Evolution of eukaryotic genomes Lecture 5 Chromosomal mutations
- Slides: 23
Evolution of eukaryotic genomes Lecture 5 – Chromosomal mutations II
Chromosome mutations Important › › Insight into gene function Insight into meiosis and chromosome architecture Useful tools for genetic manipulation Insight into evolutionary processes Cause abnormalities in cell and organisms function Usually divided into to groups › Changes in structure Rearrangements of existing DNA › Changes in number Numbers of DNA molecules change
Changes in chromosome number Aberrant euploidy › changes in whole chromosome sets Aneuploidy › changes in parts of chromosome sets
Abberant euploidy
Aberrant euploidy Euploid -organisms with basic chromosome set Normal euploid organisms have both diploid (somatic) haploid (gametes) cells Monoploid – only one chromosome set in a usually diploid organism Polyploids -more than two chromosome sets Name Designatioin Constitution No. of chromosomes Monoploid n ABC 3 Diploid 2 n AABBCC 6 Triploid 3 n AAABBBCCC 9 Tetraploid 4 n AAAABBBBCCCC 12 Monosomic 2 n-1 ABBCC 5 AABBC 5 AAABBCC 7 AABBCCC 7 Trisomic 2 n+1
Aberrant euploidy - Monoploids Male bees, wasps and ants are monoploid Males develop by parthenogenesis from unfertilized egg cells Meiosis is abnormal – no pairing partners Monoploids are usually sterile Bees, wasps and ants produce gametes by mitosis
Aberrant euploids - polyploids Correlation between copies of chromosome set and organism size Higher the ploidy level, larger the size Autopolyploids › Multiple sets from within one species Allopolyploids › Sets from two or more different (but closely related) species › Chromosomes are only partly homologous -homeologous
Aberrant euploids - autopolyploids Triploids usually autopolyploids Arise spontaneously Can be constructed by crossing a tetraploid (4 n) and a diploid (2 n) Usually sterile due to pairing problems at meiosis Pairing only takes place between two of the 3 chromosomes of each type Chromosome numbers between haploid and diploid number › Aneuploids › not usually fertile
Aberrant euploidy - autotetraploids Doubling of 2 n to 4 n Spontaneous or chemically induced –colchicine S-phase occurs, but not segregation or division A nuclear membrane forms around the double set of chromosomes Treatment with colchicine for another cell cycle results in octaploids (8 n)
Aberrant euploids – autotetraploids Since 4 is even – meiosis can be normal Depends on pairing Viable gametes are diploid Upon fusion, zygote will again be autotetraploid
Aberrant euploids - allopolyploids Hybrid of two or more species with two or more copies of the input genomes Karpechenko (1928) crossed cabbage (Brassica) with radish (Raphanus) Both species had 18 chromosomes and are closely related Hybrid was functionally sterile since 9 chromosomes did not synapse Part of the hybrid produced seeds which developed into fertile plants with 36 chromosomes Result of spontaneous chromosomal doubling to 2 n 1 + 2 n 2 in one region of the hybrid
Aberrant euploids - allopolyploids Major force in speciation of plants Brassica – 3 species naturally hybridized to form new amphidiploid species
Aberrant euploids - allopolyploids Bread wheat (Triticum aestivum) 6 n=42 Two sets of 3 ancestral genomes Pairing is between homologues from within an ancestral genome Always 21 bivalents
Aberrant euploidy – Polyploidy in animals Reproduce by parthenogenesis Multiple modes of reproduction including a sexual cycle Triploid oysters (sterile) used since do not go through a spawning cycle which makes diploids inedible salamander flatworm leech brine shrimp oyster
Aneuploidy
Aneuploidy Chromosome number is abnormal Chromosome number differs from wild type by part of the chromosome set Can have either more or less chromosomes than wild type Monosomic › 2 n-1 › one copy of one chromosome, rather than normal diploid Trisomic 2 n+1 Nullisomic 2 n-2 Disomic n+1 Refers to autosomes Sex chromosomes – listed as multiple letters › › XXY XYY XXX XO
Nondisjunction During meiosis is the usual cause of aneuploidy Failure of chromosomes or chromatids to segregate properly Produces gametes with abnormal cell numbers Occurs spontaneously due to a failure of cellular processes Usually occurs during meiosis I Bivalents joined by CO Mutagens which dec. CO also inc. nondisjunction
Monosomics (2 n-1) Monosomics for all human autosomes die in utero Sex chromosome monosomy (44 +1 X) gives Turner syndrome Sterile females, short, web of skin between neck and shoulders 1 in 5000 female births
Trisomics (2 n+1) May be viable and fertile Trisomic chromosomes form a group of three during meiosis Klinefelter syndrome › XXY – sterile Down syndrome › Trisomy 21 › Life expectancy 17 yrs Patau syndrome › Trisome 13 › Life expectancy 130 days Edwards syndrome › Trisomy 18 › Life expectancy few weeks
Gene Balance Plants more tolerant of aneuploidy than animals Datura stramonium (jimsonweed) › n = 12 › polyploid (n=24) is larger › aneuploidy of each of the 12 different chromosomes results in a unique change
Gene Balance Aneuploids much more abnomal than polyploids Aneuploidy for different chromosomes has different effects Monosomics usually more affected than trisomics Why? ? ?
Gene balance Euploid ratio of genes on any one chromosome to another chromosome is 1: 1 (100%) Holds true for monoploids, diploids tetraploids etc. But not aneuploids › 50% difference from WT for monosomics › 150% difference from WT for trisomics › Aneuploid genes are out of balance
Gene balance Rate of transcription directly proportional to number of DNA molecules More copies = more transcripts = increased gene dosage Monosomy worse than trisomy since deleterious alleles often expressed (del mutations) X chromosomes naturally aneuploid Drosophila (♀XX ♂XO and XY) › X chromosome of male is hyperactivated Mammals (♀XX ♂XY) › only one X active › Dosage compensation
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- Types of chromosomal mutations
- Robertsonian translocation
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- Deletion chromosomal mutation
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