in mammals SINES, short interspersed elements in mammals ~ 300,000 copies in human genome ~ 300,000 copies in human genome ~ 7% of genome are LINES and SINES ~ 7% of genome are LINES and SINES

32 Retroposons generate an RNA that encodes a reverse transciptase like enzyme Two types Two types Poly-A tail at 3’ end of RNA-like DNA strand Poly-A tail at 3’ end of RNA-like DNA strand Long terminal repeat (LTRs) oriented in same direction on either end of element Long terminal repeat (LTRs) oriented in same direction on either end of element Fig a

33 Fig b

34 The process of LTR transposition Fig

35 Transposons encode transposase enzymes that catalyze events of transposition Fig a

36 P elements in Drosophila After excision of P element transposon, DNA exonucleases first widen gap and then repair it After excision of P element transposon, DNA exonucleases first widen gap and then repair it Repair uses sister chromatid or homologous chromosome as a template Repair uses sister chromatid or homologous chromosome as a template P strains of Drosophila have many copies of P elements P strains of Drosophila have many copies of P elements M strains have no copies M strains have no copies Hybrid dysgenesis – defects including sterility, mutation, and chromosomal breakage from cross between P and M strains Hybrid dysgenesis – defects including sterility, mutation, and chromosomal breakage from cross between P and M strains Promotes movement of P elements to new positions Promotes movement of P elements to new positions

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38 Genomes often contain defective copies of transposable elements Many TEs sustain deletions during transposition or repair Many TEs sustain deletions during transposition or repair If promoter needed for transcription deleted, TE can not transpose again If promoter needed for transcription deleted, TE can not transpose again Most SINES and LINES in human genome are defective TEs Most SINES and LINES in human genome are defective TEs Nonautonomous elements – need activity of nondeleted copies of same TE for movement Nonautonomous elements – need activity of nondeleted copies of same TE for movement Autonomous elements – move by themselves Autonomous elements – move by themselves

39 TEs can generate mutations in adjacent genes spontaneous mutations in white gene of Drosophila Fig

40 TEs can generate chromosomal rearrangements and relocate genes Fig

41 The loss or gain of one or more chromosomes results in aneuploidy

42 Autosomal aneuploidy is harmful to the organism Monosomy usually lethal Monosomy usually lethal Trisomies – highly deleterious Trisomies – highly deleterious Trisomy 18 – Edwards syndrome Trisomy 18 – Edwards syndrome Trisomy 13 – Patau syndrome Trisomy 13 – Patau syndrome Trisomy 21 – Down syndrome Trisomy 21 – Down syndrome

43 Humans tolerate X chromosome aneuploidy because X inactivation compensates for dosage Fig

44 Mitotic nondisjunction Mitotic nondisjunction Failure of two sister chromatids to separate during mitotic anaphase Failure of two sister chromatids to separate during mitotic anaphase Generates reciprocal trisomic and monosomic daughter cells Generates reciprocal trisomic and monosomic daughter cells Chromosome loss Chromosome loss Produces one monosomic and one diploid daughter cell Produces one monosomic and one diploid daughter cell Fig a

45 Mosaics – aneuploid and normal tissues that lie side-by-side Mosaics – aneuploid and normal tissues that lie side-by-side Aneuploids give rise to aneuploid clones Aneuploids give rise to aneuploid clones Fig b

46 Gynandromorph in Drosophila results from female losing one X chromosome during first mitotic division after fertilization Fig

47 Euploid individuals contain only complete sets of chromosomes

48 Monoploid organisms contain a single copy of each chromosome and are usually infertile Monoploid plants have many uses Monoploid plants have many uses Visualize recessive traits directly Visualize recessive traits directly Introduction of mutations into individual cells Introduction of mutations into individual cells Select for desirable phenotpyes (herbicide resistance) Select for desirable phenotpyes (herbicide resistance) Hormone treatment to grow selected cells Hormone treatment to grow selected cells

49 Fig

50 Treatment with colchicine converts back to diploid plants that express desired phenotypes Fig c

51 Polyploidy has accompanied the evolution of many cultivated plants 1:3 of flowering plants are polyploid 1:3 of flowering plants are polyploid Polyploid often increases size and vigor Polyploid often increases size and vigor Often selected for agricultural cultivation Often selected for agricultural cultivation Tetraploids - alfalfa, coffee, peanuts Tetraploids - alfalfa, coffee, peanuts Octaploid - strawberries Octaploid - strawberries Fig

52 Triploids are almost always sterile Triploids are almost always sterile Result from union of monoploid and diploid gametes Result from union of monoploid and diploid gametes Meiosis produces unbalanced gametes Meiosis produces unbalanced gametes Fig

53 Tetraploids are often source of new species Tetraploids are often source of new species Failure of chromosomes to separate into two daughter cells during mitosis in diploid Failure of chromosomes to separate into two daughter cells during mitosis in diploid Cross between tetraploid and diploid creates triploids – new species, autopolyploids Cross between tetraploid and diploid creates triploids – new species, autopolyploids a

54 Maintenance of tetraploid species depends on the production of gametes with balanced sets of chromosomes Maintenance of tetraploid species depends on the production of gametes with balanced sets of chromosomes Bivalents- pairs of synapsed homologous chromosomes that ensure balanced gametes Bivalents- pairs of synapsed homologous chromosomes that ensure balanced gametes Fig b

55 Fig c

56 Some polyploids have agriculturally desirable traits derived from two species Amphidiploids created by chromosome doubling in germ cells Amphidiploids created by chromosome doubling in germ cells e.g., wheat – cross between tetraploid wheat and diploid rye produce hybrids with desirable traits e.g., wheat – cross between tetraploid wheat and diploid rye produce hybrids with desirable traits Fig


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