Biology
7. Nucleus, cell division, and cell death
Doc. RNDr. Jan Hošek, Ph.D.
hosekj@pharm.muni.cz
Department of Molecular Pharmacy
FaF MU
Genophores
• Nucleoid
• Plasmids
• Nucleus
• Mitochondrial and
chloroplast DNA
2
https://sciencenotes.org/prokaryotic-vs-eukaryotic-similarities-and-differences/
Prokaryotic „nucleus“ - nucleoid
• Formed by DNA, RNA and proteins
• It is NOT covered
• Usually 1 circular dsDNA molecule
• DNA is strongly condensated and
organised into 3D structure by
proteins (Nucleoid-associated
proteins - NAPs)
3
https://www.sciencephoto.com/media/209697/view/c
oloured-tem-of-dna-from-e-coli-bacterium
https://doi.org/10.1038/nrg3375
Plasmids
• Extrachromosomal circular dsDNA
• occurrence in many bacterial species
• 1,000 to 200,000 bp in size
• carry only genes encoding secondary features (e.g.,
resistance to antibiotics)
• autonomous replication
• replication cycle synchronized or unsynchronized
• must contain its origin of replication (ori locus)
4
Plasmid types
• F-plasmid
– fertilization plasmid for "sexual"
reproduction of bacteria
– transmitted by conjugation
• R-plasmid
– carries antibiotic resistance genes
• Col plasmids
– they carry genes for the production of
bactericidal peptides
• Plasmids with genes for
metabolisation of atypical
substrates
• Plasmids with genes for virulence
Organisation of eukaryotic nucleus
• Outer membrane – bound on rough
ER
• Inner membrane – binds lamines –
DNA-binding proteins
• Nuclear pores
• Nucleolus
– Genes for rRNA
– Functional region formed by sequences of
satellites of acrocentric chromosomes
• Nucleoplasm – contains DNA and
proteins 6
https://www.wikiskripta.eu/w/Bun%C4%9B%C4%8Dn
%C3%A9_j%C3%A1dro#/media/Soubor:Diagram_hum
an_cell_nucleus.svg
Organisation of eukaryotic nucleus
DOI: 10.3389/fcell.2021.761469
Chromosomes
Chromosome consists of chromatin. It is formed:
❖ long linear molecule of DNA
❖ proteins, which are bound to DNA
- helps DNA to be packed (histones)
- participate on gene expression
- participate on replication and DNA correction
DNA is divided into sets of chromosomes. Genes are
stored on chromosomes linearly in a precise position =
locus
Chromosomes look different in interphase (loose)
and mitosis (highly condensed)
Chromosomes
We distinguish between homologous (autologous) chromosomes
that are paired. Humans have 22 pairs of chromosomes+ XX nebo XY
X and Y = sex (non-homologous ;
heterologous) chromosomes
Human chromosomes in mitosis. The colors used usually distinguish
sequences rich in A-T pairs from sequences with C-G pairs.
One chromosome in a pair
is always paternal (P), the
second maternal (M)
Chromosomes
Artificially arranged
chromosomes of one cell
into pairs =
KARYOTYPE
How chromosomes look like? – update VI/2024
Barley (Hordem vulgare) mitotic metaphase chromosomes
observed by A-ESEM, secondary electron detector. (a)
Overview of a chromosome with protrusions covering it’s the
entire body, including centromeric region, top view. (b)
Histogram of chromosome length distributions as determined
using A-ESEM (95 measurements). (c) Detailed view of the
protrusions on the terminal telomeric chromosome region,
with the sizes of the protrusions indicated (yellow bars). (d)
Histogram of the protrusion widths (183 measurements). (e)
Close-up of a chromosome region showing ~ 12 nm features,
which may represent nucleosome fibers. (f,g) The ~ 12 nm
features form ~ 37 nm structures (yellow bars), whose
molecular composition is not clear (see the text for more
details).
Human chromosome banding
Chromosomes are stained in early stage of mitosis
(condensed)
Based on the centromere position we distinguish:
metacentric; submetacentric; acrocentric
Short arm = p (petit) Long arm = q (queue)
Giemsa staining . Dark bands = high
content of A-T base pairs
The protrusions contain genes for
large ribosomal RNA (rRNA)
➢ Location of chromosomes in the nucleus is not random
➢ Clustering of areas with the same function and activity
DOI: 10.1038/nsmb.2474
E. Lieberman-Aiden et al., Science 326, 289-293
(2009)
https://unlockinglifescode.org/the-genome-ball
A cell reproduces by carrying out an ordered sequence of
reactions = CELL CYCLE
It is the basic mechanism by which all living
things reproduce.
