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Genetics (B 252) Lecture 4 2015-2016 Phenotypic effects and variations in the genetic material (part 1) "Phenotype" is an organism's actual observed characters, such as morphology, development, or behavior. Changes in the genetic material that affect the phenotype of any eukaryotic organisms may be classified into: I. Chromosomal Aberrations (abnormalities or anomaly) They include the variation at the Chromosomal level. These changes may be as missing, extra, or irregular segment of chromosome. It can also be from an atypical number of chromosomes or a structural abnormality in one (within the chromosome) or more chromosomes (between chromosomes). Chromosome anomalies usually occur when there is an error in cell division following meiosis or mitosis. It is the most dramatic examples of genomic instability. II. Mutation (or point mutation)…..next lecture They include variations at the DNA level. It is a permanent change of the nucleotide sequence within a single gene of the genome of an organism, virus, or extrachromosomal DNA (DNA of plastids and mitochondria) or other genetic elements. Mutations result from damage to DNA which is not repaired either due to errors in the process of replication, or in segments of DNA by mobile genetic elements. Mutations in genes can have either no effect, alter the product of a gene, or prevent the gene from functioning properly or completely. I. Chromosomal Aberrations A. Variation in chromosome number The variation in the number of sets of chromosomes (ploidy) is common in nature. Changes in chromosome number can occur by the addition or loss of all or part of a chromosome (aneuploidy), the loss of an entire set of chromosomes (monoploidy) or the gain of one or more complete sets of chromosomes (euploidy). Most of these conditions is a variation on the normal diploid (2n) number of chromosomes so, cause drastic effects on phenotypic expression. I. Monoploidy: Organisms have a single set of chromosomes i.e one set of chromosomes (n) in the nuclei of their body cells. Meiosis cannot take place normally in the germ cells of the monoploid, because each chromosome lacks a pairing partner and hence they are usually sterile. Most micro-organisms (e.g., bacteria. fungi and algae); gametophytic generation of bryophytes; sporophytic generation of some higher angiospermic plants (e.g., Sorghum, Triticum, Hordeum, Datura, etc.) and certain hymenopteran male insects (e.g., wasps, bees, etc.) are monoploid organisms. Monoploid human somatic cells are lethal. Monoploids plants are often weak, less vigorous and sterile (seedless) eg: those produced from cells in the anthers (pollen), figure below. The reason of plant sterility is that the chromosomes have no regular pairing partners (homologous chromosomes) during meiosis, and meiotic products are deficient in one or more chromosomes. For instance, a haploid in maize (2n=20) will have 10 chromosomes and the number of chromosomes in a gamete can range from 0-10. Consequently, considerable sterility will be found in a monoploid maize. 2 How to create a monoploid plant? In some plant species, monoploids can be artificially derived from the products of meiosis in a plant's anthers (pollen grains, n) by tissue culture. After having been moved to an agar plate medium containing different plant hormones, haploid embryoids (a small dividing mass of cells) will formed and grow to in vitro plantlets. These plantlets will then planted into soil to form mature monoploid plants with roots, stems, leaves, and flowers. They are important in plant breeding for the selection of desired properties as herbicide resistance (figure below). 3 Difference between Haploid and Monoploid? Monoploid refers to the number of chromosomes of the normal cell, meaning that the cell has only one chromosome from every one of the "n" pairs, while Haploid strictly refers to the number of chromosomes in the germ cells (sperms and eggs), which is half of the number of chromosomes normal cells have. Note: (read only) Colchicine is a toxic chemical that is often used to induce polyploidy in somatic cells of plants or microorganisms. Basically, the colchicine prevents the microtubule formation during cell division so, arrest of cell division at prometaphase, thus the chromosomes do not pull apart like they normally do in anaphase. II. Euploidy: Organisms with multiples of the monoploid number of chromosomes are called euploid. Diploids are normal euploidy, so, euploid types that have more than two sets of chromosomes are called polyploid. The polyploid types are named triploid (3n), tetraploid (4n), pentaploid (5n), hexaploid (6n), and so forth (reported previously in Lecture 2). Polyploid is considered aberrations because they differ from the previous normal (monoploid and 4 diploid). Triploids and pentaploid are characteristically sterile because no regular pairing partners during meiosis. For example, triploid arises naturally or constructed by geneticists from the cross of a 4n (tetraploid) and a 2n (diploid). The 2n and the n gametes unite to form a 3n triploid. Many species of plants, microorganisms and animals have clearly arisen through polyploidy, so evidently evolution can take advantage of polyploidy when it arises. Several species are polyploidy: 1. Haloarchaea species: Halobacterium salinarum, Haloferax mediterranei, and Haloferax volcanii. 