barcode reader using vb net source code VA R I A T I O N I N CHROMOSOMAL STRUCTURE in Software

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VA R I A T I O N I N CHROMOSOMAL STRUCTURE
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In general, chromosomes can break due to ionizing radiation, physical stress, or chemical compounds. When a break occurs in the chromosome before DNA replication, during the S phase of the cell cycle (see g. 3.6), the break itself is replicated. After the S phase, any breaks that occur affect single chromatids. Every break in a chromatid produces two ends.These ends have been described as sticky, meaning simply that enzymatic processes of the cell tend to reunite them. Broken ends do not attach to the undamaged terminal ends of other chromosomes.(Normal chromosomal ends are capped with structures called telomeres see chapter 15.) If broken ends are not brought together, they can remain broken. But, if broken chromatid ends are brought into apposition, they may rejoin in any of several ways. First, the two broken ends of a single chromatid can reunite. Second, the broken end of one chromatid can fuse with the broken end of another chromatid, resulting in an exchange of chromosomal material and a new combination of alleles. Multiple breaks can lead to a variety of alternative recombinations. These chromosomal aberrations have major genetic, evolutionary, and medical consequences. The types of breaks and reunions discussed in this chapter can be summarized as follows: I. Noncentromeric breaks A. Single breaks 1. Restitution 2. Deletion 3. Dicentric bridge B. Two breaks (same chromosome) 1. Deletion 2. Inversion C. Two breaks (nonhomologous chromosomes)
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Tamarin: Principles of Genetics, Seventh Edition
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II. Mendelism and the Chromosomal Theory
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8. Cytogenetics
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Variation in Chromosomal Structure
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Centric fragments
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Acentric fragments
having this chromosome and a normal homologue will have, during meiosis, a bulge in the tetrad if the deleted section is large enough ( g. 8.4). The bulge also appears in the paired, polytene giant salivary gland chromosomes of Drosophila. (Note that when a bulge like that illustrated in gure 8.4 is seen in paired chromosomes, it indicates that one chromosome has a piece that is missing in the other. In our example, the bulge resulted from a deletion in one chromosome; it could also result from an insertion of a piece in the other chromosome.)
Inversion
Dicentric chromosome
e f e f
Acentric fragment
Figure 8.1 Chromosomal break with subsequent reunion to form a dicentric chromosome and an acentric fragment.
Two Breaks in the Same Chromosome
Deletion
Figure 8.3 shows two of the possible results when two breaks occur in the same chromosome. One alternative is a reunion that omits an acentric fragment, which is then lost. The centric piece, missing the acentric fragment (e-f-g in g. 8.3), is a deletion chromosome. An organism
a b c
d e f g h h
Break
a b c
d e f g h h g f e
gh duplication
gh deletion
Figure 8.2 Breakage of a dicentric bridge causes duplications and additional de ciencies.
Two breaks in the same chromosome can also lead to inversion, in which the middle section is reattached but in the inverted con guration (see g. 8.3). An inversion has several interesting properties. To begin with, fruit ies homozygous for an inversion show new linkage relations when their chromosomes are mapped. One outcome of this new linkage arrangement is the possibility of a position effect, a change in the expression of a gene due to a changed linkage arrangement. Position effects are either stable, as in Bar eye of Drosophila (to be discussed), or variegated, as with Drosophila eye color. A normal female y that is heterozygous (XwX ) has red eyes. If, however, the white locus is moved through an inversion so that it comes to lie next to heterochromatin ( g. 8.5), the y shows a variegation patches of the eye are white. This is presumably caused by a spread of the tight coiling of the heterochromatin, turning off the expression of the locus. In a heterozygote, if the turned-off allele is the wild-type, the cell will express the normally recessive white-eye allele. Depending on what happens in each cell, patches of red and white eye color result. When synapsis occurs in an inversion heterozygote, either at meiosis or in the Drosophila salivary gland during endomitosis, a loop often forms to accommodate the point-for-point pairing process ( gs. 8.6 and 8.7). An outcome of this looping tendency is crossover suppression. That is, an inversion heterozygote shows very little recombination of alleles within the inverted region. The reason is usually not that crossing over is actually
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