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Unformatted text preview: Chromosome Terminology
chromosome number – generally a constant for a species
haploid – the number of chromosomes found in a gamete (n)
diploid – the number of chromosomes found in a zygote (2n)
chromosome size – relative length is one way to classify
chromosomes within a species
chromosome homologues – members of a chromosome pair, found in
the diploid state
telomere – the termination of the linear chromosome, marked by
long, repetitive sequences
centromere – a constriction along a chromosome with specific
repetitive sequences, which organize its kinetochore.
kinetochore – a protein structure which attaches a chromosome to
the spindle fibers during cellular division
chromosome morphology – another way to classify chromosomes
within a species using specific stains during periods of cellular
metacentric – describing chromosomes with centrally located
acrocentric – describing chromosomes with centromeres towards
telocentric – describing chromosomes with terminally located Cytogenetics Cytogenetics is the study
of cells at the
historically been studied
by microscopic analysis
during mitotic metaphase.
Banding of the chromatin
is revealed using the
Giemsa staining complex.
Images of the
chromosomes are sorted in
a chart to form what is
known as a karyotype. Polyploidies Polyploidy is the state of a nucleus having three or more
sets of the genome.
In organisms that can tolerate polyploidy, even-numbered
ploidies are fertile, and odd-numbered are not.
Tetraploid yeasts, fish, and amphibia have all been
observed in nature and may represent increased potential
Endopolyploidy is polyploidy of a specific tissue within a
diploid organism. The human liver, for example, will contain
tetraploid and octoploid cells.
Polyploidy in plants is common. They are usually more robust and larger than their diploid
Triploid and pentaploid crops produce seedless varieties of fruit
and can be cultivated vegetatively. An alloploid individual contains the genomes of two or more
Autopolyploidy refers to an increase in the number of
chromosome sets within a species. Human Triploidy Individuals with three
copies of each
Triploidy accounts for
17% of all
and 3% of stillbirths.
Causes: Two spermatozoa
fertilize one oocyte.
A haploid spermatozoon
fertilizes an oocyte that
undergoes a complete
chromosomes. Classes of Aneuploidy
abnormality description monosomy one missing chromosome trisomy one extra chromosome deletion portion of a chromosome missing duplication portion of a chromosome present twice inversion portion of a chromosome reversed in
directionof two chromosomes exchanged
portions translocation Autosomal monosomies and trisomies are deleterious due
to imbalance in relative gene dosages.
Duplications are generally less harmful than deletions.
Inversions and translocations are often associated with
position effects. Partial Meiotic Nondisjunctions
in meiosis I Anaphase I Anaphase II Nondisjunction
in meiosis II Gametes Abnormal gametes Abnormal gametes Normal gametes Mosaic Aneuploidy When an aneuploid gamete is fertilized by a
normal gamete, an aneuploid zygote is produced.
Partial nondisjunctions can also occur during
mitosis, resulting in a somatic mutation and
A mosaic can also be produced from a trisomic
zygote if an extra chromosome is lost in mitosis
during development, correcting the aneuploidy
for that cell’s progeny.
Mosaic Down Syndrome, for example, has far
more variability in symptoms than typical Down
Syndrome, depending on what portions of the
body are affected. Autosomal Monosomies and
Trisomies Most autosomal monosomies and
trisomies are inviable.
Trisomies of some of the smaller
autosomes can survive birth but will
manifest syndromic disorders. Trisomy 13 (Patau Syndrome), ~1/20,000 births, less than
20% live past first month. Symptoms include mental
retardation, microcephaly, seizures, and cyclopia.
Trisomy 18 (Edward Syndrome), ~1/8,000 births, less than
50% live past first month. Symptoms include mental
deficiencies, micrognathia, heart and kidney abnormalities.
Trisomy 21 (Down Syndrome), ~1/800 births, over 85% live
past first year. Symptoms include retardation of mental
development, social skills, and physical growth, facial
abnormalities such as flattened nose and epicanthal folds. Alterations in Sex Chromosome
Number Sex chromosomal aberrations are better tolerated
due to the small size of the Y and a natural
mechanism for correcting relative gene dosage
for the X – Lyonization or X-chromosome
inactivation. Triple-X syndrome: Occurring in 1:2000 live births,
these females may be asymptomatic but some may
have tall stature, learning disabilities, delayed motor
coordination, and infertility.
Turner syndrome (XO): Occurring in 1:2000 live births,
these females may show short stature, webbed neck,
Klinefelter syndrome (XXY): Occurring in 1:2000 live
births, these males may be asymptomatic but some may
have tall stature, feminized body shapes, and infertility.
Jacob syndrome (XYY): Occurring in 1:2000 live births,
these males are usually asymptomatic but some may Other Chromosome
Aberrations Misaligned or unequal crossover events lead
to deletions and duplications.
Crossover events between non-homologous
chromosomes leads to translocation.
Crossover events within a chromatid results in
inversions. Evolution by Gene
Genes can be duplicated
by a number of
mechanisms: genome duplication
mobile element expansion
unequal crossing over Unequal crossover results
in tandem gene
Over the course of many
mutations and evolve into
Paralogues are genes
within a species sharing a
common ancestral gene, ...
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This note was uploaded on 04/15/2008 for the course CELL 211 taught by Professor Dotson during the Fall '07 term at Tulane.
- Fall '07