1. CELLULAR CHECKPOINTS
AND MITOSIS
Aballe, Karen Marie L.
Lamberte, Joshua L.
Ateneo de Zamboanga University
1.31.2013
2. WHY MITOSIS?
- The main purpose of mitosis is to segregate SISTER
CHROMATIDS into two nascent cells, such that each
daughter cell inherits one complete set of
chromosomes.
- Dividing cells progress through a series of phases
known as interphase and the mitotic phases
namely, Prophase, Prometaphase, Metaphase,
Anaphase, Telophase.
3. INTERPHASE
• Interphase is the phase of the
cell cycle in which the cell
spends the majority of its time
and performs the majority of
its purposes including
preparation for cell division.
• increases its size and makes a
copy of its DNA
• considered to be the 'living'
phase of the cell.
• CELL CYCLE CHECK POINTS
(G1, S, G2)
4. NORMAL CELL CYCLE
• A combination of 2 substances signal the
cellular reproduction.
1. Cyclins (Proteins)
2. CDKs (Cyclin-dependent kinases)-
enzymes.
• Different cyclin/CDK combinations signal
other activities, including DNA replication,
protein synthesis, and nuclear division
throughout the cell cycle.
5. A simplified view of the core of the
cell-cycle control system
Cdk associates successively with
different cyclins to trigger the different
events of the cycle.
Cdk activity is usually terminated by
cyclin degradation.
6.
7. G1 (Gap 1)
- The cell grows in size and synthesizes mRNA and
proteins that are required for DNA synthesis.
- G1 phase and the other subphases of the cell cycle
may be affected by limiting growth factors.
- G1 phase is particularly important in the cell cycle
because it determines whether a cell commits to
division or to leaving the cell cycle.
- *While in the G1 phase of the cell cycle, a cell is
diploid or 2n
8.
9. S phase (Synthesis)
• S-phase (synthesis phase) is the part of the cell
cycle in which DNA is replicated.*
• Damage to DNA is detected and fixed during S-
phase.
10. G2 (Gap 2 of pre-mitotic phase)
• G2 phase is a period of rapid cell growth and
protein synthesis during which the cell readies itself
for mitosis.
• Cyclin B-cdc2 complex activation causes
breakdown of nuclear envelope and initiation of
prophase.
11. MITOSIS?
• is the process by which a cell separates the
chromosomes in its cell nucleus into two identical
sets in two nuclei.
• It is generally followed immediately by cytokinesis,
which divides the nuclei, cytoplasm, organelles and
cell membrane into two cells containing roughly
equal shares of these cellular components.
• Only comprises 10% of the cell cycle.
12. PROPHASE
-Chromatin in the nucleus
begins to condense and
becomes visible in the light
microscope as chromosomes
- The nucleolus disappears.
- Centrioles begin moving to
opposite ends of the cell
- In the prophase, the
chromosome double, from 46
(23 pairs) chromosomes to 92
(46 pairs)
- The chromosomes are
attached to each other at
the center, forming an x;
these are known as
centromeres
13. • Chromosomes condense
to form compacted
mitotic chromosomes.
• - HOW?
• Activated M-Cdk
phosphrylates condensin
subunits, triggering the
assembly of condensin
complexes on DNA and
condensation of the
chromosome. The
condensin can use energy
of ATP hydtolysis
topromote DNA coiling(in
vitro)
14. • The sister chromatids are glued
together by multisubunit protein
complex called cohesins.
• Centrosome duplicates at S, and
separates to form mitotic spindle
at the beginning of prophase
15. PROMETAPHASE
The role of prometaphase is completed when all of the kinetochore
microtubules have attached to their kinetochores, upon which
metaphase begins.
16. • Golgi, ER etc.
disperse to form
vesicles;
kinetochore
assembly
17. METAPHASE
• the chromosomes align along the cell equator.
• Metaphase accounts for approximately 4% of the
cell cycle's duration.
