Download Eucharyotic Chromatin Organization

Aim: How is DNA organized
in a eukaryotic cell?
Why is the control of gene expression more
complex in eukaryotes than prokaryotes ?
 Eukaryotes have:
 1)more functional genes to regulate.
(estimated 35,000 genes in the human
genome) (4300 genes in E.coli bacteria)
 2) introns that must be excised
before functional mRNA is made.
 3) DNA wrapped in histone protein
complexes called nucleosomes.
Why is the control of gene expression more
complex in eukaryotes than prokaryotes ? (2)
 4) cells that require cell specialization or
differentiation (if multicellular)



a) genes expressed in nerve cells may be
different from genes expressed in muscle cells
(only 3% - 5% of genes expressed at any one
time)
b) genes may be amplified (many copies)
c) genes are affected by the way DNA is
packaged in the nucleus


Densely packed areas are inactivated.
Loosely packed areas are being actively transcribed
How is DNA packaged in a eukaryotic
genome ?
 Eukaryotic DNA combines with histone
proteins to form chromatin fibers.
 - Histone proteins are small and contain a
high proportion of positively charged
amino acids (arginine & lysine) which bind
tightly to negatively charged DNA.
 - Histones are universal in eukaryotes.
How is DNA packaged in a eukaryotic
genome ? (2)
 - Heterochromatin remains highly
condensed throughout the cell cycle
and is not transcribed. It is found in
Barr bodies.
 - Euchromatin is less condensed
during interphase and is actively
transcribed. It becomes highly
condensed during mitosis.
Levels of DNA packing (smallest to largest)
 1) Exposed DNA
(2nm)
 2) Nucleosomes
(11nm) (beads) basic unit of DNA
packing.


(DNA + 4 pairs of
histone proteins)
A fifth histone attaches
to the “bead” further
condensing the
nucleosome.
Levels of DNA packing (smallest to largest)
 Nucleosomes may
control gene
expression by
controlling access
of transcription
proteins to DNA.
Levels of DNA packing (smallest to largest)
 3) Chromatin fiber
(30 nm)
 4) Looped domains –
chromatin fibers that
fold into loops.
 A nonhistone protein
‘scaffold’ maintains
the loop and keeps it
attached to the
nuclear membrane.
Levels of DNA packing (smallest to largest)
 5) Metaphase
chromosome (1400
nm).
How do noncoding sequences and gene
duplications account for much of the eukaryotic
genome ?
 DNA in eukaryotes is organized differently from
prokaryotes.
 Prokaryotes – most DNA codes for protein
(mRNA), tRNA, or rRNA.

Small amounts of noncoding DNA consist mainly of
control sequences like promoters and operators.
 Eukaryotes – most DNA does NOT encode
protein or RNA.

Coding sequences may be interrupted by long stretches
of noncoding DNA (introns or tandemly repetitive DNA)
 Interphase chromosomes have areas that
remain highly condensed, heterochromatin,
and less compacted areas, euchromatin.
What are some levels of tandemly repetitive
DNA ?
 I: Highly repetitive DNA (10 – 25 % of the
genome)

A) also called satellite DNA
 B) never transcribed into protein
 C) multiple copies (thousands)
 II: Moderately repetitive DNA (20% of
genome)
 III: Single gene DNA (70% of genome)
Types of highly repetitive DNA
 1) centromeres (chromosome
centers)
 2) telomeres (chromosome ends).
-
may facilitate cell division
 - maintains chromosomes stability on
lagging strand
 - in humans, composed of 250 – 1500
repetitions of the base sequence
TTAGGG
Types of highly repetitive DNA
 3) pseudogenes – nonfunctional gene
that has DNA sequence similar to a
functional gene; but because of a
mutation, lacks sites necessary for gene
expression.
 4) Transposons – generally nonfunctional
but may be disease-causing if they
interrupt an essential gene.
Types of highly repetitive DNA
 5) Genes coding for very small
rRNA units (5SrRNA) (25000 copies)
-
adjacent to pseudogenes
 - needed in developing eggs so that
ribosomes can be manufactured quickly
 - gene sequence reads: 5SrRNA--pseudogene---spacer---5SrRNA-----pseudogene---spacer---5SrRNA---etc.
Moderately Repetitive DNA
 1) repeats hundreds of times
 2) codes for 3 other rRNA
 sequence reads: 18SrRNA---6SrRNA---
28SrRNA---SPACER---18SrRNA---6SrRNA--28rRNA---spacer
 3) moderately repetitive DNA makes many
copies of itself while not affecting the rest
of the chromosome.

These multiple copies are called polytene
regions and exist in what are called lampbrush
chromosomes.
Single gene DNA
o 1) only 1% of the eukaryotic
chromosome ever codes for mRNA that
is actually translated.
2)The other 99% may be composed of
intron genes, pseudogenes or
transposons that are rarely transcribed
into functional mRNA.
What are multigene families ?
 Multigene families – a
collection of genes
that are similar or
identical in sequence
 a) possibly from
same ancestor

probably arose by
transposition.
 b) may be clustered or
dispersed in the
genome
What are multigene families ?
 Example: globin gene
family (alpha and beta
polypeptide subunits
of hemoglobin.
 embryonic and fetal
hemoglobins have a
higher affinity for
oxygen than the adult
forms, allowing
efficient oxygen
exchange between
mother and
developing fetus