Human Chromosomes

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• Human Chromosomes
• Karyotypes Structures

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Chromosome Stability Centromeres

• Centromeres are important to the separation of
  chromosomes at anaphase during cell division
• Dicentric chromosomes contain two centromeres
  abnormal separation
• Acentric chromosomes contain no centromere and
  are often lost in cell division since they cannot
  attach to spindle fibers

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Chromosome Morphology

• Metacentric Centromere located in middle of
  chromosome
• Submetacentric Cent. located closer to one end
  of chromosome
• Acrocentric Cent. located near one end of
  chromosome

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Chromosome Polyploidy (???)

• Monoploidy basic set of chromosomes that is
  multiplied in a polyploid
• Haploid set of unpaired chromosomes found in
  gametes
• Diploid set of paired homologous chromosomes
  found in most cells total number is fixed for a
  species
• Polyploidy genome composed of multiple complete
  sets of chromosomes occurs in plants

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Polyploidy
Chromosome numbers in Chrysanthemum
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Extra or Missing Chromosomes

• Polysomy extra copies of single chromosomes in
  a cell
• Trisomy an extra copy of a chromosome
• Trivalent abnormal pairing of trisomic
  chromosomes in cell division
• Univalent extra chromosome in trisomy is
  unpaired in cell division

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Polysomy Trisomy
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Human Chromosomes

• Humans have 46 chromosomes, including 22 pairs of
  homologous chromosomes and two sex chromosomes
• Karyotype stained and photographed preparation
  of metaphase chromosomes arranged in homologous
  pairs in descending size order

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A Karyotype of a Normal Human Male
Chromosome painting
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Human Chromosomes

• Chromosome maps are prepared by dividing the
  chromosome into two regions (arms) separated by
  the centromere
• p short arm (petite) q long arm
• p and q arms are divided into numbered bands and
  interband regions based on pattern of staining

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Chromosomal Bands Interbands
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Human X-Chromosome

• Females 2 copies of X chromosome
• One copy of X is randomly inactivated in all
  somatic cells (dosage compensation)
• Females are genetic mosaics for genes on the X
  chromosome, since different X- linked alleles
  will be expressed in different cells (single
  active X principle)
• Barr body inactive X chromosome seen
• during the interphase.

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Abnormal Chromosome Number

• Trisomy of chromosome 21 most common autosomal
  (non-sex chromosome) aneuploidy
• Down Syndrome genetic disorder due to
  chromosome imbalance aneuploidy
• Sex chromosome aneuploidies include XXX (trisomy
  X), XXY (Kleinfelter syndrom), XYY (double Y) and
  X0 (Turner syndrom)

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Down Syndrome Ch. 21 trisomy
15 of all pregnancies end up spontaneous
abortions, which in turn show
chromosomal abnormalities in 50.
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Chromosome Deletions

• Deletions missing chromosome segment
• Large deletions are often lethal
• Polytene chromosomes of Drosophila can be used to
  map physically the locations of deletions
• Testcross mapping shows which wild-type alleles
  are lost by deletion

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Salivary Gland Polytene Nuclei
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Gene Duplications

• Duplication chromosome segment present in
  multiple copies
• Tandem duplications repeated segments are
  adjacent
• Tandem duplications often result from unequal
  crossing-over due to mispairing of homologous
  chromosomes during meiotic recombination

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Red-Green Color Vision Genes

• Green-pigment genes may be present in multiple
  copies on the X-chromosome due to mispairing and
  unequal crossing-over
• Unequal crossing-over between these genes during
  meiotic recombination can also result in gene
  deletion and color-blindness

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Unequal Cross-over of Tandem-repeats
Tandem-repeat cross-over in color blindness
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Chromosome Inversions

• Inversions genetic rearrangements in which the
  order of genes is reversed in a chromosome
  segment
• Inversions do not alter the genetic content but
  change the linear sequence of genetic information
• At synapsis, inversion loops form after
  homologous pairing.

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Chromosome Inversion Inversion Loop
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Reciprocal Translocations

• Reciprocal translocations exchange of genetic
  segments between non-homologous (unrelated)
  chromosomes
• There is no loss of genetic information, but the
  functions of specific genes may be altered
• Reciprocal translocation may affect one or both
  pairs of chromosomes

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Reciprocal Translocation

• Heterozygous translocation only one pair of
  non-homologous chromosomes is affected
• Homozygous translocation both pairs of
  non-homologous chromosomes are affected
• Synapsis involving heterozygous reciprocal
  translocation results in pairing of four pairs of
  sister chromatids quadrivalent
• Chromosome pairs may segregate in several ways
  during meiosis, with varying genetic outcomes in
  gametes

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Robertsonian Translocation

• Robertsonian translocation fusion of two
  acrocentric chromosomes in the centromere region
• Translocation results in apparent loss of one
  chromosome in karyotype analysis
• Genetic information is lost in the tips of the
  translocated acrocentric chromosomes

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Robertsonian Translocation
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Abnormal Chromosomes Cancer

• Cancer cells contain many genetic abnormalities
  including aneuploidies, translocations, deletions
  and duplications of chromosome segments
• Cancer is a clonal expansion of neoplasm
• Philadelphia chromosome first genetic
  abnormality linked with a specific type of
  cancer, chronic myelogenous leukemia (CML)

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Chromosomal Break Points Oncogenes
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Abnormal Chromosomes Cancer

• Philadelphia chromosome result of reciprocal
  translocation between chromosomes 9 and 22
• Retinoblastoma gene tumor suppressor gene
  contains a deletion in an inherited form of
  cancer
• Oncogenes are mutated forms of growth control
  genes