Genome Evolution
Evolution of Gene clusters
Why are genes are
arranged in clusters?
• Many genes are arranged in
groups of related genes along a
chromosome - gene clusters
• Eukaryotic ribosomal RNA
genes: tandem repeats
45S
ITS 28S
NTS
ETS
18S ITS 28S
NTS
Gene Families
• Related genes may be organized
is several clusters at different
locations These are known as
gene families e.g. Globin genes
Gene Families
Gene duplication and
misalignment
• The misalignment of genes
during recombination is the
most likely cause of gene
duplication and clustering.
• Once repeats exist the
probability of misalignment
increases.
Misalignment - simple
case
A B
A B
A B B
A
Misalignment - Globin
But not all DNA codes
for genes
• Single copy DNA can be used as a
rough guide to the amount of the
DNA that actually codes for genes.
• % of single copy DNA ranges from
95% in some arthropods to 12% in an
Amphibian (newt)
• Human have 64% SC DNA
Types of repetitive DNA
• Tandem Repeats
– Microsatellites (repeats 2-5 b
pairs) 100 repeats per loci
– Minisatellites (repeats 15 base
pairs) average length 500-2000
nucleotides
– Satellite DNA (5-100 base pairs)
found in blocks of 1000 or more
repeats
Types of repetitive DNA
• Scattered repeats - Longer
sequences (100 or more base
pairs) scattered through the
genome usually as single copies
Selfish DNA - tandem
repeats
• Most non-coding DNA is selectively
neutral
• It will passively replicate and may be
duplicates as with coding tandem
repeats
• It will continue to accumulate until
it’s mass slows the cell cycle then it
will be selected against
Selfish DNA - Scattered
Repeats
• Transposable elements
– Transposition by reverse
transcription - disrupts genome
and increase number of copies of
self
– Transposition without reverse
transcription - disrupts genome
Selfish DNA - Scattered
Repeats
• Origin - are they ancient and
very successful virus?
The C-factor Paradox
• Why is there excess DNA?
• Selfish DNA provides an
attractive hypothesis to solve
this question.
• It would also explain why
apparently functionless DNA is
functionless