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Advanced Genetics
Lecturer:
Dr. Winston Elibox
Instructor: Ms. Gabrielle Holder
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Course Assessment
Final Theory Paper = 50%
In-course = 50%
Three in-course tests = 30%
Tutorials = 10%
Labs = 10%
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LECTURE-1
PART- I CYTOGENETICS
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Chromosome theory of inheritance
States that chromosomes are the
unit of inheritance
Proposed by Walter Sutton and
Theodor Boveri (1907)
Behavior of chromosomes duringmeiosis paralleled that of Mendels
particles
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Chromosome theory of inheritance
Meiosis: Segregation and
Independent assortment
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ChromosomesMendels principles
Chromosome theory of inheritance
Pairs of homologous
chromosomes
Homologous
chromosomes separate
and go into gametes at
equal frequency
Different pairs ofchromosomes segregate
independently
Paired particles
Paired particles segregate
into gametes at equal
frequency
Different particles assort
independently
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Chromsome theory of inheritance
Chromosomes became the centre of
interest following the Chromosometheory of inheritance
Chromosomes are bodies that take up stain during
cell division.
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What is Cytogenetics ?
The study of the genetic constitutionof cells through the visualisation andanalysis of chromosomes.
Primarily concerned with genome and chromosome
characterization so that any changes to the
organization or structure can be detected andcorrelated with behavioural changes and
evolutionary leaps.
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What is Cytogenetics ?
Cytogenetics is the study of
Nuclear organisation of chromosomes
Macromutations that affect chromosome
structure or number
Effect of macromutations on
chromosomal behavior during meiosis
Effect of macromutations on phenotye
and evolution
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Chromosome organization
Chromosome number
Total number of chromosomes in asomatic cell
Basic chromosome number (x)
Number of unique chromosomes in asomatic cell (Monoploid number)
Basic chromosome set
The unique chromosome set of asomatic cell
Human genome
X = 23
2x = 46
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Genome characterisation
A genome can be characterisedbased on:
(a) Basic chromosome set
(b) Ploidythe number of repetitionsof the basic chromosome set
- Euploidy-whole set repetitions
- Aneuploidy-repetitions that are notwhole number replicates of the basicchromosome set.
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Genome characterisation
Size: 0.5 - 400 M length (A - G groups)(http://www.accessexcellence.org/AE/AEPC/WWC/1993/karyoteype.php)
Banding pattern:Imparted by staining. Provides a means of uniquelyidentifying each chromosome
Shape: - Position of centromere (primaryconstriction; non-stainable) = telocentric,
acrocentric, mesocentric or metacentric
- Position of secondary constrictions
(satellites and nuclear organisers)
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Genome characterisation
Position of nucleolarorganizer:
Represents the regionof the chromosomethat has the rRNAgene cluster
Forms a secondaryconstriction
The region isassociated with thenucleolus which
stores the rRNA
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Genome characterisation
Satellites (satellite DNA): Secondary constrictions in eukaryotic DNA (5 to 200pb) thatconsists of short, tandem repeated non-coding sequences ofnucleotide pairs, often found near the region of the centromereand occupying the majority of the heterochromatin.
Secondary
constriction
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Genome characterisation :
Banding patterns
Quinacrine stain = Q bands
Impermanence fluorescent bands;
fades quickly
Giemsa staining = G bands
Dark permanent bands (area of most
coiling)
Reverse Giemsa
R bands- complimentary
to G bands
C bands (Giemsa fixed with alkali-
only heterochromatic region is seen)
Dark bands- heterochromatin
Light bands- euchromatin
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Genome characterization :
Giemsa staining
Giemsa stain = G bands.
Most popular staining method (permanent).
Reveals areas of most coiling- dark bands associated with heterochromatin.
Lighter bands are the euchromatin.
Also reveals regions called chromomeres- heavily stained regions associated with
genes (associated with the euchromatic regions).
If Giemsa stain is fixed with alkali, a different banding pattern called C bands revealsonly the heterochromatic regions.
If the chromosomes are heated in a phosphate buffer, then treated with Giemsa stain,
an R banding patterns occurs- that is the reverse of that produced in G-banding.
Cytogeneticists use stains to differentiate between chromosomes.
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Ultrastructure of a chromosome
One chromosome = One molecule
of double helical DNA packaged in
a lattice work of histone proteins
Unineme model- each chromosome comprises 1 DNA double helix extending from one end of the
chromosome to the other.
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Ultrastructure of a chromosome
Chromosomes are nucleoproteins- have both DNA
and proteins. Proteins form the structural framework
for the DNA.
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Ultrastructure of a chromosome
Primary structure
Nucleosome
- The unit of packaging of a chromosome
- Nucleosome = Histone core (2 H2A, 2 H2B, 2 H3 and 2 H4) +
two turns of double helical DNA (140 bp)
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The Nucleosome threadThe primary structure of the chromosome (100 Ao) = 0.000001 cm. (1 Angstrom
= 10-8 cm).
The nucleosomes are linked together by linker DNA and
stabilized by H1 protein to form the nucleosome thread.
Ultrastructure of a chromosome
Primary structure
1 nm = 10Ao
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Ultrastructure of a chromosome:
Secondary structure
Chromatin
- The secondary structure of the chromosome
- The nucleosome thread is thrown into coils to form a solenoid
structure (200-300 Ao
)- approximately six nucleosomes per turn.
Meiotic chromosomes
- Chromatin is thrown into tertiary and quaternary coilingduring mitosis and meiosis.
