Dr Snehal Moghe Assistant Profeesor Department of Botany
Dr Snehal Moghe Assistant Profeesor Department of Botany Bilasa Girls Govt PG College
Chromosomal Aberrations
Types : v Structural Aberrations – Change in structure of chromosomes v Numerical Aberrations – Change in number of chromosomes
Structural Aberrations Deletion Duplication Pericentric Inversion Paracentric Translocation
Duplication: Duplications are mutations at the DNA level by which new copies of previously existing genes or regions are made. Involves the production of extra copies of parts of the chromosome. Duplications may affect phenotype by altering gene dosage.
The evolutionary significance of gene duplication was first recognized by Haldane (1932) and Muller (1935). They suggested that a redundant copy of a gene may acquire divergent substitutions and eventually emerge as a new gene. Drosophila Bar eyes Percentage of genes that belong to identifiable families of duplicates in Saccharomyces is 30%, in C. elegans is 48%, in Arabidopsis is 60%, in Drosophila is 40% and in humans is 38% of the genome.
Deletion : Involves the loss of part of a chromosome. TWO TYPES OF DELETIONS: • End missing (terminal) • Piece within chromosome missing (interstitial)
Deletions involve the loss of DNA sequences. Phenotypic effects of deletions depend on the size and location of deleted sequences on the genome Deletions that span a centromere result in an acentric chromosome that will most likely be lost during cell division. Deletions can also affect gene dosage and thus the resulting phenotype. The larger the deletion, the more genes are likely to be involved, and the more drastic the resulting defect is likely to be. Diseases caused by deletions have phenotypes, which are more severe than those caused by duplications. Cytogenetically visible deletions occur in 1 in approximately every 7, 000 live births (Jacobs et al. , 1992).
Translocation: When one part of a nonhomologous chromosome breaks off and attaches to another chromosome. q Translocations can be either balanced or unbalanced depending if genetic material is lost. q A translocation in balanced form results in no excess or deficit of genetic material and causes no health difficulties. q In unbalanced forms, translocations cause chromosomal deletions or duplication and result in syndromes of multiple malformations. Reciprocal translocation Example: t(8; 14) (q 24; q 32) – Burkitt’s Lymphoma t(11; 14) (q 13; q 32) – Mantle cell Lymphoma First plant with spontaneous translocation – Oenothera
Numerical Aberrations Euploidy Aneuploidy Monoploidy (n) Hyperploidy Hypoploidy Diploidy (2 n) Polyploidy (>2 n) Auto Allo Trisomy (2 n+1) Tetrasomy (2 n+2) Monosomy (2 n-1) Nullisomy (2 n-2)
Aneuploidy Hyperploidy Trisomy (2 n+1) Balanced Nullisomic Tetrasomy (2 n-2+2) Hypoploidy Monosomy (2 n-1) Double trisomy (2 n+1+1) Double monosomy (2 n-1 -1) Tetrasomy (2 n+2) Nullisomy (2 n-2)
Trisomy in Datura stramonium n- 12 Primary trisomy Secondary trisomy Tertiary trisomy Wheat (n-21) All Nullisomy, tetrasomy And Nullisomic-tetrasomy Are made
Euploidy (true multiples) § § An organism (diploid or polyploid) with exact multiples of a basic chromosome number Polyploids are not common in animal systems These are very common in plants, especially in angiosperms. From 30% to 70% of today's angiosperms are thought to be polyploid. Origin of Polyploidy The process can begin if diploid (2 n) gametes are formed. These can arise in at least two ways. q q The gametes may be formed by mitosis instead of meiosis. Plants, in contrast to animals, form germ cells (sperm and eggs) from somatic tissues. If the chromosome content of a precursor somatic cell has accidentally doubled (e. g. , as a result of passing through S phase of the cell cycle without following up with mitosis and cytokinesis), then gametes containing 2 n chromosomes are formed.
Examples Polyploid types are labeled according to the number of chromosome sets in the nucleus: triploid (three sets; 3 x), for example seedless watermelons, Banana, Apple, Ginger tetraploid (four sets; 4 x), for example the cotton, Apple, Durum wheat, Potato, Tobacco, Peanut pentaploid (five sets; 5 x), for example Kenai Birch hexaploid (six sets; 6 x), for example wheat, kiwifruit octaploid (eight sets; 8 x), for example dahlias, Strawberries, Sugercane decaploid (ten sets; 10 x), for example certain strawberries
Autopolyploid: § A cell or individual whose several chromosome sets, three or more, are all homologous. § These sets arise within a species via a process of genome multiplication. § Autopolyploids can be fully fertile (potato (2 n = 4 x = 48), alfalfa (2 n = 4 x = 32) § Colchicine is a chemical used to “double” chromosome numbers. It interferes with spindle fiber formation Allopolyploid: § A cell or individual with genetically different chromosome sets derived from two or more species. § Allopolyploids arise through interspecific hybridization and spontaneous chromosome doubling. § Allopolyploids can also be created by crossing two different species and doubling the chromosome number of the F 1.
Origin of cultivated Banana Similarly seedless watermelon is also developed.
ADVANTAGE OF POLYPLOIDY • Enlargement and increased vigour • Creation of sterile triploids • Restoring fertility in hybrids • Enhancing pest resistance and stress tolerance. • There are many advantages of polyploidy that offer great insights and benefits if these secrets can be unlocked from the plants’ DNA.
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