Structural changes in chromosomes

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2. Chromosomal Aberrations The somatic (2n) and gametic (n) chromosome numbers of a species ordinarily remain constant. This is due to the extremely precise mitotic and meiotic cell division. Somatic cells of a diploid species contain two copies of each chromosome, which are called homologous chromosome. Their gametes, therefore contain only one copy of each chromosome, that is they contain one chromosome complement or genome. Each chromosome of a genome contains a definite numbers and kinds of genes, which are arranged in a definite sequence. Sometime due to mutation or spontaneous (without any known causal factors), variation in chromosomal number or structure do arise in nature. - Chromosomal aberrations . Chromosomal aberration may be grouped into two broad classes: 1. Structural and 2. Numerical 2

3. INTRODUCTION: Chromosome structure variations result from chromosome breakage. Broken chromosomes tend to re-join; if there is more than one break, rejoining occurs at random and not necessarily with the correct ends. The result is structural changes in the chromosomes. Chromosome breakage is caused by X-rays, various chemicals , and can also occur spontaneously. There are four common type of structural aberrations: 1.Deletion or Deficiency, 2.Duplication or Repeat 3.Inversion, and 4.Translocation 3

4. DELETION OR DEFICIENCY Loss of a chromosome segment is known as deletion or deficiency It can be terminal deletion or interstitial or intercalary deletion. A single break near the end of the chromosome would be expected to result in terminal deficiency. If two breaks occur, a section may be deleted and an intercalary deficiency created. Terminal deficiencies might seem less complicated. But majority of deficiencies detected are intercalary type within the chromosome. Deletion was the first structural aberration detected by Bridges in 1917 from his genetic studies on X chromosome of Drosophila. 4

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6. 6 Chromosome with deletion can never revert back to a normal condition and transmitted to next generation. In intercalary deletion, broken acentric fragment of chromosomes appear as small chromatin bodies in cells known as Micronuclei. Homo zygous deletion is lethal. Heterozygous deletions can revealed a phenomenon known as Pseudodominance / One copy of a gene is deleted So the recessive allele on the other chromosome is now expressed Eg ., sex linked lethal and dominant notch-wing etc. in Drosophila. Cri-du-chat (Cat cry syndrome) of babies results from the chromosome deficiency in the short arm of chromosome 5 .

7. 7 Deletion can be recognized during meiotic pairing of homologus chromosome and during somatic pairing in specialized tissue like salivary gland of Drosophila or during pachytene in Maize. Due to intercalary deletion unpaired loop is formed.

8. DUPLICATION The presence of an additional chromosome segment, as compared to that normally present in a nucleus is known as Duplication. In a diploid organism, presence of a chromosome segment in more than two copies per nucleus is called duplication. Four types of duplication: 1. Tandem duplication 2. Reverse tandem duplication 3. Displaced duplication 4. Translocation duplication 8

9. The extra chromosome segment may be located immediately after the normal segment in precisely the same orientation forms the tandem . When the gene sequence in the extra segment of a tandem in the reverse order i.e , inverted , it is known as reverse tandem duplication. In some cases, the extra segment may be located either on the same chromosome or on a different one but away from the normal segment –termed as displaced duplication. Later condition is also termed as translocation duplication . 9

10. ORIGIN Origin of duplication involves chromosome breakage and reunion of chromosome segment with its homologous chromosome. As a result, one of the two homologous involved in the production of a duplication ends up with a deficiency , while the other has a duplication for the concerned segment. Another phenomenon, known as unequal crossing over , also leads to exactly the same consequences for small chromosome segments. For e.g., duplication of the band 16A of X chromosome of Drosophila produces Bar eye. This duplication is believed to originate due to unequal crossing over between the two normal X chromosomes of female. 10

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 are similar to each other derived from a common gene ancestor ex- globin gene family whose Genes encode proteins that bind oxygen Tandem duplications play a major role in evolution, because it is easy to generate extra copies of the duplicated genes through the process of unequal crossing over . These extra copies can then mutate to take on altered roles in the cell, or they can become pseudogenes , inactive forms of the gene, by mutation. 11

