Download Compartimentation, biological membranes

II. INTERNAL ORGANIZATION
OF EUKARYOTIC CELLS
Structure of a „typical” animal cell
Structure of a „typical” plant cell
II.1.4. Nucleus
Biological significance: storage, expression and transmission
of the genetic information.
Segmented nuclei of leukocytes
neutrophils
Structural components
II.1.4.1. Nuclear membrane (envelope)
cytoplasm
nuclear membrane
nuclear lamina
chromatin
nuclear pore
Biological significance: limiting layer between the inner part of the
nucleus and the cytoplasm. Specific transport
of micro- and macromolecules.
The nuclear envelope is formed from two concentric membranes
that are continuous with the endoplasmic reticulum (ER). The
space between the two membranes is the perinuclear space
which is continuous with the lumen of the ER.
The inner nuclear membrane contains proteins which enable it to
bind to the nuclear lamina (network of intermediate filaments),
which binds the chromatin.
The double-membrane envelope is penetrated by nuclear pores.
Nuclear pore
Construction
EM image
surface wiev
side wiev
Relationship of the nucleus and the cytosol
II.1.4.2. Chromatin
The chromatin is composed of deoxyribonucleic acids and proteins. In interphase
they are arranged in a relatively loose , „network” form. EM shows that there are
two different forms of the chromatin, one appearing as a light element
(euchromatin), the other one being dark (heterochromatin).
Euchromatin:
Here the chromatin is loosely
arranged. The DNA double helices
are partially split to single strands
by RNA polymerases.
Heterochromatin:
Closely packed chromatin .
The DNA is organized by
histon and non-histone
proteins.
Euchromatic nucleus of neuron
Structure
The chromatin is a DNA-protein complex About half of the chromatin is protein:
they are either histones or non-histone proteins.
Histones are small, positively charged proteins, which bind non-specifically to
DNA end compensate the negative charge of phosphate groups of the DNA.
Histone classes: H1, H2A, H2B, H3 and H4.
1. Level of organisation: Nucleosome
H2A, H2B, H3 and H4 (two molecules of each) are
building the Histone-Octamer . The DNA double
helix makes two turns around this octamer (166
base pairs). H1 binds to theis complex.
(packing level: 6-7x)
Octamer
H1
2. Level of organisation : 30 nm chromatin fiber
The nucleosomes are spirally organized around the H1 core (packing level: ~ 40 x)
This structure is broken to segments by binding of non-histone proteins (e.g. gene
regulator proteins).
3. Level of organisation: 300 nm chromatin fibers
20 000-30 000 base pairs
30 nm
chromatin fiber
loop domains
Chromosomes
W. Waldeyer, 1888:
" …thick chromatin structures which can be seen during mitosis and
meiosis...„
Name: chroma: color and soma: body
Boveri and Sutton, 1905. Chromosome theory: the chromoses are
the carriers of the genetic information..
Chromosomes exists only during cell division as microscopically
detecteble units.
At the start of the mitosis (M phase) each chromose has replicated
and consists of two sister chromatids joined together
Fine structure
Chromatids: 300 nm chromatin fiber form spirals (packing level: 10 000 x)
700 nm
300 nm
Staining of the chromosomes
• Fluorescent stains (e.g. Quinacrin):
Q-bands
specific for AT-rich DNA regions
• Giemsa-staining:according to the technique applied,
either
G-bands ("Giemsa-positiv")
specific for AT-rich DNA-regions
or
R-bands ("Giemsa-negativ")
specific for GC-rich DNA-regions
Chromosomes
The human karyotype
Chromosomal organization of different species
baker’s yeast
corn
rice
wheat
tobacco
32
20
24
42
48
fruitfly
house fly
ant
frog
carp
dog
cat
mouse
rat
cattle
rhesus monkey
human
8
12
48
26
104
38
64
40
42
60
42
46
Variability in chromosomal organization.
Chromosomal organization (karyotype) can show high
variability even in closely related species.
Reeves
deer
The number of
genes is equal!
Indian
deer
Structure of the chromosomes
X
4
5
Scanning-EM,
human
chromosomes
Schematic
view of
chromosomal
regions
2
1
1
2
sister
chromatids
5
4
1 2 31
1
2
3
centromer
1
1
2
3
shorter
arm „p”
longer
arm „q”
T
e
l
o
m
er
Telomers, aging, cancer (and Dolly)
Human telomer: 15 000 base pairs of repeated TTAGGG DNA
sequences. Shortening: 50-200 base pairs/division.
In germline cells: telomere terminal transferase (telomerase):
ribonucleoprotein reverse transcriptase.
Binding of the chromosomes to the mitotic spindle
Attachment site:
kinetochor at the
centromer
Preparation of chromosomes, karyotype
determination
Blood
colchicin
blood cells
culture (phytohaemagglutinin)
separation of erythrocytes and white
fixation
staining
microphoto
arrange
according to
size, centromer
position and
banding
pattern