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Chapter 9

55-211 Chapter Notes - Chapter 9: Robertsonian Translocation, Chromosomal Translocation, Chronic Myelogenous Leukemia

Biological Sciences
Course Code
BIOL 2111

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Chapter 9 Mutation at the Chromosomal Level
At the DNA level, there is a close similarity of nucleotide sequence across hundreds of
thousands of base pairs; but at the chromosomal level, mouse and human karyotypes
bear littler resemblance to each other
These early genomic analyses focused on considerable effort on the sequencing of
regions encompassing more than 2000 kb of mouse and human DNA containing a
complex of genes that encode immune response proteins known as T cell receptors
Nucleotide sequences of T cell receptor genes are similar, in the 2 species, as are
the order of the genes and the relative positions of a variety of noncoding
sequences along the chromosome
Data showed that each mouse chromosome consists of pieces of different human
chromosomes, and vice versa
These blocks represent syntenic segments in which the identity, order, and
transcriptional direction of the genes are almost exactly the same in 2 genomes
2 types of events that reshape genomes
1. Rearrangements, which reorganize the DNA sequences within one or more
2. Changes in chromosome number involving losses or gains of entire
chromosomes or sets of chromosomes
These may affect gene activity or gene transmission by altering the position, order, or
number of genes in a cell
Karyotypes generally remain constant within a species, not because rearrangements and
changes in chromosome number occur infrequently, but because the genetic
instabilities and imbalances produced by such changes usually place individual cells or
organisms and their progeny at a selective disadvantage
Despite selection against chromosomal variations, related species alsmot always have
different karyotypes, with closely related species diverging by only a few
rearrangements and more distantly related species diverging by a larger number of
9.1 Rearrangements of DNA Sequences
Deletions Remove Material From the Genome
Deletions remove one or more contiguous base pairs of DNA from a chromosome
o May arise from errors in replication, from faulty meiotic or mitotic
recombination, and from exposure to X rays or other chromosome damaging
agents that break the DNA backbone
o Use the symbol Del to designate a chromosome that has sustained a deletion
Larger deletions are sometimes identifiable because they affect expression of 2 or more
adjacent genes
Very large deletions are visible at the relatively low resolution of a karyotype, showing
up as the loss of one or more bands form a chromosome
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Lethal Effects of Homozygosity for A Deletion
Because many of the genes in a genome are essential to an individuals survival,
homozygous (Del/Del) or hemizygotes (Del/Y) for most deletion bearing chromosomes
do not survive
o Rare cases where the deleted chromosomal region is devoid of genes essential
for viability, however, a deletion hemi or homozygote may survive
o Drosophila males hemizygous for an 80 kb deletion including the white (w) gene
survive perfectly well in the lab; lacking the w+ allele required for red eye
pigmentation, they have white eyes
Detrimental Effects of Heterozygosity for A Deletion
Usually, the only way an organism can survive a deletion of more than a few genes is if it
carries a nondeleted wildtype homologue of the deleted chromosome
o Known as a deletion heterozygote
Newborn humans heterozygous for a relatively small deletion from the short arm of
chromosome 5 have cri du chat syndrome
Changes in gene dosagethe number of times a given gene is present in the cell
nucleus can create a genetic imbalance
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Imbalance in gene dosage alters the amount of a particular protein relative to all other
proteins and this alteration can have a variety of phenotypic effects
o In drosophila, a single does of the locus known as Triplolethal (Tpl+) is lethal in
other wise diploid individuals
o In drosophila containing only one copy of the wild type notch gene have visible
wing abnormalities but otherwise seem to function normally
Although a single dose of any one gene may not cause substantial harm to the
individual, the genetic imbalance resulting from a single does of many genes at the same
time can be lethal
Humans cannot survive, even as heterozygotes, with deletions that remove more than
about 3 percent of any part of their haploid genome
With only one remaining wild type copy of gene, a cell is more vulnerable to subsequent
mutation of that remaining copy
Thus, individuals born heterozygous for certain deletions have a greatly increased of
losing both copies of certain genes and developing cancer
One case in point is retinoblastoma (RB) the most malignant form of eyes cancer
Effects of Deletion Heterozygosity on Genetic Map Distances
Because recombination between maternal and paternal homologues can occur only at
regions of similarity, map distances derived from genetic recombination frequencies in
deletion heterozygotes will be aberrant
During the pairing of homologues in prophase of meiosis 1, the orphaned region of the
non-deleted chromosome forms a deletion loop an unpaired bulge of the normal
chromosome that corresponds to the area deleted from the other homologue
The progeny of a Del/+ heterozygote will always inherit the markers in a deletion loop as
a unit
o These genes cannot be separated by recombination, and the map distances
between them, as determined by the phenotypic classes in the progeny of a
Del/+ individual, will be zero
o Genetic distanced between loci on either side of the deletion will be short than
expected because fewer crossovers can occur between them
Uoeig Gees i Deletio Heteozgotes
If normal chromosome carries a mutant recessive allele of one of these genes, the
individual will exhibit the mutant phenotype
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