2. yeasts of the Saccharomyces genus 3. Oomycetes, which are non-true fungi members, such as within the Phytophthora genus 4. Green algae Oedogonium and Cladophora 5. Potato plants 6. Amphibians and Fishes Note: 5 III. Aneuploidy: The abnormal conditions were one or more chromosomes of a set of chromosomes are missing (-) or present (+) in more than their usual number of copies. These variations happen during the formation of gametes. Rarely, at either the 1st or 2nd meiosis, separation (disjunction) of the chromatids of a tetrad does not occur. Instead, both members move to the same pole during anaphase i.e. fail to disjoin. Such event is called non-disjunction. The different forms of aneuploidy are: 1. Hypoploidy: These are when an organism missing one or pair of chromosomes. a. Nullisomics – (2n-2) the loss of a pairs of homologous chromosomes. Nullisomy is a lethal condition in diploids. Some polyploids, however, can lose 2 homologous chromosome of a set and still survive. Eg: Bread wheat, a hexaploid (6n) which behaves meiotically like a diploid and can tolerate nullisomy (due to the presence of multicopy of the lost chromosomes) but 6 exhibit reduced vigor and fertility = Nullosomic hexaploid wheat (6n-2). b. Monosomics – (n-1, 2n-1) Only one copy of a specific chromosome is present instead of the usual two found in its diploid cell. Monosomic chromosome complements are generally deleterious مؤذas the missing chromosome perturbs the overall gene balance in the chromosome set. Nondisjunction is a failure of this disjoining process, and two chromosomes (or chromatids) go to one pole and none to the other. In meiotic nondisjunction, the chromosomes may fail to disjoin at either the first or second division. Either way, n + 1 and n − 1 (monosomic) gametes are produced. If an n − 1 gamete is a monosomic (2 n fertilized by an n gamete, − 1) zygote is produced. When a monosomic of a diploid plant undergoes meiosis, haploid (n) and nullisomic (n-1) gametes are produced. In plant the n-1 gametes rarely function. Eg: Monosomic plants are produced in large number after certain treatment through regeneration by tissue culture every time we need it as it is not transmitted by diploid organism. Eg: In human, Turner syndrome. 7 2. Hyperploidy: These are an organism that gains extra copy of one or pair of chromosomes. a. Disomics (n+1) the gain of an extra copy of a chromosome. A disomic is an aberration of a haploid organism. Eg: In fungi, they can result from meiotic nondisjunction. In the fungus Neurospora (a haploid), an n − 1 meiotic product aborts and does not darken like a normal ascospore; so we may detect MI and MII nondisjunctions by observing asci 8 with 4:4 and 6:2 ratios of normal to aborted spores, respectively, as shown below. In these organisms, the disomic (n + 1) meiotic product becomes a disomic strain directly. The abortion patterns themselves are diagnostic for the presence of disomics in the asci. b. Trisomics – (2n+1) the gain of an extra copy of a chromosome so, the individual will have 3 copies of a certain chromosome. Nondisjunction is a failure of this disjoining process, and two chromosomes (or chromatids) go to one pole and none to the other. In meiotic nondisjunction, the chromosomes may fail to disjoin at either the first or second division. Either way, n + 1 (disomic) and n − 1 (monosomic) gametes are produced (previous figure). If an n + 1 gamete is fertilized by an n gamete, a trisomic (2 n + 1) zygote is produced. 9 Eg: trisomics of the Jimson weed Datura stramonium Eg: In human, Klinefelter, Down, Patu and Edwards syndromes. c. Double Trisomics- (2n+1+1) the gains of 2 different extra copy of chromosome, so 2 different chromosomes are present in triplicate. d. Tetrasomics- (2n+2) the gain of an extra pair of homologous chromosomes, so a chromosome are present in 4 copies. Note: B. Variation in chromosome structure In these cases, the number of chromosomes remains the same but their genetic material becomes modified through the loss, grain or 10 rearrangement of particular sections. Such structural changes are caused by breaks in the chromosome or the chromatid. Each break produces 2 ends which may join either as following: - Remain ununited: lead to loss of chromosomal segment due to absence of centromere. - Immediate reunion: reunion of the same broken