mutations gene mutations occur within a gene and result in a change in the nucleotide sequence....
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Mutations
• Gene mutations occur within a gene and result in a change in the nucleotide sequence.
• Chromosomal mutations affect all or part of a chromosome.
• Two types of chromosomal mutations:– Structural– Numerical
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The collection of all the chromosomes is referred toas a Karyotype:
Human Karyotype as shown by traditionalchromosome banding.The chromosomes are defined by size, position of the centromere and location of the stained bands.
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Prenatal diagnosis: amniocentesis
• sampling cells from amniotic fluid• usually done ~ 15-18 weeks
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Preparing a karyotype
• harvest cells• culture cells 1-2 days• arrest cells in metaphase with colchicine
• “spread” cells on slide and stain• count chromosomes in 20 representative cells • capture image of 5 “best” cells and construct karyotypes for each
metaphase
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Pair homologous chromosomes and cut out images.
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Completed normal human karyotype
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Abnormal karyotype
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• Nondisjunction occurs when problems with the meiotic spindle cause errors in daughter cells.– This may occur if
tetrad chromosomes do not separate properly during meiosis I.
– Alternatively, sister chromatids may fail to separate during meiosis II.
1. Alterations of chromosome number some genetic disorders
Copyright © 2002 Pearson Education, Inc., publishing as Benjamin Cummings
Fig. 15.11
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MEIOSIS I NONDISJUNCTION
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MEIOSIS II NONDISJUNCTION
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• As a consequence of nondisjunction, some gametes receive two of the same type of chromosome and another gamete receives no copy.
• Offspring results from fertilization of a normal gamete with one after nondisjunction will have an abnormal chromosome number or aneuploidy.
Copyright © 2002 Pearson Education, Inc., publishing as Benjamin Cummings
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Types of aneuploidy– Monosomic cells have only one copy of a
particular chromosome type and have 2n - 1 chromosomes. (Turner’s syndrome: X0)
– Trisomic cells have three copies of a particular chromosome type and have 2n + 1 total chromosomes. (Down Syndrome: 3 chromosome 21’s)
• If the organism survives, aneuploidy typically leads to a distinct phenotype.
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SYNDROMES ASSOCIATED WITH ANEUPLOIDY OF AUTOSOMAL
CHROMOSOMES
Karyotype Syndrome Clinical Features
47,XX, +13 Patau Severe mental retardation andphysical deformities.
47,XX,+18 Edward Severe mental retardation andphysical deformities.
47,XX,+21 Down Mental retardation, flat face, simiancrease.
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• Down syndrome, is due to three copies of chromosome 21.
• It affects one in 700 children born in the United States.• Although chromosome 21 is the smallest human chromosome, it
severely alters an individual’s phenotype in specific ways.
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Down Syndrome Features
•Brushfield spots •Upslanting palpebral fissures •Epicanthic folds•Flat nasal bridge
SinglePalmar (simian)Crease
Short broad hands
5th finger clinodactyly
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Potter’s Atlas of Fetal and Infant Pathology, 1998
Trisomy 18 Phenotype
Very Hypertonic – note contractures
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Trisomy 18 Phenotype
• Characteristic Clenched Hands
• Short sternum
• Cardiac Anomalies
• Single umbilical artery
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Rocker Bottom Feet in +18
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Trisomy 13 (Patau) Syndrome47,XY,+13
• Severe mental
retardation
• Physical
abnormalities
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Trisomy 13 Phenotype
• Craniofacial anomalies– Sloping forehead– Cleft lip and palate– Bulbous nose– Micropthalmia –
very small eyes
• Holoprosencephaly• Microcephaly
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See polydactyly and very malformed ears
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• Nondisjunction of sex chromosomes produces a variety of aneuploid conditions in humans.
• Unlike autosomes, this aneuploidy upsets the genetic balance less severely.– This may be because the Y chromosome
contains relatively few genes.– Also, extra copies of the X chromosome
become inactivated as Barr bodies in somatic cells.
Copyright © 2002 Pearson Education, Inc., publishing as Benjamin Cummings
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• Klinefelter’s syndrome, an XXY male, occurs once in every 2000 live births.– These individuals have male sex organs, but are
sterile.– There may be feminine characteristics– Their intelligence is normal.
• Males with an extra Y chromosome (XYY) tend to somewhat taller than average.
• Trisomy X (XXX), which occurs once in every 2000 live births, produces healthy females.
• Monosomy X or Turner’s syndrome (X0), which occurs once in every 5000 births, produces phenotypic, but immature females.
Copyright © 2002 Pearson Education, Inc., publishing as Benjamin Cummings
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• Jacobs Syndrome-caused by nondisjunction of the Y chromosome during the second phase of meiosis giving a 47 XYY karyotype. Occurence is 1/1000 live male births. Men with this karyotype are tall and have low mental ability.
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• Organisms with more than two complete sets of chromosomes, have undergone polypoidy.
• This may occur when a normal gamete fertilizes another gamete in which there has been nondisjunction of all its chromosomes.– The resulting zygote would be triploid (3n).
• Alternatively, if a 2n zygote failed to divide after replicating its chromosomes, a tetraploid (4n) embryo would result from subsequent successful cycles of mitosis.
Copyright © 2002 Pearson Education, Inc., publishing as Benjamin Cummings
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• Polyploidy has been induced in certain plants that have become major food crops.
• EX. Wheat, Corn, Strawberries.
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Structural Chromosomal Mutations
• Breakage of a chromosome can lead to four types of changes in chromosome structure.– Deletion– Duplication– Inversion– Translocation
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• A deletion occurs when a chromosome fragment lacking a centromere is lost during cell division.– This chromosome will be missing certain
genes.
• A duplication occurs when a fragment becomes attached as an extra segment to a sister chromatid.
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46,XY,der(8)t(8;9)(q24.3;q22.1)
partial trisomy or duplication due to 2:2 segregation
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46,XY,der(9)t(8;9)(q24.3;q22.1)
partial monosomy or deletion
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• An inversion occurs when a chromosomal fragment reattaches to the original chromosome but in the reverse orientation.
• In translocation, a chromosomal fragment joins a nonhomologous chromosome.– Some translocations are reciprocal, others are
not.
Copyright © 2002 Pearson Education, Inc., publishing as Benjamin Cummings
Fig. 15.13c & d
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46,XY,t(8;9)(q24.3;q22.1)
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• Chromosomal translocations between nonhomologous chromosomes are also associated with human disorders.
• Chromosomal translocations have been implicated in certain cancers, including chronic myelogenous leukemia (CML).– CML occurs when a fragment of chromosome
22 switches places with a small fragment from the tip of chromosome 9.
• Some individuals with Down syndrome have the normal number of chromosomes but have all or part of a third chromosome 21 attached to another chromosome by translocation.
Copyright © 2002 Pearson Education, Inc., publishing as Benjamin Cummings
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Chronic myelogenous leukemia
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abl/bcr Genes on Diploid Cells
and Ph Positive CML Cells
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• Not all of a eukaryote cell’s genes are located in the nucleus.
• Extranuclear genes are found on small circles of DNA in mitochondria and chloroplasts.
• These organelles reproduce themselves.• Their cytoplasmic genes do not display
Mendelian inheritance.– They are not distributed to offspring during meiosis.
Extranuclear genes exhibit a non-Mendelian pattern of inheritance
Copyright © 2002 Pearson Education, Inc., publishing as Benjamin Cummings