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Primary Mechanism of Duplication : Unequal Crossing Over
Crossing overBetween Daughter
Strands
Addition (duplication)
Deletion
(tandom duplications)
Crossing overBetween Daughter
Strands
Addition
Deletion
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Polyploidization : complete genome duplication
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Processed Pseudogene
Creation of a processed pseudogene.
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Duplication
Speciation
TD
TS
and are said to be paralogous genes
species1 and species2 are said to be orthologous genes
Species 1 Species 2
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Examples of Duplicated Genes
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Fate of Duplicated Loci
(1) Both copies retain original functione.g. rRNA genes
(2) Gain new function through mutation and selectione.g. globin genes
(3) Become functionless pseudogenese.g globin genes
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All vertebrates but jawless fish have and
Figure 1. Phylogenetic relationship of amphibians (Xenopus and Ambystoma) to other model vertebrate
species and groups of the phylum Chordata.
Million years ago 500 400 300 200 100 0
XenopusAmbystoma
Chicken
PufferfishZebrafish
AmphioxusAgnathans
MouseHumana e
b
c
d
gf
ha = mammalsb = teleostsc = amphibiansd = amniotese = tetrapodsf = gnathostomesg = vertebratesh = chordates
H1: Two independent gene losses
H2: Duplication
Time of duplication may be estimated by observing thephylogenetic distribution of genes
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Repeated Duplication Creates Gene Families
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Globins are differentially expressed during development
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Point mutations generally arise during DNA synthesis or repair
Error rate of DNA polymerasesand the
Efficiency of DNA repair are heritable traits.
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Chromosomes, Genes, and Alleles
Traits are determinedto a great extent by genes
Different forms of a gene
are called alleles
Many different genes (loci)per chromosome
Locus A
Locus B
Locus C
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Basic Unit of DNA
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Nucleic acids are long chains of nucleotides
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The Watson-Crick Model of DNA
In humans 35,000?? different proteins are
specified by the simple linear arrangement
of four bases
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Nucleotide Bases
Adenine
Guanine
Cytosine
Thymine
}
}
Purines
Pyrimidines
Complimentary
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5’ P---dR---P---dR---P---dR---P---dR---OH 3’ | | | | G A A C
3’ OH---dR---P---dR---P---dR---P---dR---P 5’
C T T G | | | |
Schematic Representation of DNA
G A A C
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Classes of Gene
• Protein-coding genes : transcribed and translated
• RNA specifying genes : transcribed
• Untranscribed genes
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Intron 1 Intron 2
Exon 1 Exon 2 Ex 3 3’ 5’
Initiationcodon
GT AG GT AG
RegulatorySequences
Stopcodon
5’ UTR 3’ UTR
Schematic of Eukaryotic Protein-Coding Locus
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How does mRNA specify the amino acid sequence of the protein?
Every three nucleotides forms a codon : each codon codes for an amino acid
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Note : different codons specify the same amino acid
Figure 4.3 The Genetic Code (Codons of mRNA)
(i.e. Code is Degenerate)
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Example of RNA specifying gene :rDNA array
18S 28SNTS
ETS ITS-1
5.8S
ITS-2
1 array in Tetrahymena19,300 copies in Amphiuma200 copies in Human
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ITS NTS ETS 18S ITS 28S NTS
28S
28S
28S
28S
28S
human
chimp
pygmy chimp
gorilla
a BamHIb HindIc BgIIId HpaI
d
d
d
d
a
a
a
a
b
b
b
b
c c
c
c
c
c
c
c
Arnheim, 1983