cell biology

1. Cell Biology
S. Rahgozar,PhD
University of Isfahan
Faculty of Science
3. The nucleus
3.2. Internal organization and rRNA processing
1392-93

2. Internal organization of the nucleus
Lamin-dependent complexes form nuclear bodies that have roles in
DNA repair, chromatin organization, gene regulation and
signal transduction
Heterochromatin in interphase nuclei
 Chromosomes and higher-order chromatin structure
Types of heterochromatins
 Constitutive
 Facultative
The amount of facultative heterochromatin
varies depending on the transcriptional activity
of the cell

3. o Interphase chromatin is not randomly distributed within the nucleus
1885, Carl Rabl: Chromosome organization in
salamander. A) Complete chromosome, B)
Telomeres only
1984, Chromosome organization in Drosophila.
Rather than randomly winding around one another,
each chromosome occupies a discrete region of
the nucleus. B) Topological separation between
chromosomes.

4. o Individual chromosomes occupy distinct
territories within the nuclei of mammalian cells
Location of actively transcribed genes, where newly
transcribed RNAs will be released.
Location of heterochromatin bound to the matrix of
the nuclear lamina
 the chromosome location differ upon the
species and tissues.
 chromatin reorganizes during cell differentiation
and gene expression by means of nuclear actin and
myosin.

5.  Looped domains contain
50-100kb of DNA. They
appear to represent
discrete functional units,
which independently
regulate gene expression.
Lampbrush chromosome, amphibian oocyte

6. From 30nm fiber (0.1 cm) to μm

7. Structure of a lampbrush chromosome

8.  Sub-compartments within the nucleus
Early and late distribution in DNA synthesis. Newly synthesised
DNA is labeled with bromodeoxyuridine, a thymidine analog,
then detected by staining with a fluorescent antibody.
DNA synthesis starts at:
1- Perinucleolar sites ; 20 clusters distributed around
the nucleolus and associated with nuclear lamina
2- Origins of replication; about 4000 sites in a mammalian
cell, organized into distinct functional bodies called
replication factories.
 Nuclear speckles
Storage sites of mRNA
splicing components (20-50
within a nucleous).

9.  PML bodies
Sites of accumulation of
transcription factors and chromatin-
modifying proteins (such as histone
deacetylases)
 Cajal bodies
Sites of small RNPs assembly
and processing.
Contain Coilin protein.
Green: Coilin
Red: Fibrillarin

10.  Nucleolus and rRNA processing
 Nucleolus is the site of
rRNA transcription and processing and ribosome assembly
RNA modification
 Actively growing mammalian cells contain 5-10 million
ribosomes that must be synthesized each time the cell
divides.
Structure of the nucleolus
FC: fibrillar center
DFC: dense fibrillar component
G: granular component
Nucleoli in amphibian oocyte
Amlified rRNAs are
clustered in multiple nucleoli

11. o Ribosomal RNA genes and the organization of the nucleolus
RNA polymerase I
(200 copies)
Ch13,14,15,21,22
o Following each cell division,
nucleoli become associated
with the related chromosomal
regions. Therefore called
nucleolar organizing regions.
o 5S is present only on ch1
o 5S is transcript by RNA
polymerase III elsewhere

12. o Transcription and processing of rRNA
Electron micrograph of nucleolar chromatin showing 3 rRNA genes surrounded by growing RNA chains
Processing of pre-rRNA
ETS: external transcribed
spacers
ITS: internal transcribed spacers
snoRNPs U3
U8
U22

13.  Pre-rRNA Processing (cont)
~300 proteins and 200 snoRNAs are located
in nucleoli and involved in pre-RNA
processing.
Role of snoRNAs in base modification of pre-RNA
 snoRNAs contains short sequences of ~15
nucleotides that are complementary to 18S or 28S
rRNAs

14. o Ribosome assembly