Chromosomal mutations (chromosome rearrangements) Chromosomes throughout cell cycle cell division {mitosis) cycle begins cell prepa to divide cell grows two chromatids one chromatid replication of DNA cell decides whether to continue Chromosomes and chromatids during mitosis and meiosis 1 chromatid 2 chromatids 1 chromatid Mitosis 1 chromatid CenťoBorre ------- Spirdlů microtubule 2 chromatids 1 chromatid Chromosome mutations are variations in: 1. Chromosome structure (chromosomal rearrangements) ^ • deletions • duplications • translocations • inversions • transpositions 2. Chromosome number • aneuploidy * abnormal euploidy Chromosome rearrangements Chromosome rearrangements are caused by breakage of DNA double helices in the genome at two different locations, followed by a rejoining of the broken ends to produce a new chromosomal arrangement of genes, different from the gene order of the chromosomes before they were broken. Most common chromosome rearrangements are (i) deletions (ii) duplications imbalanced - change the gene dosage of a part of the affected chromosomes, similar to aneuploidy for whole chromosomes (the loss of one copy or the addition of an extra copy of a chromosome segment can disrupt normal gene balance) (iii) inversions , ., , . , y ' balanced - change the chromosomal gene order ,. v , .. but do not remove or duplicate any of the DNA of (iv) translocations J K y the chromosomes Chromosome rearrangements: points to remember • each chromosome is a single double-stranded DNA molecule (DNA helix) • the first event is the generation of two or more double-strand breaks (DSBs) in the chromosomes • DSBs are potentially lethal, unless they are repaired • repair systems in the cell correct the DSBs by joining broken ends back together • if the two ends of the same break are rejoined >> the original DNA order restored, if ends of two different breaks are joined together (= mis-repair of DNA demage) >> chromosomal rearrangement • segment of DNA lost or duplicated in the rearrangement cannot be "too large" (gene balance); the larger the segment of a chromosome lost or duplicated, the more likely will it cause phenotypic abnormalities How double-strand breaks are generated DSBs are caused by several factors: • arrest of replication and restart of DNA synthesis (replication forks tend to stall in regions of repeat elements - e.g. tRNA genes, retroposons, and telomeres); major ansposon excision _ _ __ Ionizing radiation Endonuclease source of DSBs! during meiotic recombination . . , ■ ■ , , ° abcdef ghijkl • mechanical pulling (e.g. in dicentric chromosomes) DSB • experimentally (radiation by X-rays, DNA transposons, rare cutting restriction endonucleases) ♦♦♦ in vegetative (mitosis) and generative cells (meiosis) ♦♦♦ DSBs have to be repaired before genomes are replicated (S phase) ♦♦♦ in plants, errors in DSB repair (DSBs misrepair) can have the evolutionary significance because changes in meristematic cells can be transferred to the offspring >>> chromosome rearrangements 88 Origins of chromosome rearrangements (a) By breakage a rid rejoining 10-',2 3,V, 4 "if I Loss 12 3-1 3 A4 2 o ■■, 3 6 7_i>_8 1C 1 DalHtiůn 4 1 Deletion 2 4 1 2 3 3 4 1 Duplication 3 2 4 Inversion 1 2 6 9 10 5 6 7 3 4 Reciprocal translocation Key: l r h r A = break rcpditivc DNA rejoining segments X- crossover In organisms with repetitive DNA, homologous repetitive segments within one chromosome or on different chromosomes can act as sites for illegitimate crossing-over. Deletions, duplications, inversions, and translocations can all be produced by such crossing-over. (b) By crossing-over between repetitive DNA 1 2 3 4 O I [32] 4 1 S3 l 4 1 2 3 4 t i 2 3 ^_ 1 2 3 4 s o ľ1/ e & io 1 4 Lost Deletion Dolenon 12 3 2 3 4 i i * Duplication 1 3 2 4 Inversion 1 2 B 910 5 e 7 3 4 ricdprocal translocation Chromosome rearrangements Deletions m r iQi Duplications * . Q ! ,t! « • ■ * - 1 S S 3 4, 5 6 7 fl Q ) Inversions • * * »<^ -• ».'» * -» Translocations i___3 I C. D El"! '.nn.....™~E...........s...........ž.........h...........aHD.................} Deletion formation by breakage and rejoining = deficiencies = losses of chromosome segments • can occur terminally or internally, e. g. caused by... • breakage and rejoining within one chromosome: r X rays break both strands of DNA I Deletion of region CDE v_JL segmer to be deletei ° S ■4= Q. » < ) j i. Mechanical shear 2. Radiation 3. Transposable elements Deletion formation by intra-chromosomal crossover another example... "Direct" sequence repeat I | a « U ~ t 1 2 r-* 6 7 S 9 U ) Del e ti oil with one copy of direct-repeat sequence Deletion (and duplication) formation by unequal cross-over Sometimes during meiosis two chromatids from homologous chromosomes (A) are misaligned during a cross-over event (B) as a result, one chromatid gained a duplicated region and the another lost a deleted region (C). The duplication as well as the deletion are inherited by resulting gametes. G Direct" sequence repeat l J a Ž o L-L 3 misaligned homologous chromosomes J. ( : > T 5 (J ) Deletion of [3 4] segment sl^>s$ 4 