Zellbiologie

M Phase: Mitosis (division of nucleus) and cytokinesis (cell splits in two) During mitosis, there is a so
called “checkpoint” which makes sure, that the replicated chromosomes are properly attached to the
mitotic spindle before the spindle pulls the chromosomes apart and distributes them into the two
daughter cells. The three following phases (G 1 , S, G 2 ) all belong to the interphase, during which the cell
increases its size. At the end of the interphase, the next M phase starts.

G 1 (Gap) Phase: Check, whether the environment is favourable (i.e. sufficient nutrients and specific
signal molecules present) for cell proliferation. If it is, the cell goes on to the S phase. If not, the
progress through G 1 can be delayed by the cell and it can even enter a specialised resting state (G 0 ).

S (synthesis) Phase: The cell replicates its
nuclear DNA

G 2 Phase: Check, whether all damaged DNA
has been repaired and DNA replication is
complete. Only if everything is ok, the cell enters mitosis

- Prophase. Condensation of the replicated chromosomes and assembly of the mitotic spindle.
- Prometaphase: Breakdown of nuclear envelope, chromosomes attach to spindle microtubules via their
kinetochores.
- Methaphase. Chromosomes are aligned at the equator of the spindle, the paired kinetochore
microtubules on each chromosome attach to opposite poles of the spindle
- Anaphase: Sister chromatis are separated and pulled toward the spindle pole they are attached to.
Spindle poles also move apart.
- Telophase: Chromosomes arrive at the poles of the spindle and a new nuclear envelope reassembles
around them, assembly of contractile ring begins.
-Cytokinesis: Cytoplasm is divided in two by the contractile ring.

Condensins: Protein complexes that help carry out chromosome condensation. They make the mitotic
chromosomes more compact, reducing them to small physical packets that can be more easily segregated.

Contractile Ring: Consists mainly of actin filaments and myosin filaments arranged in a ring around the equator
of the cell. It starts to assemble just beneath the plasma membrane toward the end of mitosis. As the ring
contracts, it pulls the membrane inward, thereby dividing the cell in two.

here are several key proteins and protein complexes which are
cyclically activated and deactivated during the cell-cycle in order to initiate or regulate DNA replication, mitosis
and cytokinesis.

To stop the cell cycle at a specific point, the cyclins which promote the processing to the next phase are either
marked with ubiquitin chains and destroyed by proteasomes or the assembly or activity of cyclin-Cdk
complexes is blocked by Cdk inhibitor proteins.

S-Cdk: During early G 1 phase, a regulatory protein called Cdc6 binds together
with other proteins to an ORC (origin recognition complex), which is bound to
the origin of replication on the DNA.

p53: DNA damage found by the G 1 checkpoint causes an increase in both the
concentration and activity of the protein p53

M-Cdk: Triggers the condensation of the replicated chromosomes into compact, rod-
like structures, readying them for segregation and also induces the assembly of the
mitotic spindle that will separate the condensed chromosomes

Necrosis: Cell death due to unexpected and accidental damage.

Apoptosis (Programmed Cell Death): The cell shrinks and condenses. The cytoskeleton collapses, the nuclear
envelope disassembles, and the nuclear DNA breaks up into fragments. Most importantly, the cell surface is
altered

Caspase: A protease, which is usually held inactive (procaspases) by some
prodomains until there is a signal that induces apoptosis

Embryonic    stem    cells    (ESC)    are    pluripotent,    i.e.    they    can    differenBate    into    
all    cell    types    (expect    placenta)    of    the    three    cell    layers    and    form    teratomas    
if    transplanted    into    animals/humans   

Many    cancer    cells    mutate    at    the    p53    protein    circumvenBng    DNA    damage    
protecBon

Induced    pluripotent    cells        (iPS)    are    highly    similar    to    ESC    and    are    potenBally    
very    useful    for    the    regeneraBon    of    may    Bssue    types

Cancer    cells    arise    from    accumulaBon    of    several    mutaBon    events    in    a    single    
somaBc    cell    lineage

Integrated in the plasma membrane of the cell binds huge, hydrophilic signal
molecules. The three main classes are ion-channel-coupled receptors, G-protein-coupled receptors and
enzyme-coupled receptors.

Intracellular Receptors: In the cell plasma or even inside the nucleus bind small, hydrophobic signal
molecules which are able to pass the plasma membrane.

neurotransmitter acetylcholin

Intracellular signaling proteins can relay, amplify, integrate and distribute the incoming signal

cAMP cGMP

Ion-Channel-Coupled Receptors: Open/close as a reaction to the binding of a
signal molecule and allow/permit a flow of ions across the plasma
membrane

G-Protein-Coupled Receptors: When a signal molecule binds, they activate a protein (G-protein) which binds
GTP, which itself activates either an enzyme or an ion channel in the plasma membrane, initiating a cascade of

G-Protein: Consists of three subunits (α, β, γ) which can detach after the binding of GTP to the α-subunit and activate different target proteins. The G-protein is inactivated if it hydrolyses its bound GTP molecule

After binding a signal molecule, they either act themselves as enzymes or associate with enzymes inside the cell to activate a variety of intracellular signaling pathways.

• Receptor Tyrosine Kinase (RTK): Largest class of enzyme coupled receptors. The activation of an RTK stimulates the assembly of an intracellular signaling complex
• Ras: A small GTP-binding protein that is activated by almost all RTKs
• PI 3-kinase (phosphoinositide 3-kinase): An enzyme which is recruited by RTK if this RTK is activated by an extracellular survival signal

Intermediate Filaments: Ropelike fibres with a diameter of around 10 nm. One type of intermediate filament forms a meshwork called the nuclear lamina just beneath the inner nuclear membrane. Other types extend across the cytoplasm, giving cells mechanical strength. In an epithelial tissue, they span the cytoplasm from one cell-cell junction to another,  thereby strengthening the entire epithelium.

Microtubules: Long, hollow cylinders made of the protein tubulin. With an outer diameter of 25 nm, they are much more rigid than actin filaments. Microtubules are long and straight and typically have one end attached to a single microtubule-organising centre (MTOC) called a centrosome. Microtubule have a polarity: the α-tubulin end (minus end) is usually found in the centrosome and the β-end (plus end) is usually outside the centrosome.

Growing of Microtubule: The centrosome contains many γ-tubulin rings, which serve as nucleation site for the
microtubule. It grows outward from the organising centre by addition of free, GTP containing αβ-tubulin
subunits (strong binding). The GTP is hydrolysed shortly after binding, the end of a growing microtubule is
therefore composed entirely of GTP-tubulin subunits (GTP-cap). As long as this GTP-cap is present, the
microtubule will continue to grow.