Each cell comes from only one other cell.
Cell doctrine R.Virchow 1858
Cell cyclus
❖ doubling of cell mass
❖ cell genome replication
❖ own division of the mother cell into two
daughter cells
This is how genetic information is transferred
to the next generation of cells
The cell cycle includes the events that occur:
Cell division in multicellular organisms does not only
occur during the formation of a new individual, but
also during life, with different types of cells at
different rates
They usually do not divide at all: nerve, muscular cells
Minimal rate of division: hepatocytes (1x per year)
They divide intensively: gut epithelial cells, blood stem cells (more than 1x
a day), cells of hair folikul
Each of us creates a million new cells every
second, the stop dividing leads to death.
It has a single circular chromosome
that is attached to the plasma
membrane and remains attached
during chromosome replication.
Both chromosomes are separated by
cell growth. The cell wall and the
plasma membrane are inserted
between the two chromosomes → two
cells are formed.
= BINAR DIVISION
Bacterial cell division (E. coli)
https://genesdev.cshlp.org/content/12/7/1036/F6.expansion
https://www.sciencephoto.com/
media/12477/view/tem-of-
dividing-e-coli-bacterium
M-PHASE + INTERPHASE (G1, S, G2)
G1 and G2 phases
are the phases when the cell grows and the cytoplasmic organelles duplicate
Cell cycle of eukaryotic cells
https://www.vce.bioninja.
com.au/unit-one/area-of-
study-1-cell-develo/cell-
cycle.html
S-phase (syntetic)
❖ nuclear DNA replication
❖ histone synthesis
G1-phase (presyntetic)
duplication processes of ribosomes, ER, mitochondria, synthesis of enzymes,
nucleotides take place
M-phase
(mitotic)
G2-phase (postsyntetic)
❖ proteins, RNA synthesis
Mitosis (karyokinese) = nukleus
division
Cytokinesis = cytoplasmic division
❖G0-phase (quiescent)
❖ only basal metabolism
maintained
❖ It occurs only in some types of
cells, especially those that are
already terminally differentiated
(neurons, erythrocytes)
INTERPHASE
Cell cycle of eukaryotic cells
The length of the cell cycle varies
Cell type Length of cell cycle
Cells of an early frog embryo 30 min
Yeast cells 1,5 – 3 h
Intestinal epithelial cells 12 h
Mammalian fibroblasts in culture 20 h
Human hepatocytec 1 year
Control of cell division and cell growth
Cell division and growth is regulated by extracellular
signaling molecules (usually peptides), which mediate their
effect through specific receptors.
❖ Mitogens
These proteins can be divided into three main classes:
❖ Growth factors
❖ Survival factors
❖Mitogens – stimulate cell division
by triggering G1/S-Cdk activity, which
"unlocks" intracellular negative
control mechanisms that block cell
division without these mitogens.
❖Growth factors – they stimulate
cell growth (increase the cell mass) by
promoting the synthesis of proteins
and other macromolecules and
inhibiting their degradation.
❖Survival factors – they help the
cell to survive by suppressing
events leading to apoptosis.
ENTRANCE TO
G1 PHASE
Presence of
mitogens and
growth factors
Presence only of
growth factors
ENTRANCE TO G0
PHASE
The cell cycle must be well regulated and coordinated
in a multicellular organism → ensuring continuity
and sequence of individual steps and processes
❖ activate and inactivate the relevant enzymes
❖ enable cell cycle regulation through chemical signals
(signal molecules)
❖ use of so-called molecular brakes (Rb-protein, p53,
p21) to stop the cell cycle at so-called checkpoints
Animal cells have an intrinsically limited
number of cell divisions they can go through
This phenomenon is called cellular senescence, and the length
varies from cell type to cell type. Even though the relevant
factors are present, the cell stops responding to them.
The gradual shortening of telomeres (small structures at the end
of chromosomes) with each cell division is considered the most
important cause of cell aging. A cell is unable to replicate telomeres
without the enzyme telomerase.
Some cells do not have this enzyme at all, or its activity may change
due to age.
