Transcript Chromosomes_posted

Cytogenetics: Chromosome
Mutations, Aberrations &
Evolution
Chromosomes
Prokaryote
Eukaryote
Eukaryotic chromosomes
Human karyotype
Why do we care?
Many diseases and birth defects are a
direct result of missing, broken, or extra
chromosomes.
• Down Syndrome
• Cri du chat Syndrome
• Patau Syndrome
Mutations at the level of the
homologous pair
• EUPLOIDY: "true" ploidy, meaning two
members of each homologous pair.
• ANEUPLOIDY: "not true" ploidy, meaning
more or fewer members than two of each
homologous pair.
– MONOSOMY - one homolog; partner is missing
– TRISOMY - three homologs
– NULLISOMY- one entire homologous pair is
missing.
Monosomy and Trisomy
Down Syndrome
How does it happen? Nondisjunction
Each chrom.
has two
chromatids
Trisomy: Patau Syndrome
• 1/20,000 births
• severe mental
retardation
• heart and organ
defects
• polydactyly
• death by the
age of one year
Chromosomal Abnormalities Occurring in Human Fetuses
% spontaneously aborted
% fetuses with the
fetuses with the
abnormality that
Type of Abnormality
abnormality
survive to term
All abnormalities
50
5
Autosomal Trisomies
16
7.5
0
13, 18, & 21
4.5
15
All others
13.8
0
Trisomies of Sex c hromosomes
XXX, XXY, XYY
0.3
75
Monosomy for X (XO)
8.7
1
Structural A bnormalities
20
45
Structural Changes
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•
•
•
Deletions (deficiencies)
Duplications
Inversions
Translocations
Deletions (deficiencies)
w
How can chromosomes break?
Ionizing radiation (production of free radicals,
which act like little atomic "cannon balls",
blasting through strands of DNA or c'somes.
Chemical insult.
Why do they rejoin?
Break points of chromosomes are highly reactive
("sticky"), whereas normal ends of c'somes are
capped by telomeres, which do not readily bond
to other molecules.
• Breaks that occur ______
__________________
__________________
will affect both newly
formed chromatids, & all
daughter cells arising
from them.
• Breaks that occur ______
____________________
____________________
may affect only one
chromatid. (Thereafter,
only the progeny carrying
the broken chromatid will
be affected.)
Cri-du-chat Syndrome
_________________
_________________
Mental retardation
Slow motor skill
development
Low birth weight and slow
growth
Small head (microcephaly)
Partial webbing of fingers or
toes
Wide-set eyes
(hypertelorism)
High-pitched cry
Structural Changes
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•
Deletions (deficiencies)
Duplications
Inversions
Translocations
Duplication
Duplications
Bar eye: caused by duplication
Duplications: source of
evolutionary novelty?
Duplication is a source of new genes over
evolutionary time: e.g., gene families like globins
and MHC genes
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______________
Structural Changes
•
•
•
•
Deletions (deficiencies)
Duplications
Inversions
Translocations
Inversions