Genetics
of Chromosomes:
Genome Organization
In
Principle:
Chromosomes each comprise a single,
continuous
DNA molecule
DNA is systematically packaged into chromosomes
through several levels of coiling
Most DNA does not code for proteins
Non-coding
DNA occurs in several classes
Chromosome architecture at the
molecular level
OR: Whatcha gonna do
with all that junk inside your genes?
How is DNA packed into chromosomes?
1o coiling of
DNA helix
2o
coiling of DNA +
histone core = nucleosome
histone octamer:
(H2A, H2B, H3, H4) x 2
histone H1 + linker
DNA
3o coiling
of core + linker
forms Solenoid
six H1 subunits
4o
coiling of solenoid
loops to non-histone Scaffold
scaffold
supercoil
chromatin thread chromosome
Fine-scale DNA structure of
chromosomes [Chromosome 11
CSHL
Animation]
Protein-coding genes
have introns, exons,
promoters, enhancers
'exon shuffling' hypothesis
exons as protein domains
exon
recombination
occurs
via transposable elements (Bio4241)
Human genomes include ~20,050
protein-coding "genes" in <5% of DNA
Telomeric DNA caps chromosomes (IG1 11.1
Research Briefing)
may limit lifespan of cells
in vitro and in vivo (aging) [Nobel Prize, 2009]
Repetitive DNA accounts
for >>50% of
human genome, within and
between coding
gene loci
Repeats may be dispersed or tandem
Tandem repeats - multiple copies arranged
end-to-end
satellite DNA - variable [G+C] content
NORs (nucleolus-organizer
regions) comprise multiple rDNA genes
5S, 18S & 28S rRNA gene clusters
multiple rDNA genes allow
more efficient production of rRNA
VNTRs (variable number of tandem repeats)
micro- & minisatellites - 2 or 4~10bp repeats
Dispersed repeats
-
multiple
copies scattered throughout chromosomes
SINEs (short interspersed
elements) may occur within
introns
Alu family
- 200 ~ 300 bp
x 105 copies
Alphoid family - centromeric
heterochromatin, 170 bp x ~103
copies
LINEs (long interspersed
elements) occur between
genes
LI family - 6,400
bp x 104 copies
All text
material ©2016 by Steven M. Carr