Checkpoints of cell cycle
1) before entering S-phase
= initiation of DNA replication
→ DNA REPLICATION
Cell cycle control is ensured
by three checkpoints:
Is the envinment hospitable?
Is the cell big enough?
Isn't the DNA damaged?
Checkpoints of cell cycle
2) before entering M-phase
= mitosis initiation →
FORMATION OF THE
MITOTIC SPINDLE
Is the environment hospitable?
Is all DNA replicated?
Cell cycle control is ensured
by three checkpoints:
Checkpoints of cell cycle
3) At the interface of
metaphase / anaphase =
anaphase initiation→
completion of DIVISION of
the nucleus and
subsequently the cell
Are all chromosomes attached
to the spindle?
Cell cycle control is ensured
by three checkpoints:
Cell cycle control
It is done by activating and deactivating the
respective CYCLIN-DEPENDENT KINASES.
They are activated by regulatory proteins
„CYCLINES“
These activated kinases then catalyze the
phosphorylation of the respective proteins and control
the passage of the cell through the phases of the
cycle.
After cyclins form a complex with Cdk, the kinase is activated and able to
initiate the appropriate part of the cell cycle.
Without cyclins, Cdk is inactive.
Cell cycle control
• Entry into the individual phases of the cell cycle is determined
by the concentration of cyclins and the activity of CdK
https://mysciencesquad.weebly.com/ib-hl-16u5.html
https://facts.net/science/biology/15-astounding-facts-about-cyclin-
dependent-kinases-cdks/
Cell cycle control
https://doi.org/10.3390/ijms21061960
INTERPHASE
mitotic profase mitotic
prometaphase
mitotic
metaphase
mitotic
anaphase
mitotic
telophase
MITOSIS – individual phases
early mitotic
prophase
late mitotic prophase
(prometaphase)
Prophase
Condensation of
chromosomes occurs
A mitotic spindle begins to
form outside the nucleus
Briefly about the individual phases of mitosis
Metaphase
Chromosomes group together in the equatorial plane
and thus form the metaphase plate.
Briefly about the individual phases of mitosis
Anaphase
Sister chromatids are separated
Briefly about the individual phases of mitosis
Telophase
A new nuclear envelope forms around each set of
chromosomes and two daughter nuclei are formed
Briefly about the individual phases of mitosis
Prophase
1. At the end of S-phase, DNA replication is finished, the centrosome is also
duplicated (first they are together at one pole)
2. Condensation and spiralization of chromosomes (50,000x shortening). Sister
chromatids are joined together along their entire length.
Participation of the structure of the cytoskeleton: centrosome, microtubules, kinetochore and
molecular motors: kinesin, dynein
Prophase
3. Both centrosomes begin to move to opposite poles of the nucleus –
the movement occurs along microtubules and is controlled by
molecular motors. ATP is consumed in the process.
Prophase
4. A bundle of microtubules is
organized around each
centrosome (at each pole).
These interact to form the
mitotic spindle.
Kinetochore = the protein structure through which the chromosomes are
attached to the mito. spindle – is completed in prometaphase
Prometaphase (late prophase)
1. Breakdown of the nuclear envelope into membrane vesicles.
It begins with the phosphorylation of nuclear lamins (= protein subunits of
intermediate filaments) and the subsequent disintegration of the nuclear lamins
Nuclear lamins are located under the nuclear envelope (stabilize it). This brings
the spindle microtubules into contact with the chromosomes.
Prometaphase
2. Chromosomes attach to the
microtubules of the mitotic
spindle
Microtubules attach to chromosomes
via special protein complexes called
kinetochores.
Kinetochores form on
chromosomes during late
prophase.
Each sister
chromatid has its
own kinetochore in
the region of the
centromere, which
connects it to the
kinetochore
microtubule.
The kinetochore is encoded by a special centromere DNA sequence.
Its removal means that kinetochores cannot form and chromosomes
cannot segregate correctly during mitosis
20-40 microtubules
are attached to the
human kinetochore
Metaphase
1. The beginning of metaphase is defined by the formation of the metaphase plate.
Chromosomes are aligned in the equatorial plane between the poles. Also, the kinetochores of
all chromosomes are aligned in a plane.
Metaphase
2. The chromosomes in the
metaphase plate are held
together by considerable force.
Both microtubular molecular motors (motor proteins) and the gradual growth and
degradation of microtubules (tubulin units are either added or removed, leading to
movement) are involved in the creation and maintenance of this state.