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Ultrastructure of a chromosome
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chromatin -secondary
structure of
chromosome
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Chromatin:
Euchromatin vs heterochromatin
Nuclei consist of chromatinstrands as loosely packed
euchromatin or densely packed
heterochromatin
Heterochromatin
Euchromatin
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Chromatin:
Euchromatin vs heterochromatin
Heterochromatin
Heavily coiled functionally inactive regions of the chromosome
Constitutive heterochromatin
Associated with centromere, telomere and intercalary (betweencentromere and tip) regions = represent highly repetitive non-coding regions
Condensed heterochromatin
Distributed differently from tissue to tissue and appears during cell
maturation. Reflects permanent turning off of certain genes duringdifferentiation
Facultative heterochromatin
reflects regulatory devices designed to adjust the dosage ofcertain genes
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Position effects
Expression of genes in the euchromatic region can be affectedby adjacent heterochromatic regions.
The highly coiled regions of the heterochromatin affecttranscription machinery from accessing the genes fortranscription.
The nearer a gene is to the heterochromatic region, the more its
expression is affected by the heterochromatic region.
The spreading suppressing influence of the heterochromaticregion on genes in the euchromatin region is referred to as
position effect.
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Position Effects on Gene
Expression
Heterochromatin: condensed
Euchromatin: loose
Position effects
Yeast
Fruit Fly
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Karyotyping
Diagrammaticrepresentation ofchromosomes of asomatic cell at the
mitotic metaphasearranged inhomologous pairsofdecreasing size.
Indicates landmark
features that allowchromosomes to beuniquely identified.
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How do you do a Karyotype?
1. Growing root tips squashed and stained
2. Identify mitotic metaphase in transverse (cross sectional) view
3. Take microphotograph
4. Enlarge photograph
5. Cut and paste in descending order of size
6. Indicate landmark features.
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Uses of a Karyotype
1. Provides a means of identifying chromosomal aberrations fromthe type (normal) karyotype.
Identifies changes in chromosomes structure, size and
chromosome number (Down syndrome-trisomy 21).
2. Comparison of karyotypes of different species allowdetermination of taxonomic relationships.
Helps reform and correct taxonomic relationships.
3. Enables the understanding of evolution, where smallchromosomal changes accumulate over time in a linear fashion.Fewer changes imply recent divergence. Can help us construct
phylogenetic trees.
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Karyotype: Polytene chromosomes
The nuclei in the salivary glands of Dipteran insects (true flies e.g.Drosophila) show enlargement due to extra replication ofchromosomesendopolyploidy orpolyteny
The replicated chromosome do not separate but stay in stacks to formthick giant chromosomes called polytene chromosomes.
The heterochromatic regions which are visible to the naked eye inthese chromosomes.
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Karyotype: Polytene chromosomes
chromocenter
http://www.google.tt/url?sa=i&rct=j&q=polytene+chromosomes&source=images&cd=&cad=rja&docid=_FILasP1ZYZQSM&tbnid=UCZAvmOfW1gpXM:&ved=0CAUQjRw&url=http%3A%2F%2Fsekelsky.bio.unc.edu%2Fresearch%2Fgenes.html&ei=JT0SUvSxDIapyAHcg4Fg&bvm=bv.50768961,d.b2I&psig=AFQjCNG83Q99MXAdEifhfjJvl3m6_ICmjA&ust=1377013131346301http://www.google.tt/url?sa=i&rct=j&q=polytene+chromosomes&source=images&cd=&cad=rja&docid=_FILasP1ZYZQSM&tbnid=UCZAvmOfW1gpXM:&ved=0CAUQjRw&url=http%3A%2F%2Fsekelsky.bio.unc.edu%2Fresearch%2Fgenes.html&ei=JT0SUvSxDIapyAHcg4Fg&bvm=bv.50768961,d.b2I&psig=AFQjCNG83Q99MXAdEifhfjJvl3m6_ICmjA&ust=13770131313463017/29/2019 1. Introduction to Cytogenetics
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Karyotype: Polytene chromosomes
Polytene chromosomes begin as normal chromosomes.
Undergo repeated rounds of DNA replication without cell division.
They become large, banded chromosomes.
Centromeric regions do not endoreplicate very well.
Centromeres of all the chromosomes bundle together in a mass calledthe chromocenter.
Found in larvae of the insects and promote faster growth anddevelopment than the diploid state.
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Karyotype: Polytene chromosomes
The active region (chromomeres- thickened,tightly coiled DNA, stains darker than therest of the DNA and associated withhighly expressive genes) form puffs,
while other regions are condensed(Drosophila: 5000-6000 puffs)
In Drosophila all the four chromosomes areconnected together at the chromocenter
Chromosomal aberrations can be seen asloops
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Karyotype: Lampbrush chromosomes
Found in the oocytes of some amphibiawith yolky eggs.
Occurs during the prolonged prophase
during the first meiotic division
The meiotic chromosomes reach 1000 umin thickness with long lateral loops
Each loop emerges from one chromomereby duplication (puff)
Some loops are pinched off as balbianirings, which can also independentlyexpress gene products.
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Karyotype: Lampbrush chromosomes
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A model for the structure of a lampbrush
chromosome
Karyotype:
Lampbrush
chromosomes
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Notes
60% of DNA in higher eukaryotes is junk DNA.
Important for pairing of chromosomes and crossovers inconstitutive heterochromatin
Coding regions (genes) are not highly repetitive or may be unique.
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Summary
Chromosome organization
Chromosome theory of inheritance
Chromosome organisation
Basic chromosome set
Ploidy
Karyotyping allow characterisation of the basic chromosome set
Length, shape, special features (satellites, nucleolar organiser)
Various types of banding patterns by differential staining
Ultrastructure of the chromosome
Primary secondary, tertiary and quaternary packaging of DNA
Heterochromatin vs euchromatin
Position effects
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