y*^i 4r^5" Duplication of [3 4] segment \J —U 9 How deletions can be identified by finding a visible change in chromosome structure: Normal homolog Deletion loop Homolog with a deletion polytene chromosome 50A B C Drosophila deletion heterozygote • polytene chromosomes d 51A Missing most of 5OD »r Duplications (a) Tandem duplication Normal chromosome Same order Reverse order * Fi r i r " F A E G Nontandem (dispersed) duplications Same order t* k t: n F F Reverse order ^^^^^jjj (b) X rays break one chromosome in two places Nontandem duplication X rays break homologous chromosome in one place Duplication (and deletion ) formation by unequal cross-over • common mechanism of duplications Sometimes during meiosis two chromatids from homologous chromosomes (A) are misaligned during a cross-over event (B) as a result, one chromatid gained a duplicated region and the another lost a deleted region (C). The duplication as well as the deletion are inherited by resulting gametes. ''Direct" sequence repeat l l____________ t\ ana (~rm r-> t\ > b / b d o ) misaligned homologous chromosomes r^i—^r—>b b i b y u i Deletion of [3 4] segment Duplication of [3 4] segment : Consequences of duplications • most duplications have no phenotypic consequence • sometimes effects can be seen due to increased gene dosage • play a very important role in evolution: • increase gene number • evolution of new genes (paralogs!) i i 1970 Susomo Ohno - "Evolution by Gene Duplication" • evolution of RNA genes Gene duplication produces a reservoir of genes • in particular rRNA genes (rDNA) from which to evolve new ones. Why reinvent the . 5_10 copies/bacterial genome wheel from scratch? • c. 130 copies/Drosophila genome • Xenopus c. 400 copies/genome (but the oocyte may have 1500 micronuclei, each with a NOR, it is c. 600,000 copies of rDNA) Inversions Inversions as balanced rearrangements are generally viable and show no particular abnormalities at the phenotypic level. Many inversions can be made homozygous. Inversion heterozygote - cells that contain one normal haploid chromosome set plus one set carrying the inversion. Microscopic observation of meioses in inversion heterozygotes reveals an inversion loop. ^_L segment to be inverted si p > co ±±± meiotic inversion loop Two types of inversions Paracentric inversion Breaks Arm ratio unchanged Pericentric inversion j^0 £ —-,y\_f Breaks Arm ratio changed Copyright© 2006 Pearson Prentice Hall, Inc. mechanism of inversion formation: breakage and rejoining Inversion formation by intra-chromosomal crossover inverted repeats How the chromosomes pair in an inversion heterozygote? (paracentric) inversion heterozygote normal G 7------5-----W inversion <' * a 9 s I- , B b (J -) by forming an inversion loop... Inversion loops in para- and pericentric inversion heterozygotes (_£ CZ paracentric inversion é" pericentric inversion When no recombination occurs, 50% of gametes have inversion. Next two slides show what happen if a recombination event does occur in the inversion loop... A crossover within the inversion loop of a heterozygote for a pericentric inversion v A B A B a c 1 d b C D c a b d c D Wl ild tyj: :>e duplication /deletion (gametes/zygotes not viable) inversion duplication/deletion (gametes/zygotes not viable) D <33=C A D Crossover in loop B ! C B E Pairing Paracentric ft inversion helero/ygute two nonsister chromatids cross over within the loop CAccentric fragment (lost) B E Dicentric bridye freaky randomly B h" ^ B C D E f A crossover within the inversion loop of a heterozygote for a paracentric inversion Crossing-over within the inversion loop connects homologous centromeres in a dicentric bridge while also producing an acentric fragment - one without a centromere. Anaphase I - the acentric fragment cannot align itself and it is lost - tension eventually breaks the dicentric bridge, forming two chromosomes with terminal deletions Normal product A B C D E J. B c D Deletion product I in: A Deletion product Inversion product 0 c G E Gametes containing deleted chromosomes may be inviable, but, even if viable, the zygotes that they eventually form will probably be inviable. Crossing-over generates here lethal products. The overall result is a lower recombinant frequency. Inversions affect recombination in another way, too. Inversion heterozygotes often have mechanical pairing problems in the region of the inversion, which reduces the opportunity for crossing-over in the region. Consequences for speciation. Inversions and recombination: evolutionary significance Can be "adaptive" when it stabilizes a superior combination of alleles on a chromosome (examples seen in Drosophila) Position effect in Drosophila YV r+ ild-tv wuci-tvpe yp w mutant Inversions are crossover suppresors - evolutionary consequences (more information on the role of inversions in speciations in next lecture) Translocations: nonreciprocal and reciprocal (d) Nonreciprocal translocation of A-B A J H 1 B 1 / 1 C 1 D J Ml £ 1 Ĺ I Z1 1 M 11 C 1 B C D Nonreciprocal ^ translocation + 4 B H I J K L M attachment of chromosome fragment to a non-homologous chromosome (leading to deletions and duplications in progeny) (e) Reciprocal translocation of A-B and H-!