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Chapter 7
Clusters and Repeats
Jocelyn E. Krebs
Figure 07.CO: Scanning electron micrograph (SEM) of red blood cells.
© Sebastian Kaulitzki/ShutterStock, Inc.
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A gene family consists of related genes that arose by duplication and
variation from a single ancestral gene.
Duplicated genes may diverge to generate different genes or one
copy may become an inactive pseudogene.
Translocation
Gene cluster
translocation
inversion
Figure 07.01: Chiasma formation can result in the generation of recombinants.
Figure 07.02: Recombination involves pairing between complementary strands of the
two parental duplex DNAs.
Satellite DNA, minisatallite DNA, microsatallite DNA
Figure 07.03: Unequal crossing-over results from pairing between nonequivalent
repeats in regions of DNA consisting of repeating units.
6.2
Gene Duplication Is a Major Force in Evolution
Figure 07.04: Gene number can be changed by unequal crossing-over.
6.3 Globin Clusters Are Formed by Duplication and
Divergence
• All globin genes are descended by duplication and
mutation from an ancestral gene that had three
exons.
• The ancestral gene gave rise to myoglobin,
leghemoglobin, and α and  globins.
Nonallelic copies
HbH disease; b4
hemoglobin
Hydrops fetalis; complete
deletion of a-chain
Figure 07.05: α-thalassemias result from various deletions in the α-globin gene cluster.
Hb lepore
Hb anti-lepore
Hb kenya
Figure 07.06: Deletions in the β-globin gene cluster cause several types of thalassemia.
6.7 Genes for rRNA Form Tandem Repeats
Including an Invariant Transcription Unit
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Ribosomal RNA is coded by a large number of identical genes that are tandemly
repeated to form one or more clusters.
Each rDNA cluster is organized so that transcription units giving a joint precursor to
the major rRNAs alternate with nontranscribed spacers.
Figure 07.07: A tandem gene cluster has an alternation of transcription unit and
nontranscribed spacer and generates a circular restriction map.
Figure 07.08: The nucleolar core identifies rDNA under transcription, and the
surrounding granular cortex consists of assembling ribosomal subunits.
Photo courtesy of Oscar Miller
Figure 07.09: Transcription of rDNA clusters generates a series of matrices, each
corresponding to one transcription unit and separated from the next by the
nontranscribed spacer.
Photo courtesy of Oscar Miller
Figure 07.10: The nontranscribed spacer of X. laevis rDNA has an internally repetitious
structure that is responsible for its variation in length.
Crossover fixation-identical repeated seq
Concerted evolution; two genes in single locus
Gene conversion
Crossover fixation
Figure 07.11: Unequal recombination allows one particular repeating unit to occupy
the entire cluster. The numbers indicate the length of the repeating unit at each stage.
Simple sequence DNA; short
repeated units of DNA seq.
Satellite DNA
Cryptic satellite-repeated seq in high
dense-DNA
Heterochromatin-centromeric locus
euchromatin
Figure 07.12: Mouse DNA is separated into a main band and a satellite by
centrifugation through a density gradient of CsCl.
In situ hybridization
FISH
Figure 07.13: Cytological hybridization shows that mouse satellite DNA is located at
the centromeres. Photo courtesy of Mary Lou Pardue and Joseph G. Gall, Carnegie
Institution.
Courtesy of Mary Lou Pardue and Joseph G. Gall, Carnegie Institution
Figure 07.14: Satellite DNAs of D. virilis are related. More than 95% of each satellite
consists of a tandem repetition of the predominant sequence.
7.7
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Mammalian Satellites Consist of Hierarchical
Repeats
Mouse satellite DNA has evolved by duplication and mutation of a short repeating
unit.
 This gives a basic repeating unit of 234 bp in which the original half, quarter,
and eighth repeats can be recognized.
Figure 07.15: The repeating unit of mouse satellite DNA contains two half-repeats,
which are aligned to show the identities (in blue).
Figure 07.16: The alignment of quarter-repeats identifies homologies between the
first and second half of each half-repeat.
Figure 07.17: The alignment of eighth-repeats shows that each quarter-repeat consists
of an α half and a β half.
Figure 07.18: The existence of an overall consensus sequence is shown by writing the
satellite sequence in terms of a 9 bp repeat.
Figure 07.19: A dimer occurs when one unit has lost the site, a trimer is generated
when two adjacent units have lost the site, and so on.
Microsatellite DNA
Variable number tandom repeat (VNTR)
DNA fingerprinting
Figure 07.20: Alleles may differ in the number of repeats at a minisatellite locus, so
that cleavage on either side generates restriction fragments that differ in length.
Figure 07.21: Replication slippage occurs when the daughter strand slips back one
repeating unit in pairing with the template strand.