Colchicine = mitotic spindle poison blocks the addition of microtubule subunits
Anaphase
1. The connection between sister chromatids is broken by proteolytic enzymes.
Each chromatid (daughter chromosome) moves toward the spindle pole to
which it is attached.
Anaphase
2. This segregation of
chromosomes leads to the
division of chromosomes into
two identical sets at opposite
ends of the mitotic spindle.
Chromosome movement speed 1μm per minute.
Movement is the result of two independent
processes (anaphase A - anaphase B)
Three types of mitotic spindle microtubules
Kinetochore microtubules connect chromosomes to both poles
Chromosomes are
pulled forward
The poles move away
from each other
A: Molecular motors (motor proteins) of the kinetochore
"walk" even with the attached chromosome along the
kinetochore microtubule
Chromosomes are
pulled forward The poles move away
from each other
B: Motive forces provided by sets
of "molecular motors"
they are on long polar microtubules
they are on microtubules extending from the spindle pole
Chromosomes are
pulled forward
The poles move away
from each other
B: The spindle poles move away is accompanied
by the elongation of the polar microtubules - at
their plus ends, new subunits polymerize
Telophase
A new nuclear envelope begins to form around each set of chromosomes
and two daughter nuclei are formed.
Vesicles of the nuclear membrane cluster around individual chromosomes and then fuse to form the
nuclear envelope. Nuclear lamins that were phosphorylated in prometaphase are dephosphorylated and
reassociate back into the nuclear lamin, which is under the nuclear envelope (has an inner and outer
nuclear membrane)
Telophase
Additional nuclear proteins enter
the nucleus through pores in the
newly formed nuclear envelope
and the nucleus grows.
Chromosomes decondense into the so-called interphase
state, so gene transcription can resume. Mitosis ends.
Cytokinesis
Cytokinesis is the division of the cytoplasm and all its components. It starts
already in anaphase - a dividing groove is formed perpendicular to the
longitudinal axis of the mitotic spindle. In anaphase, the contractile ring also
begins to form.
Cytokinesis
The contractile ring is formed from bundles of actin and myosin filaments. It is
attached to proteins associated with the inner side of the membrane and is able
to exert a great force.
The movement of actin filaments against myosin filaments is similar to the contraction
of a muscle. However, the ring structure is only temporary !! It disappears.
During rapid division,
sometimes
cytokinesis does not
immediately follow
mitosis, the so-called
SYNCYTIUM
(multinucleated cell)
Membranes are then formed
simultaneously in coordinated
cytokinesis =
CELLULARISATION
Cells in animal tissues are usually in firm adhesive contact
with their neighbors, flattened and adhered to the substrate.
As soon as the cell enters the M-phase, the
phosphorylation of integrins (responsible for the mutual
cohesion of cells in tissues) and the weakening of these
interactions and bonds, the cell becomes rounded.
After cytokinesis is completed, the cells flatten again and mutual
adhesion forces are restored. The cell has thus rearranged its
contacts with neighboring cells - this allows the incorporation of new
cells into the tissues.
Differences in cytokinesis in plants
A plant cell not only has a plasma membrane, but
also a solid cell wall.
Daughter cells are not separated by a contractile ring, but by a
newly forming cell wall.
It begins to form at the beginning of telophase and its formation
is controlled by a structure called FRAGMOPLAST.
It is formed from the remnants of polar microtubules in the
equatorial plane of the mitotic spindle.
Small membrane-bound vesicles derived from the Golgi apparatus with
polysaccharides and glycoproteins travel along the microtubules to the
phragmoplast. They are necessary for the formation of the cell wall.
It was described in 1883. Meios = decreasing
It is cell division that takes place during the
formation of gametes (specialized cells intended
for reproduction).
Gametes are HAPLOID = have only one set of
chromosomes. Other human cells are DIPLOID =
have two sets of chromosomes.
From father From mother
Meiosis
Difference between meiosis and mitosis
1) The replicated chromosomes line up randomly at
the metaphase plate
2) The sister chromatids then separate from each
other to form separate chromosomes.
3) The resulting daughter cells each have one copy of
each maternal and one copy of each paternal
chromosome.
