Structural changes in chromosomes


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Structural changes in chromosomes

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  2. 2. Chromosomal Aberrations• The somatic (2n) and gametic (n) chromosome numbers of aspecies ordinarily remain constant.• This is due to the extremely precise mitotic and meiotic celldivision.• Somatic cells of a diploid species contain two copies of eachchromosome, which are called homologous chromosome.• Their gametes, therefore contain only one copy of eachchromosome, that is they contain one chromosome complement orgenome.• Each chromosome of a genome contains a definite numbers andkinds of genes, which are arranged in a definite sequence.• Sometime due to mutation or spontaneous (without any knowncausal factors), variation in chromosomal number or structure doarise in nature. - Chromosomal aberrations.• Chromosomal aberration may be grouped into two broad classes:1. Structural and 2. Numerical 2
  3. 3. INTRODUCTION:• Chromosome structure variations result fromchromosome breakage.• Broken chromosomes tend to re-join; if there is morethan one break, rejoining occurs at random and notnecessarily with the correct ends.• The result is structural changes in the chromosomes.• Chromosome breakage is caused by X-rays, variouschemicals, and can also occur spontaneously.• There are four common type of structural aberrations:1.Deletion or Deficiency,2.Duplication or Repeat3.Inversion, and4.Translocation 3
  4. 4. DELETION OR DEFICIENCYLoss of a chromosome segment is known as deletionor deficiencyIt can be terminal deletion or interstitial orintercalary deletion.A single break near the end of the chromosomewould be expected to result in terminal deficiency.If two breaks occur, a section may be deleted and anintercalary deficiency created.Terminal deficiencies might seem less complicated.But majority of deficiencies detected are intercalarytype within the chromosome.Deletion was the first structural aberration detectedby Bridges in 1917 from his genetic studies on Xchromosome of Drosophila. 4
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  6. 6. 6Chromosome with deletion can never revert back to a normalcondition and transmitted to next generation.In intercalary deletion, broken acentric fragment ofchromosomes appear as small chromatin bodies in cells knownas Micronuclei.Homozygous deletion is lethal.Heterozygous deletions can revealed a phenomenon known asPseudodominance / One copy of a gene is deleted So the recessive allele on the other chromosome is nowexpressedEg., sex linked lethal and dominant notch-wing etc. inDrosophila.Cri-du-chat (Cat cry syndrome) of babies results from thechromosome deficiency in the short arm of chromosome 5 .
  7. 7. 7•Deletion can be recognized during meiotic pairing ofhomologus chromosome and during somatic pairing inspecialized tissue like salivary gland of Drosophila or duringpachytene in Maize.•Due to intercalary deletion unpaired loop is formed.
  8. 8. DUPLICATIONThe presence of an additional chromosomesegment, as compared to that normally present in anucleus is known as Duplication.In a diploid organism, presence of a chromosomesegment in more than two copies per nucleus iscalled duplication.Four types of duplication:1. Tandem duplication2. Reverse tandem duplication3. Displaced duplication4. Translocation duplication8
  9. 9.  The extra chromosome segmentmay be located immediately afterthe normal segment in preciselythe same orientation forms thetandem . When the gene sequence in theextra segment of a tandem in thereverse order i.e, inverted , it isknown as reverse tandemduplication. In some cases, the extra segmentmay be located either on thesame chromosome or on adifferent one but away from thenormal segment –termed asdisplaced duplication. Later condition is also termed astranslocation duplication. 9
  10. 10. ORIGIN Origin of duplication involveschromosome breakage andreunion of chromosome segmentwith its homologous chromosome. As a result, one of the twohomologous involved in theproduction of a duplication endsup with a deficiency, while theother has a duplication for theconcerned segment. Another phenomenon, known asunequal crossing over, also leadsto exactly the same consequencesfor small chromosome segments. For e.g., duplication of the band16A of X chromosome ofDrosophila produces Bar eye. This duplication is believed tooriginate due to unequal crossingover between the two normal Xchromosomes of female. 10
  11. 11. GENETIC EFFECTS Majority of small duplications have no phenotypic effect However, they provide raw material for evolutionary change Lead to the formation of gene families. A gene family consists of two or more genes that aresimilar to each other derived from a common geneancestor ex- globin gene family whose Genes encodeproteins that bind oxygenTandem duplications play a major role in evolution, becauseit is easy to generate extra copies of the duplicated genesthrough the process of unequal crossing over. These extracopies can then mutate to take on altered roles in the cell, orthey can become pseudogenes, inactive forms of the gene, bymutation. 11