-J A B C D E F C + H I J K L M H A 1 B Reciprocal , translocation K + f~ i o M *\ f\ c Q exchange of chromosome fragments between non-homologous chromosomes Reciprocal translocations Reciprocal translocations result from crossover events between nonhomologous chromosomes Two ways of segregation: a) translocation chromosomes segregate together (balanced translocation) b) translocation chromosomes are separated > > gametes with duplications and deletions (imabalanced gametes) > > 50% of the gametes are not viable (= semisterility) (a) Possible origin of a reciprocal translocation A O A O K$%D K H HD (b) Synapsis of translocation heterozygote (c) Two possible segregation patterns leading to gamete formation Normal Meiosis I and II Meiosis I and II Balanced translocation Duplicated and deficient Duplicated and deficient Nonreciprocal translocation and its consequences (familial Down syndrome) (a) fusions of long arms of two Breaks acrocentric chromosomes (13, 14, 15, 21 and 22) -* * *fc « .. * V) ^ Centric fusion Acentric fragments 3=> Nonreciprocal translocation and its consequences (familial Down syndrome) • Most of long arm from chromosome 21 translocated to 14 (14/21 translocation) • Fusion occurs at two rDNA regions on the chromosomes • about 20% rDNA copies lost • carrier still normal • trisomy 21 (Down) 21 14 Normal l í Translocation carrier 14/21 Gamete formation I T Gametes 1 } Normal Translocation Trisomy 21 Monosomie carrier (Down) (lethal) 46 45 46 45 Chromosome number Reciprocal translocation: homozygotes Normal segregation during meiosis Secondary reciprocal translocation i normal ("wildtype") karyotype (n=3) 2 types of translocation heterozygotes crossing meiosis: crossover (X) between partially homologous chromosomes - the new chromosome segregates together with the other two translocation chromosomes to one pole (arrows down), on the other pole the wildtype karyotype is reconstituted (arrows up) Robertsonian translocations - ROBs (centric fusions) • type of a reciprocal translocation between two acrocentric chromosomes • also called whole-arm translocations or centric-fusion translocations • named after the American insect geneticist W. R. B. Robertson, who first described a Robertsonian translocation in grasshoppers in 1916 • evolutionary significance >>> chromosome number reduction (from 2 acrocentric chromosomes one metacentric chromosome) Dicentric ROB (more frequent) fl ^=> lost Monocentric ROB n ^=> Robertsonian translocations are the most common recurrent structural anomaly in humans, with about 1 in 1000 individuals carrying this rearrangement. The carriers of ROBs have 45 chromosomes instead of the normal 46. lost Permanent translocation heterozygotes Translocation heterozygotes segregate the chromatides from the tetrades in such a way that there are two kind of gametes (a non-translocated set of chromosomes and a translocated set of chromosomes). Example: evening primrose (Oenanthera). normal chromosomes Each chromosoma have exchanged an arm with the next chromosome IB) Metaphase I: a ring of chromosomes, each arm is paired with its proper partner (each chromosome consists of two chromatids) Alternate segregation yields two types of gametes: gametes with all normal chromosomes, and gametes with all translocated chromosomes Breakage-fusion-bridge cycle (BFB cycle) • described by B. McClintock (1940) in maize (Zea mays) • chromosomes are broken by various means (asymetrie translocation) • the adhesive broken ends fuse with one another • a chromatid bridge is produced as the two centromeres of the terminally united chromosomes pass to opposite poles in this mitotic anaphase • the tension on the anaphase bridge due to the poleward migration of the centromeres, results in breakage s- • chromatids with broken ends enter sister telophase nuclei • the cycle can be stopped by addition of telomeric sequences at the breakpoints • in maize, BFB cycle only in endosperm, not in zygote (broken ends healed by telomeric sequences) Barbara McClintock asymmetric translocations Breakage-fusion-bridge cycles (BFB cycles) Chromatide type Replication and fusion Bridge h, n i [ r i: a b L L i j i 1 4 ' A \i chromosome type Dicentric chromosome I Replication Broken ends rejoin Criss-cross segrega tiůn Parallel segregation Intact dicentric Double-bridge brol> stable monocentric chromosomes Breakage-fusion-bridge cycle (BFB cycle) and chromosomal evolution asymmetric reciprocal translocation 1st division n <4I dicentric chromosome 2nd division f ^ A W • BFB cycles can alter chromosome size and shape via random disruption of dicentric chromatids (which result from asymmetric reciprocal translocation) during anaphase. • Such disruptions yield deletion, duplication or inversion through fusion of broken ends after replication and another breakage during the next nuclear division (shown only for the upper product of the first bridge). The cycle might stop by healing of breaks when telomeric sequences become attached.