Download Edouard van Beneden (Belgian, 1883)

Chapter 10: Gene & Genome
Objectives
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Early genetic principles (Mendel).
Crossing over (recombination) and linkage.
Structure of the DNA double helix.
DNA supercoiling and topoisomerases
Genome complexity and repetitive DNA.
DNA denaturation, renaturation, hybridization
DNA sequence function
Mechanisms for gene duplication
Mobile DNA and consequences of transposition
Restriction Enzymes, RFLPS, Genome Maps
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Karp/CELL & MOLECULAR BIOLOGY 3E
Genetic Principles
Genes, Chromosomes,
Recombination, and Inheritance
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Figure 10.1
Mendel and Peas
• Easily defined characteristics (phenotypes)
• Focused on 7 clearly definable traits, height
& flower color,
• Each trait had 2 alternate & clearly
identifiable forms
• Inbred (purified) lines
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Mendel's conclusions
• Characteristics governed by units of inheritance
(genes)
• Each organism has 2 copies of gene
• The two genes (may be same or not - alleles)
• Dominant alleles mask recessive alleles
• Gametes have only 1 copy of gene
• Law of Segregation - alleles separate (segregate)
randomly
• Law of Independent Assortment – true for 2 or
more genes
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Discovery of chromosomes
(colored bodies, 1888)
• Walther Flemming, early 1880s –
– During cell division, nuclear material organized
into visible threads called chromosomes
– Chromosomes appear as doubled structures,
split, then doubled before next division
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Fertilization and meiosis
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Sperm & egg are very different cells
Sperm & egg equally important genetically
Most obvious shared character: nuclei
Theodore Boveri (German) - sea urchin polyspermy
– Results in disruptive cell division & early death
– Daughter cells receive variable numbers of
chromosomes
– Each chromosome possesses different qualities
• Edouard van Beneden (Belgian, 1883)
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Fertilization and meiosis
• Edouard van Beneden (Belgian, 1883)
– Discovered Ascaris eggs & sperm had 2
chromosomes each
– Somatic cells had four chromosomes
• August Weismann (German biologist, 1887)
– meiosis involved reduction division where
chromosome number was cut in half before forming
gametes
– If not, chromosome number would increase with
every succeeding generation
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Chromosomes as genetic
information
• Walter Sutton (Columbia, 1903)
– Studied grasshopper sperm formation
– Saw 2 types of division by spermatogonia
– Mitotic divisions where spermatogonia make more
spermatogonia
– Meiotic division where spermatogonia differentiate into
sperm
– 11 homologous chromosome pairs (look alike) & extra (X)
– Correlated with Mendel's inheritable pairs of factors
– Hypothesized chromosomes have Mendel's factors
– First meiotic division separated pair members
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Karp/CELL & MOLECULAR BIOLOGY 3E
Figure 10.3
Chromosomes as linkage groups
• How did Mendel's factors assort
independently?
– Mendel owed his results to good luck or lack of
interest in traits that did not fit his predictions
– All Mendel’s traits were unlinked or nearly so,
but…
– Flower color & pollen shape on same
chromosome
– Crossing over (more later)
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Drosophila Genetics– T.H. Morgan
• Fruit flies were ideal for genetics
– Short generation time (14 days)
– Produce up to 1000 eggs in a lifetime
– Small; easy to maintain & breed;
inexpensive
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Figure 10.5
Drosophila Genetics– T.H. Morgan
• by 1915 he found many mutants (85)
– Localized to 4 linkage groups, one with few
mutants
– On rare occasions, mutation occurred
– Mutation + Selection = Evolution
– Mechanism for new species to slowly
emerge
– Linkage groups = chromosome pairs (4),
one small
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Crossing over & recombination
• F. A. Janssens (1909)
– observed interaction of homologs
– hypothesized breakage & exchange of pieces
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Morgan (1911) – this could explain recombination
Crossing over makes linkage incomplete
Can separate genes on same chromosome
Can reshuffle genes
– recombination percent constant for given gene pair
– but different between different gene pairs
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Karp/CELL & MOLECULAR BIOLOGY 3E
Copyright, ©, 2002, John Wiley & Sons, Inc.,
Karp/CELL & MOLECULAR BIOLOGY 3E
Figure 10.7
Crossing over & recombination
• Therefore position (locus) of genes fixed
– Recombination percentage is a measure of distance
– Bigger distance means more crossovers
– Big distance = independent assortment
– Small distance = tightly linked
• Alfred Sturtevant (1911)
– realized recombination could be used to map
– constructed maps of the 4 fruit fly chromosomes
– since used to map genes in many organisms
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Mutagenesis & giant
chromosomes
• Mutants valuable for genetic analysis
• Herman Muller (Indiana U., 1927)
– Genetic material can be damaged by X-rays
– Sublethal dose raises mutation frequency >100 fold
– Other mutagenic agents: UV irradiation, ENU
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Mutagenesis & giant
chromosomes
• Theophilus Painter (U. of Texas, 1933)
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Rediscovers Drosophila polytene chromosomes
Arise in salivary gland cells; Strands stay in register
As much as 1,024 times normal copy number
Cell stops dividing, but keeps growing
Allows cell to maintain high rate of secretion
Exhibit constant banding patterns (~5000 bands)
Bands correlate with specific genes
• Visualize Deletion by X-rays
• Visualize species specific evolutionary change
• Active genes puff out (RNA Synthesis)
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Karp/CELL & MOLECULAR BIOLOGY 3E
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Figure 10.8a