Transcript Chromosomes - TeacherWeb

Chromosomes
• Cytogenetics: study which
links chromosome
variations to specific traits
• Chromosome:
 Dark heterochromatin (repetitive
DNA)
 Light euchromatin (proteinencoding genes)
 Telomeres: tips (repeats of
TTAGGG)
 Centromere: constriction that
attaches to spindle
 Repeats of a 171 base DNA sequence
(alpha satellite)
 Replicated during the end of S phase
• Karyotype: a chromosome chart
(22 pairs of autosomes/1 pair of
sex chromosomes)
• Chromosome Studies:
 Amniocentesis: removal of small
sample of fetal cells and fluid from the
uterus
 Between 14-16 weeks gestation (Usually
pregnant women over 35)
 Can detect at least 800 of 5,000
biochemical problems
 Most common abnormality detected –
trisomy
 Chorionic Villus Sampling: cells
from chorionic villi (develop into
placenta
 Earlier results (10-12 weeks)
 Less accurate than amniocentesis
 Disadvantage: spontaneous abortion
 Fetal Cell Sorting: separates fetal cells from the mother’s bloodstream
uses fluorescence-activated sorter
 Safer, newer, experimental
• Ideogram: graphical representation of a specific
chromosome
 Bands appear as stripes
 FISH highlights (“paints”) individual genes (Fluorescence
In situ Hybridization)
• Abnormal chromosome numbers account for
at least 50% of spontaneous abortion
 Polyploidy: extra sets of chromosomes
 17% of spontaneous abortions
 Common among flowering plants; Colchicine (alkaloid from autumn
crocus) interferes with spindle formation
 Seedless watermelon, Winesap apples
 Aneuploidy: + or – a single chromosome (due
to nondisjunction)
Many spontaneous abortions; mental
retardation
 Monosomy: 1 less than the normal diploid number
(2N-1 = 45)
 Turner Syndrome: (45; X_) Short, sterile female; 1 in
2,000 female births; Sexually underdeveloped; normal
intelligence; skin flaps on back of neck; broad shield-like
chest
 Trisomy: 1 more than the normal diploid number
(2N+1 = 47)
 Down Syndrome: 1 infant in every 800 live births; Extra
#21; Epicanthic folds in eyelids; flat face, poor muscle
tone; short fingers; short stature; suppressed immunity;
developmental problems; mentally challenged; life span of
50 years; ovum as the source of the problem in 90% of the
cases; older adults with Alzheimer Disease; decreased risk
of cancer (extra copy of DSCRI gene suppresses
angiogenesis
 Klinefelter Syndrome: (XXY) Sterile male underdeveloped sexually (prostate,
testes); breast development (gynecomastia); 1 in 660 male births; long limbs with
large hands and feet; decreased intelligence
 Jacobs Syndrome (XYY) Increased height; acne; speech and reading problems
 Triplo-X (XXX) Taller female with menstrual irregularities; 1 in 1,000 female births;
sterile; delayed language development and motor skills
Abnormal Chromosome Structures
• Deletion: missing genes
 Cri-du-chat Syndrome: deletion of part
of #5; high pitched cry; mentally
challenged; pinched features;
gastrointestinal and cardiac
complications; abnormal development of
larynx and glottis
• Duplication: region of
chromosomes where genes are
repeated
• Translocation: pieces break off
and connect to a different
chromosome
spontaneous
abortions and birth defects
• Inversion: part of chromosome
has flipped around
• Fragile Sites in Human Chromosomes
 Susceptible to chromosome breakage
 Fragile X Syndrome (Mortin-Bell Syndrome)
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Most common mentally challenged inherited condition
Long narrow face with protruding chin, enlarged ears
Increased testicular size (gynecomastia)
Gene = FMR1 (sequence of 3 nucleotides repeated many times)
Link between an autosomal fragile site and lung cancer
Specialized Chromosomes
• Polytene Chromosomes: salivary, rectal, midgut
 Can be seen in interphase
 Individual bands called chromomeres
 Undergo uncoiling (puff) in Drosophila
• Lampbrush Chromosomes: meiotic
chromosomes
 In vertebrate oocytes and insect spermatocytes