ends to reconstruct the original chromosome structure. - Exchange: one or both ends of a particular break may join those produced by different break. Depending upon the number of breaks, their locations, and the pattern in which broken ends join together, a wide variety of structural changes are possible. These variations may be either: I. Variation in arrangement of chromosome segment II. Variation in number of chromosomal segments III. Variation in chromosome morphology I. Variation in arrangement of chromosome segment Translocation and inversion are error formed by crossing-over. i. Translocation It is the transfer of a section in the same chromosome from an arm to the other (non-reciprocal intrachromosomal translocation) or a section from one chromosome to a nonhomologous chromosome (interchromosomal translocation). There are 2 types of interchromosomal translocation: reciprocal translocations (interchanges) and Transposition (non-reciprocal = insertion). 11 a. Non-reciprocal intrachromosomal translocation This type involves the insertion of an interstitial segment produced by two breaks in one chromosomal arm to the single break in the other. b. Reciprocal translocation (interchanges) It is the most common type in which single breaks occur in two on-homologous chromosomes and producing an exchange of chromosome sections between them. Eg: Leukemia 12 c. Transposition (insertion= non-reciprocal) It may involve the attachment of a fragment from one chromosome to another non-homologous chromosome (transposition) or it involve the insertion of an interstitial segment produced by two breaks in one chromosome to the single break in the other. 13 ii. Inversions When 2 breaks occur in a chromosome, the part between them may be reattached in its original place but in a reverse order. There are 2 types: Paracentric and Pericentric. a. Paracentric inversion The inverted reattached segment does not include centromere. 14 b. Pericentric inversion The inverted reattached segment includes centromere. In meiosis alignment, loop is formed II. Variation in number of chromosomal segments There are 2 types: Deletions (Deficiency) and Duplication (Addition) Deletion and duplication are formed due to error of replication. i. Deletions (Deficiency) It means the loss of a portion of the original chromosome, these may occur either terminal or interstitial. A loss of any considerable portion of a chromosome is usually lethal to a diploid organism due to the genetic unbalance. 15 a. Terminal: A part at the end of the chromosome is lost as a result of one break. Eg: cri du chat syndrom b. Interstitial (intercalary): A chromosome break in two regions and the middle segment is lost; these may originate both a ring chromosome and acentric fragment. ii. Duplication (Addition) It happens when a segment of the chromosome is represented 2 or more times due to an unequal crossing-over between homologous chromosomes during meiosis. There are 3 types: Tandem Reverse tandem and terminal tandem. a. Tandem: Segment is repeated 2 or more times with the same arrangement as original. 16 b. Reverse tandem: Segment is repeated 2 or more times with reverse position. c. Terminal tandem: Segment is repeated 2 or more times at the end of the chromosome. Note: when join with its sister chromatid → unbalance III. Variation in chromosome morphology There are 2 types: Isochromosomes and Shifts i. Isochromosomes: 17 It rises from a break or a missdivision at the centromere. The 2 resultant telocentric chromosomes produce chromosomes with 2 identical arms. ii. Shifts: It is formed when 3 breaks occur in same chromosome and the resulted segment from the 2 break is inserted at the third single break of the same chromosome. Eg: seen in Neurospora A B C D E F G H I J K Produces either: A B C F G D E H I J K shift segment or A B C G F D E H I J K shift segment & reverse the other Importance of chromosome aberration: - They can be associated with species differences. - They have played a very important part as indicators of genetic damage in both clinical and cancer studies. 18 So, they are useful to medical and graduate students, physicians, molecular biologists, and cytogeneticists Animations: Polyploidy https://www.youtube.com/watch?v=DieKrGX90Pc Non-disjunction http://www.uic.edu/classes/bios/bios100/lectures/nondisjunction.htm http://www.sumanasinc.com/webcontent/animations/content/mistakesmei osis/mistakesmeiosis.html Translocation https://www.youtube.com/watch?v=eUZYACO236c Inversion https://www.youtube.com/watch?v=ZcnyMMHLkAw Chromosome abnormalities https://www.youtube.com/watch?v=FgMKGIED4Yo https://www.youtube.com/watch?v=WNboyqs67p4 Reference: 1. An Introduction to Genetic Analysis (2000), 7th edition, W. H. Freeman, New York. 2. Chromosome Engineering in Plants: Genetics, Breeding, Evolution, (1991), P.K. Gupta and T. Tsuchiya, 656 pp.,Elsevier. 19