= DAUGHTER CELLS ARE DIPLOID AND
GENETICALLY IDENTICAL
1) The replicated chromosomes pair with their homologue
before being arranged in the metaphase plate and create
structures = bivalents, which therefore contain four
chromatids (2x2)
2) The formation of a bivalent enables genetic recombination
between the paternal and maternal parts of the same
chromosome = CROSSING OVER
3) Bivalents diverge towards the poles
Difference between meiosis and mitosis
1) Before the second meiotic division, there is no DNA
replication, nor is interphase present.
2) Sister chromatids separate in the normal way as in
mitosis.
= A TOTAL OF FOUR HAPLOID CELLS ARE FORMED,
WHICH MAY NOT CARRY COMPLETELY IDENTICAL
GENETIC MATERIAL.
Thanks to crossing-over, the father's and mother's genes are
mixed.
Difference between meiosis and mitosis
homology
Non-sister
chromatids
Result
crossing-over
Chiasmats
Meiosis
Meiosis involves two cell divisions:
1st and 2nd meiotic division.
(prophase, prometaphase, metaphase, anaphase, telophase)
DNA replication occurs before the first meiotic division
(S-phase), but not before the second.
Meiosis
1st MEIOTIC DIVISION
The longest stage is prophase, when bivalents are formed. This stage can
last for many years. We therefore distinguish 5 stages of the first prophase:
leptotene, zygotene, pachytene, diplotene and diakinesis
At the end of prophase, the nuclear envelope breaks down, signaling the
beginning of prometaphase.
The remaining stages already take place quickly and similarly to mitosis.
LEPTOTENE: spiralization of DNA strands and chromosome differentiation.
ZYGOTENE: homologous chromosomes move closer to each other and with the help of a special
protein, bivalents are created
PACHYTENE: chromosomes complete spiralization and bivalents are observable as so-called tetrads (4chromatid
complexes. Non-sister chromatids intertwine - the formation of chiasmata (knots). In this
phase, the so-called crossing-over occurs.
DIPLOTENE: protein bonds between
homologous chromosomes loosen and
gradually move apart. Non-sister
chromatids still connected by chiasmata
(knots).
DIAKINEZE: there is a rearrangement
and separation of homologous
chromosomes. Chiasmata move to the
end of chromatids where they disappear
(chiasmata terminalization).
https://www.toppr.com/ask/question/give-an-account-of-prophase-1-of-
meiosis/
Meiosis
1st MEIOTIC DIVISION (heterotypic division)
Replicated cells diverge into daughter cells homologues (bivalents).
Haploid cells are formed.
If the homologues do not separate from each other (= nondisjunction), at
the end of the gamete arise, where one is missing and the other a certain
chromosome resides.
Sister chromatids remain connected (behave as one unit) at all times.
2nd MEIOTIC DIVISION (homotypic division)
Sister chromatids separate into daughter cells only during the
second meiotic division.
https://en.wikipedia.org/wiki/Meiosis#/media/File:Meiosis_Stages.svg
Meiotic S-phase
SEGREGACE
SESTERSKÝCH
CHROMATID V ANAFÁZI II.
SEGREGACE
SESTERSKÝCH
CHROMATID V
ANAFÁZI
Cell death
• Programmed cell
death – controlled
and planned cell
death
• Unprogrammed cell
death – necrosis,
cell death due to
significant stress
https://doi.org/10.3892/wasj.2020.40
https://doi.org/10.3892/wasj.2020.40
Apoptosis
• Controlled cell death - after
the "DIE" signal, the cell
initiates a sequence of steps
leading to the death of the
cell
• Extrinsic pathway – signal
outside the cell (e.g. immune
cells)
• Intrinsic pathway – signal
inside the cell (e.g. DNA
damage)
• Important role of caspases
(cysteinyl aspartate specific
protease)
https://doi.org/10.3390/biom11040534
Condensation and breakdown of chromatin
https://en.wikiversity.org/wiki/WikiJournal_of_
Medicine/Cell_disassembly_during_apoptosis
https://medical-junction.com/apoptosis-vs-necrosis/
https://plos.figshare.com/articles/figure/
_Confirmation_of_apoptosis_mediated_c
ell_death_in_HSC_4_cells_through_obse
rvation_of_A_DNA_laddering_using_DNA
_fragmentation_assay_on_cells_treated_
with_CEB4_for_12_and_24_h_followed_
by_analysis_of_extracted_DNA_on_0_1_
w_v_agarose_gel_electrophoresis_Smea
/416013