  12. 12. INVERSION• When a segment of chromosome is oriented in the reversedirection, such segment said to be inverted and the phenomenonis termed as inversion.• The existence of inversion was first detected by Strutevant andPlunkett in 1926.• Inversion occur when parts of chromosomes become detached ,turn through 1800 and are reinserted in such a way that the genesare in reversed order.• For example, a certain segment may be broken in two places,and the breaks may be in close proximity because of chance loopin the chromosome.• When they rejoin, the wrong ends may become connected.• The part on one side of the loop connects with broken enddifferent from the one with which it was formerly connected.• This leaves the other two broken ends to become attached.• The part within the loop thus becomes turned around or invert12
  13. 13.  Inversion may be classified into two types: PERICENTRIC - include the centromere PARACENTRIC - do not include the centromere13
  14. 14.  Individuals with one copy of a normal chromosome and one copy of an invertedchromosome Usually phenotypically normal Have a high probability of producing gametes that are abnormal in geneticcontent Abnormality due to crossing-over within the inversion interval During meiosis I, homologous chromosomes synapse with each other For the normal and inversion chromosome to synapse properly, an inversionloop must form If a cross-over occurs within the inversion loop, highly abnormalchromosomes are producedInversions in natural populations In natural populations, pericentric inversions are much less frequent thanparacentric inversions. In many sp, however, pericentric inversions are relatively common, e.g., insome Drosophila. Grasshoppers etc. Paracentric inversions appear to be very frequent in natural populations ofZea maize etc.INVERSION HETEROZYGOTES14
  15. 15. When a paracentric inversion crossesover with a normal chromosome, theresulting chromosomes are an acentric, withno centromeres, and a dicentric, with 2centromeres.The acentric chromosome isnt attached tothe spindle, so it gets lost during celldivision, and the dicentric is usually pulledapart (broken) by the spindle pulling the twocentromeres in opposite directions. Theseconditions are lethal.Eg, Dicentric bridges formed in Maizefemale tissue and pollen grains are sterile. 15
  16. 16. Crossing Over Within Inversion IntervalGenerates Unequal Sets of ChromatidsPARACENTRIC INVERSION16
  17. 17. When a pericentric inversion crosses overwith a normal chromosome, the resultingchromosomes are both duplicated for somegenes and deleted for other genes. (They dohave 1 centromere apiece though). The gametesresulting from these are aneuploid and do notsurvive. Eg, Drosophila.Thus, either kind of inversion has lethalresults when it crosses over with a normalchromosome. The only offspring that surviveare those that didnt have a crossover. Thuswhen you count the offspring you only see thenon-crossovers, so it appears that crossing overhas been suppressed. 17
  18. 18. Crossing Over Within Inversion IntervalGenerates Unequal Sets of ChromatidsPERICENTRIC INVERSION18
  19. 19. Inversions Prevent Generation of RecombinantOffspring Genotypes• Only parental chromosomes (non-recombinants) will produce normal progenyafter fertilizationPARACENTRICPERICENTRIC 19
  20. 20. TRANSLOCATIONIntegration of a chromosome segment into a nonhomologouschromosome is known as translocation.Three types:1. simple translocation2. shift3. reciprocal translocation.1. Simple translocation: In this case, terminal segment of achromosome is integrated at one end of a non-homologousregion. Simple translocations are rather rare.2. Shift: In shift, an intercalary segment of a chromosome isintegrated within a non-homologous chromosome. Suchtranslocations are known in the populations ofDrosophila, Neurospora etc.3. Reciprocal translocation: It is produced when two non-homologous chromosomes exchange segments – i.e., segmentsreciprocally transferred. Translocation of this type is mostcommon eg, Rhoeo, Oenothera, Tradescantia etc. 20
  22. 22.  In reciprocal translocations two non-homologouschromosomes exchange genetic material Usually generate so-called balanced translocations Usually without phenotypic consequences Although can result in position effect.CYTOLOGY OF TRANSLOCATION HETEROZYGOTE In simple translocations the transfer of genetic materialoccurs in only one direction These are also called unbalanced translocations Unbalanced translocations are associated with phenotypicabnormalities or even lethality Example: Familial Down Syndrome In this condition, the majority of chromosome 21 is attached tochromosome 14. 22
  23. 23.  Individuals carrying balanced translocations havea greater risk of producing gametes withunbalanced combinations of chromosomes. This depends on the segregation pattern duringmeiosis I During meiosis I, homologous chromosomessynapse with each other For the translocated chromosome to synapseproperly, a translocation cross must formBalanced Translocations and Gamete ProductionBALANCED LETHALS AND GAMETICCOMPLEXES23
  24. 24.  Meiotic segregation can occur in one of three ways 1. Alternate segregation Chromosomes on opposite sides of the translocation crosssegregate into the same cell Leads to balanced gametes Both contain a complete set of genes and are thus viable 2. Adjacent-1 segregation Adjacent non-homologous chromosomes segregate into thesame cell Leads to unbalanced gametes Both have duplications and deletions and are thus inviable 3. Adjacent-2 segregation Adjacent homologous chromosomes segregate into the samecell Leads to unbalanced gametes Both have duplications and deletions and are thus inviable24
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