BB30055: genes and genomes
MV Hejmadi (bssmvh), 2006-07
Genomes and genome projects –Lecture 2: Repeat elements
Repetitive sequences:



Tandem repeats : satellites, minisatellites and microsatellites
Interspersed repeats: LINEs, SINEs, LTR and DNA transposons
Segmental duplications: intra or interchromosomal duplications
Tandem repeats:
Blocks of tandem repeats at
 subtelomeres
 pericentromeres
 Short arms of acrocentric
chromosomes
 Ribosomal gene clusters
class
Size of
repeat
Repeat
block
Major
chromosomal
location
Satellite
5-171 bp
> 100kb
minisatellite
9-64 bp
0.1–20kb
centromeric
heterochromatin
Telomeres
microsatellites
1-13 bp
< 150 bp
Dispersed
Satellites:
Large arrays of repeats
E.g. Satellite 1,2 & 3
 (Alphoid DNA)
 satellite
Interspersed repeats or Transposon-derived repeats. They constitute 45% of genome and arise mainly as a
result of transposition either through a DNA/RNA intermediate. They can be divided into 4 main types
Science 12 March 2004: Vol. 303. no. 5664, pp. 1626 - 1632
BB30055: genes and genomes
MV Hejmadi (bssmvh), 2006-07
Segmental duplications:
 Closely related sequence blocks at different genomic loci
 Transfer of 1-200kb blocks of genomic sequence
 Segmental duplications can occur on homologous chromosomes (intrachromosomal) or non homologous
chromosomes (interchromosomal)
 Not always tandemly arranged
 Relatively recent
Interchromosomal segments
Intrachromosomal
 duplicated among non homologous chromosomes
 duplications occur within a chromosome /arm
 Prone to deletions/ duplications
 Prone to translocations
How did repeat elements originate?
Tandem repeats – replication slippage etc
Interspersed repeats – transposition events
Segmental duplications – strand exchange, recombination events
Why are repeat elements important?
Evolutionary ‘signposts’
Passive markers for mutation assays
Actively reorganise gene organisation by creating, shuffling or modifying existing genes
Chromosome structure and dynamics
Provide tools for medical, forensic, genetic analysis
References:
Chapters 9 & 11: Human Molecular Genetics 3 by Strachan and Read AND/OR
Chapter 10: Genetics from genes to genomes by Hartwell et al (2/e) pp 339-348
Nature (2001) 15Feb (409) pg 814-816 & 875-900