Chapter 7 Flashcards
Functional chromosomes have three parts
Centromere - attachment point for spindle fibers via kinetochores
A pair of Telomeres - Protect ends of the chromosome
Origins of Replication - Sites where DNA synthesis begins
Cytogenetics
The study of chromosome number and structure
Most cytological studies are done on what type of cells
dividing cells
What type of cells are used for human studies?
White blood cells
Why is WBC good to use in human studies?
They are arrested in mitosis by using chemicals that inhibit formation of the mitotic spindle (colchicine)
When they are put in a hypotonic solution the cells swell
They then can be spread on a microscope slide
What can chromosomes do so that they can be distinguished by?
They can be stained and each chromosome has a characteristic banding pattern with a particular stain
Chromosomes with Q banding are stained with?
Quinacrine - It produces a striped or banded appearance
Q banding
Bands appear under UV light
Highly reproducible
Individual chromosomes can be identified so structural abnormalities can be seen
What is Geimsa?
A non-fluorescent dye, that also produces bands
G-banding
produces bands that correspond to the bands of Q-banding
Stains areas rich in A-T pairs
Which banding stains the region around each centromere?
C-banding
R-banding
Produces bands corresponding to the light bands in Q-banding
Process of R-banding
Cells are heated in a phosphate buffer prior to Geisma staining
The cells are then stained with acridine orange
It produces the reverse of G-banding
Types of Chromosome mutations
Alterations in the structure of chromosomes
Aneuploidy
Polyploidy
What is aneuploidy?
The number of chromosomes are altered.
One or more individual chromosomes is deleted or added.
There is a genetic imbalance.
What is polyploidy?
One or more entire sets of chromosomes is added.
No genetic imbalance
What is euploidy?
The normal number of sets of chromosomes.
In humans that is diploid.
Polyploidy seems to confer…..
larger size but normal proportions.
Aneuploidy can do what to an organism..
It can alter its shape and proportions in characteristic ways
Gene dosage effect
The relationship between the number of copies of a gene and the amount of gene product made.
Chromosome Rearrangements
Duplication, Deletion, Inversion, Transocation
Tandem Duplication
The duplicated segment is immediately adjacent to the original segment
Displaced Duplication
The duplicated segment is located at a distance from the original on the same chromosome or on another
Reverse Duplication
The duplication is inverted
When can duplication cause problems?
With pairing in meiosis in the heterozygote
With a duplicate gene,they are how many times the normal gene product?
1.5
Duplication causes what type of mutations in drosophila?
Bar eye mutations
Why is gene duplications common in evolution?
It provides a significant source of variation for evolution.
Mammalian hemoglobin genes arose by tandem duplication
If a deletion occurs in a heterozygote what must happen?
In meiosis, one side of the chromosome must loop out
What happens if the centromere is deleted?
The chromosome will not segregate in mitosis or meiosis and will be lost.
What happens in homozygous deletions?
Lethal
what happens in heterozygous deletions?
may produce gene dosage imbalance.
Pseudo dominance - recessive phenotypes may be uncovered if the dominant allele is lost.
Haploinsufficient - A single copy of the gene does not produce enough product to produce the wild-type phenotype.
A syndrome caused by deletion
Cri-du-chat: Loss of segment on the short arm of one copy of chromosome 5 ( 46 (5p) )
Wolf - Hirschhorn syndrome (midline closure defect)
Deletion.
Loss of segment on the short arm of one copy of chromosome 4 (46 (4p) ).
Williams - Beuren syndrome
Loss of a small segment of chromosome 7- long arm (26 genes).
Elfin features
Inversion
A chromosomal segment is turned 180
Paracentric
Inversion that do not include the centromere. Does not change the size of the arms
Pericentric
Inversions that do include the centromere. May change the size of the arms.
What difficulties is produced for heterozygotes for inversion during meiosis.
This pairing causes stress in the chromosome.
The pairs may come apart inappropriately.
Depends on where the cut occurs.
If inversion occurs outside the areas of a gene…
Usually do not lead to phenotypic abnormalities.