12. INVERSION When a segment of chromosome is oriented in the reverse direction, such segment said to be inverted and the phenomenon is termed as inversion. The existence of inversion was first detected by Strutevant and Plunkett in 1926. Inversion occur when parts of chromosomes become detached , turn through 180 and are reinserted in such a way that the genes are 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 loop in the chromosome. When they rejoin, the wrong ends may become connected. The part on one side of the loop connects with broken end different 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 invert 12

13. Inversion may be classified into two types: PERICENTRIC - include the centromere PARACENTRIC - do not include the centromere 13

14. Individuals with one copy of a normal chromosome and one copy of an inverted chromosome Usually phenotypically normal Have a high probability of producing gametes that are abnormal in genetic content 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 inversion loop must form If a cross-over occurs within the inversion loop, highly abnormal chromosomes are produced Inversions in natural populations In natural populations, pericentric inversions are much less frequent than paracentric inversions. In many sp, however, pericentric inversions are relatively common, e.g., in some Drosophila . Grasshoppers etc. Paracentric inversions appear to be very frequent in natural populations of Zea maize etc. INVERSION HETEROZYGOTES 14

15. When a paracentric inversion crosses over with a normal chromosome, the resulting chromosomes are an acentric, with no centromeres, and a dicentric, with 2 centromeres. The acentric chromosome isn't attached to the spindle, so it gets lost during cell division, and the dicentric is usually pulled apart (broken) by the spindle pulling the two centromeres in opposite directions. These conditions are lethal. Eg , Dicentric bridges formed in Maize female tissue and pollen grains are sterile. 15

16. Crossing Over Within Inversion Interval Generates Unequal Sets of Chromatids PARACENTRIC INVERSION 16

17. When a pericentric inversion crosses over with a normal chromosome, the resulting chromosomes are both duplicated for some genes and deleted for other genes. (They do have 1 centromere apiece though). The gametes resulting from these are aneuploid and do not survive. Eg , Drosophila . Thus, either kind of inversion has lethal results when it crosses over with a normal chromosome. The only offspring that survive are those that didn't have a crossover. Thus when you count the offspring you only see the non-crossovers, so it appears that crossing over has been suppressed. 17

18. Crossing Over Within Inversion Interval Generates Unequal Sets of Chromatids PERICENTRIC INVERSION 18

19. Inversions Prevent Generation of Recombinant Offspring Genotypes Only parental chromosomes (non-recombinants) will produce normal progeny after fertilization PARACENTRIC PERICENTRIC 19

20. TRANSLOCATION Integration of a chromosome segment into a nonhomologous chromosome is known as translocation. Three types: 1. simple translocation 2. shift 3. reciprocal translocation. Simple translocation : In this case, terminal segment of a chromosome is integrated at one end of a non-homologous region. Simple translocations are rather rare. Shift: In shift, an intercalary segment of a chromosome is integrated within a non-homologous chromosome. Such translocations are known in the populations of Drosophila , Neurospora etc. Reciprocal translocation : It is produced when two non-homologous chromosomes exchange segments – i.e., segments reciprocally transferred. Translocation of this type is most common eg , Rhoeo , Oenothera , Tradescantia etc. 20

21. 21 SHIFT RECIPROCAL

22. In reciprocal translocations two non-homologous chromosomes 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 material occurs in only one direction These are also called unbalanced translocations Unbalanced translocations are associated with phenotypic abnormalities or even lethality Example: Familial Down Syndrome In this condition, the majority of chromosome 21 is attached to chromosome 14. 22

23. Individuals carrying balanced translocations have a greater risk of producing gametes with unbalanced combinations of chromosomes. This depends on the segregation pattern during meiosis I During meiosis I, homologous chromosomes synapse with each other For the translocated chromosome to synapse properly, a translocation cross must form Balanced Translocations and Gamete Production BALANCED LETHALS AND GAMETIC COMPLEXES 23

24. Meiotic segregation can occur in one of three ways 1. Alternate segregation Chromosomes on opposite sides of the translocation cross segregate 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 the same cell Leads to unbalanced gametes Both have duplications and deletions and are thus inviable 3. Adjacent-2 segregation Adjacent homologous chromosomes segregate into the same cell Leads to unbalanced gametes Both have duplications and deletions and are thus inviable 24

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