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Biology 331: Chapter 3
Chromosomal Basis of Heredity
Meiosis & Mitosis:
Review your Intro Biology notes
 If you have difficulty stop by my office
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Meiosis...a review:
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Interphase:
– Chromosomes and centrosomes replicate
• Much like mitosis
Meiosis I:
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Prophase I:
– Chromosomes begin to condense
– Synapsis: Homolgous chromosomes come together as pairs
– Tetrad: A complex of four sister chromatids (2 from each
homolog chromosome)
– Chiasmata: Where homologous chromatids "cross over"
– The exchange of genetic material
– 90% of the time for meiosis
Meiosis Diagram
Crossing Over
Meiosis I continued
Metaphase I:
 Anaphase I:
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– Paired sister chromatids of homologous
chromosomes move toward opposite poles
– Sister chromatids do not separate
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Telophase I and cytokinesis:
Meiosis Diagram
Meiosis II:
Prophase II:
 Metaphase II:
 Anaphase II:
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– Centromeres of sister chromatids separate
– Individual chromosomes move toward opposite poles
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Telophase II and cytokinesis:
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Each cell is now haploid
Meiosis Diagram
Mitosis –vs- Meiosis
Some origins of genetic variation in
sexually reproducing organisms:
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Without variation there is no Evolution!
Independent assortment of
chromosomes to gametes:
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Which of the homologous chromosomes is included in
a gamete is random
The chances are 50-50 for each chromosome
Which of your parents chromosomes goes into a gamete
100% maternal, 100% paternal, or some combination
The number of possible of possible combinations of
gamete is 2n (n is the haploid number)
For humans this is 823 or about 8 million combinations
Random fertilization:
Which sperm meets which egg?
 Multiply the possible gametes of each parent
together
 Assuming 2 parents with 8 million possible gametes
you get 64 trillion combinations
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Crossing over:
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Individual chromosomes in a gamete do not have
to be exclusively maternal or paternal
When chiasmata form portions of chromosomes
can switch places
In humans you get ~2-3 crossover events per
chromosome pair!
Number of possible combinations is nearly infinite
Crossing Over
Life Cycles:
If you get chromosomes from both parents how
do you avoid a doubling of genetic material?
 Meiosis: The process of creating gametes with
half the normal chromosome complement
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Review from Intro. Bio. I & II
Diploid Life Cycles
Common in animals
 Gametes are the only haploid cells
 Meiosis occurs during the production of gametes
 Gametes do not undergo further division until
fertilization
 Diploid zygotes undergo mitosis to form a
multicellular adult
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Diploid Life Cycle
Haploid life cycle
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Most fungi and some algae:
Gametes fuse to form a transient diploid zygote
Gametes undergo meiosis to form two haploid cells
These haploid cells undergo mitosis to form a
multicellular adult
The adult produces gametes via mitosis
Implications for genetics??
Haploid Life Cycle
Alternation of Generations
Plants and some algae:
 Sporophyte:
Multicellular diploid stage
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– Meiosis in the sporophyte produces haploid spores
– Spores give rise to multicellular adults without fusing
with another cell
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Gametophyte:
Multicellular haploid stage
– The haploid gametophyte makes gametes via mitosis
– Fertilization results in a diploid zygote
– The zygote develops into the next sporophyte
generation
Alternation of generations
Moss and Alternation of Generations
Implications for genetics??
Some genes affect the sporophyte and
others the gametophyte
 Height of gametophyte and color of
sporophyte for example
 Mendel and his peas
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– Why did it work out?
Topography of the
chromosome set:
Chromosome number:
Centromere Position
….and chromosome size
Banding patterns:
Heterochromatin patterns
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Utilizes DNA stain (like Feulgen)
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Euchromatin:
– Poorly staining
– Less densely packed
– Implications?
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Heterochromatin:
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Stains well
Densely packed
Can be constitutive or facultative
Implications?
Heterochromatin typically found at centromeres and telomeres
The whole drosophila Y chomosome is heterochromatic
G banding patterns:
Chromosomes partially digested with
proteolytic enzymes
 Stained with Giemsa reagent
 Produces light and dark banding regions
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G Banding
G-light bands:
Tend to be GC rich
 Not densely packed
 Replicate early
 mRNA label binds to these regions most
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G-dark bands:
Tend to be AT rich
 Densely packed
 mRNA does not bind well to these areas
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Implications??
Three dimensional structure of
a chromosome:
The molecule:
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Each chromosome is ONE molecule of DNA
E.coli contains about 1.3mm of DNA per cell
H. sapiens contains about 2m of DNA per cell
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Implications for cells and cell division ??
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The Chromosome
Histone Proteins and packaging:
Chromatin:
The length of a chromosome is much less
than that of a DNA molecule
 Chromosomes made up of chromatin
 Chromatin is DNA and protein
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Condensation of a chromosome
Histones:
Histones are the primary proteins
 The structure of histones is conserved
across eukaryotes
 Histones form an octamer
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The solenoid:
DNA wraps two times around each octamer
 This histone DNA spool is called a nucleosome
 The nucleosome "beaded necklace" assumes a
coiled form called a solenoid
 The solenoid is stabilized by another histone
protein (H1)
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Histones
High order coiling:
Solenoids are ~30nm across
 However, chromosomes are ~700nm across!
 Must have more structure
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Scaffold protein:
Non histone
 The scaffold forms a spiral
 The nucleosomes loop on the scaffold
 Loops attach at SARs (scaffold attachment points)
 The DNA region where SARs occur are non-coding
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Scaffold
SARs
Sequence organization:
How are genes arranged?
 What proportion of the DNA is genes?
 What is the nature of the non-coding regions?
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Organelle Genomes:
Portion of the genome outside the nucleus
Chloroplast DNA (cpDNA):
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Chloroplasts came from endosymbiotic cyanobacteria
cpDNA is a circular double helix
Haploid
Each organelle has 20-80 copies of cpDNA
Contains genes related to photosynthesis and those
required for gene expression
Has few noncoding sequences compared to nDNA
Highly conserved (even the order is conserved)
Uniparental inheritance
Angiosperms maternal
 Some gymnosperms paternal
 Biparental inheritance has been found in
some angiosperms
 Recombination is rare in the extreme
 Inherited as one allele
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Mitochondrial DNA (mtDNA)
Came from purple bacteria endosymbiont
 mtDNA is a circular double helix
 Haploid
 Codes for many proteins used in cellular respiration
 5-10 copies in each organelle
 mtDNA in plants, fungi and animals differ
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Plant mtDNA:
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Great variation in genome size
– 300-2,400 kb in melons
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Structure varies
– Can be circular or linear for example
Recombination is frequent
 Sequence divergence is low however
 Evolution is slow
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Fungal mtDNA:
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Genome size variable
– 26.7 - 115kb in one group
Coding sequences include introns
 Modification of the "standard" DNA code
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– UGA = tryptophan not "stop"
– CUN = threonine not leucine
– AUA = methionine not isoleucine
• In some cases
Animal mtDNA:
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Very Compact
– No introns and noncoding regions are limited
– In the mouse 94% of the mtDNA genome is coding
Genome size and gene order conserved
 Mutation rate is high
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– Sequence divergence is high
– Evolutionary rate 5-10 times that of nDNA
AUA = methionine not isoleucine
 UGA = tryptophan not "stop"
 Maternal Inheritance (Normally)
 No Recombination
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