Can be bred to homozygosity.
Inversion occurs within the area of a gene…
May produce a phenotypically detectable change, and may produce a new gene function.
May be a lethal gene, and cannot be bred to homozygosity
Why does inversion take place?
Irradiation, or naturally by the action of transposable elements, or may be a result of mechanical sheer
What is Position Effect?
When inversion takes place and changes the order of genes and may cause them to be expressed at inappropriate times or in the wrong tissue
Crossovers within inversions produce abnormal chromatids (from prophase 1 of meiosis)
2 normal chromatids, one with 2 centromere, and one with no centromere (4 total).
In Anaphase 1 what happens….
the centromeres separate stretching the dicentric chromatid, which breaks it. The chromosome with no centromere is lost.
Results after dicentric chromatid breaks
Two wild-type non recombinant chromosomes and two recombinant chromosomes that are missing some genes and will not produce viable offspring.
Dicentric and acentric chromosomes
Neither will segregate properly in meiosis
Dicentric bridge
Forms during anaphase of a dicentric chromosome
Can recombinant progeny be produced in a heterozygote with an pericentric inversion
Few do. No acentric chromosomes or dicentric bridges are produced. The recombinant chromosomes are missing genes or have extra genes and produce inviable offspring.
No recombinant progeny are produced in heterozygote with what?
Paracentric. Fertility is reduced for an inversion heterozygote.
Which chromosome in humans and chimpanzees differ by a pericentric inversion
Chromosome 4
ON EXAM* Translocation
Movement of genetic material between non-homologous chromosomes and from one are of a chromosome to another.
Reciprocal Translocation
An exchange of chromosomes between two non-homologous chromosomes with no loss of genetic material.
Effects of Translocation on the phenotype
Position effect.
Chromosomal breaks may occur within a gene disrupting its function.
Deletions frequently accompany translocation.
Translocation cause difficulties in what…
Disjunction in meiosis
Translocation heterozygotes formed from…
Aneuploid gametes will probably not be viable
Euploid gametes
Have the same number of chromosomes: Monoploid, diploid, triploid, or tetraploid.
Alternate disjunction
Only one that produces euploid gametes
What produces aneuploid gametes
Adjacent disjunction I and II
With euploid gametes from alternate disjunction how many gametes are viable.
Half
Robertsonian Translocation
Two non-homologous chromosomes fuse at their centromeres and the short arms are lost.
Fragile sites
Found mostly in males
Nullisomy
Loss of both members of a homologous pair (2n-2)
Monosomy
The loss of a single chromosome (2n-1)
Trisomy
Gain of a single chromosome (2n +1)
Tetrasomy
Gain of a single homologous pair of chromosomes (2n+2)
Trisomy 16
Most common human aneuploid
Partial Trisomy 16 individuals are…
Genetic mosaics in which only some of their cells have the extra chromosome
Trisomy 21
Down Syndrome
Primary Down Syndrome
Usually results from non-disjunction in Meiosis I in the female.
Not likely to reoccur in the family.
Incidence increases with maternal age.
Three complete copies of chromosomes.
Familial Down Syndrome
Have 46 chromosomes but a small extra part of chromosome 21 is attached to another chromosome through translocation.
Runs in the family.
Can’t tell apart from primary downs
Familial Downs frequently caused by which translocation?
Robertsonian Translocation between Chromosmes 21 and 14
Which arms are lost in Robertsonian?
The short arms
Description of carriers of Robertsonain?
Have 45 chromosomes but a normal phenotype.
Patau Syndrome
Trisomy 13 (cleft palate)
Edwards Syndrome
Trisomy 18
Warkany Syndrome 2
Trisomy 8 (bulbous tipped nose)
Uniparental Disomy
Both members of a homlogous pair of chromosmes are inherited from one parent
Mosaicism
Results from nondisjunction in a mitotic division . All of it’s progeny have the abnormality.
Gene dosage is not affected by….
Polyploidy
What is the key event in the formation of polyploids?
Chromosome doubling.
Mitosis without cytokinesis.
Alteration of meiosis to produce unreduced gametes.
