Chapter 9 How genes and genomes evolve

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Chapter 9How genes and genomes evolve

• Generating genetic variation
• Reconstructing lifes family tree
• Examining human genome

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The function of the germ cell line in sexually
reproducing organisms is to propagate genetic
information to the next generation Any mutation
that occurs germ cells is passed on to the
progeny Figure 9-1 9-2
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Types of genetic changes contributing to
evolution 1.Mutation within a gene an existing
gene can be modified by mutation that changes a
single nucleotide (substitution, deletion,
duplication) 2.Gene duplication a large
segment of an existing gene or even a whole gene
can be duplicated 3.Gene deletion a large
segment or the entire gene can be deleted
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4.Exon shuffling two or more existing genes can
be broken and rejoined. The new gene will contain
segments that originally belonged to separate
genes 5.Horizontal gene transfer common in
procaryots only, where a piece of genome can be
transferred from one cell to another Figure 9-3
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Gene duplication is thought to occur between two
homologus segments of DNA by unequal crossing
over On rare occasions, the chromosomes will
misalign The process of crossing over will
result in a long and a short version of the
gene If this process occurs in the germ line,
some of the progeny will inherit a short version
and some a long version of this gene Fig.9-5,
9-28, 9-29
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An example of proteins composed of repeated
sequences thought to evolve by exon duplication
(misalignment and unequal cross-over) are
collagen, albumins, immunoglobulins
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A single chain globin molecule gave rise to the
four-chain hemoglobin used by humans and other
mammals One hemoglobin molecule is a complex of
two alpha and two beta globin chains The four
oxygen binding sites in hemoglobin molecule allow
for more efficient oxygen binding than a single
chain version Figure 9-6
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The globin gene family The alpha globin genes are
located on the chromosome 16 The beta globin
genes are located on the chromosome 11 The fetal
globin genes (epsilon and gamma globin) were
formed by duplication of beta globin version The
delta globin version is found in primates
only Figure 9-7
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The example the duplication of the entire genome
is a frog genus Xenopus The genome duplication
can occur when the germ cells failed to divide
properly and are 2n instead of 1n Figure 9-8
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Exon shuffling Exon (where one exon represents
one protein domain) shuffling allows for liking
together combinations of initially separate exons
encoding different protein domains As a result,
modern proteins share similar domains that differ
in numbers and locations Figure 9-10
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Horizontal gene transfer in bacteria Horizontal
gene transfer occurs during conjugation Plasmid
DNA replicates itself (inside the donor cell) and
its copy gets transferred into the recipient
cell Antibiotic resistance in some pathogenic
bacterial strains can be transferred this way As
a result many antibiotics are no longer effective
treatment Fig. 9-13
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Comparison of genomes between humans and
chimpanzees revealed that there is only 1.2
difference The last common ancestor is thought to
have lived 10 millions years ago and the
divergence occurred 5 millions years ago Figure
9-15
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Comparison of protein coding sequences of leptin
genes (regulates food intake and energy
utilization) between humans and chimpanzees There
were only 5 nucleotide changes 4 changes did not
change the influence the amino acid sequence
(silent mutations) 1 change influenced protein
sequence Figure 9-16
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Gene organization within the chromosome is also
similar among closely related organisms Figure
9-17
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Humans and mice diverged 75 million years
ago There is 50 change in nucleotide
sequence Clearly exons have fewer changes than
introns Purifying selection-elimination of
individuals carrying mutations that interfere
with important functions Figure 9-19
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The tree of life was based on small rRNA
subunit Bacteria are clearly divided into
eubacteria and archbacteria The third major
domain are eucaryotic organisms Figure 9-23
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• Human genome
• Only few of the genome is protein coding
• 2. Average gene size is 27,000 nucleotides and
  only 1,300 nucleotides are protein coding
  (presence of many introns)
• 3. Gene regulatory sequences are spread out
• 4. The number of protein coding genes is 30, 000,
  not 100,000 as previously estimated
• Figure 9-25