18. SPINDLE ASSEMBLY
CHECKPOINT prevents
separation of the
duplicated chromosomes
until each and every
chromosome is properly
attached to the spindle
apparartus.
The main components of
the SAC are mitotic arrest
deficient 1 (MAD1) and
MAD2, BUBR1 BUB1 and
BUB3.
.
19. ANAPHASE
Loss in sister-
chromatid
cohesion.
ANAPHASE A –
Sister-chromatids
are pulled towards
the poles.
ANAPHASE B –
Poles separate
towards the cell
cortex.
20. Securin – its
degradation causes the
separation of the sister-
chromatid (yeast) in
which its degradation is
caused by ubiquitin-
dependent proteolysis.
Separase – prevented
by the securin from
abolishing sister
chromatid cohesion.
Cohesin – multiprotein
complex which
exhibits the sister
chromatid cohesion.
21. • SPINDLE POSITIONING CHECKPOINT ensure the
arrival of one end of the mitotic spindle in the
nascent daughter cell.
• It is coordinated by the Elm1 kinase
22. Degradation of M-cyclin and removal
of phosphates by phosphatases
TELOPHASE
allows telophase and cytokinesis.
Chromatids arrive at opposite poles
of cell, and new membranes form
around the daughter nuclei.
Spindle disassembles and the nuclear
envelope reforms.
Chromosome uncondense, nuclear
envelope reforms around the
individual chromosome, Golgi and ER
reconstruct, nucleolus reassemble
23. CYTOKINESIS
Cytokinesis results when a fiber
ring composed of a protein
called actin around the center of
the cell contracts pinching the
cell into two daughter cells, each
with one nucleus.
ensure that chromosome number
is maintained from one
generation to the next.
Regulation of the cell cycle involves processes crucial to the survival of a cell, including the detection and repair of genetic damage as well as the prevention of uncontrolled cell division. The molecular events that control the cell cycle are ordered and directional; that is, each process occurs in a sequential fashion and it is impossible to "reverse" the cycle.
The cell cycle control system is based on cyclically actived protein kinases---cyclin-dependent kinases (Cdks). Engine molecules for cell cycleCyclins form the regulatory subunits and CDKs the catalytic subunits of an activated heterodimer; cyclins have no catalytic activity and CDKs are inactive in the absence of a partner cyclin. When activated by a bound cyclin, CDKs perform a common biochemical reaction called phosphorylation that activates or inactivates target proteins to orchestrate coordinated entry into the next phase of the cell cycle. Different cyclin-CDK combinations determine the downstream proteins targeted. CDKs are constitutively expressed in cells whereas cyclins are synthesised at specific stages of the cell cycle, in response to various molecular signals
Paul Nurse – Nobel Prize Laureate for discover of Cdk-Cyclin interaction
* This means that it contains two sets of the organism’s DNA and that the DNA hasn’t been copied in preparation for cell division. The genetic material exists as chromatin, or loosely bundled strands of DNA.
Precise and accurate DNA replication is necessary to prevent genetic abnormalities which often lead to cell death or disease.*all of the chromosomes have been replicated, i.e., each chromosome has two (sister) chromatids. Thus, during this phase, the amount of DNA in the cell has effectively doubled, though the ploidy of the cell remains the same. During this phase, synthesis is completed as quickly as possible due to the exposed base pairs being sensitive to external factors such as any drugs taken or any mutagens (such as nicotine).
This condensation is needed to allow the chromosomes to move along the mitotic spindle without becoming tangled or broken during their distribution to daughter cells.
Condensins are required for chromosome condensation in extracts of mitotic cells and appear to function by wrapping DNA around itself, thereby compacting chromosomes into the condensed mitotic structure
**Since the genetic material has been replicated in the prior interphase of the cell cycle, there are two identical copies of each chromosome in the cell. Those copies are called sister chromatids and they are attached to each other at a DNA element present on every chromosome called the centromere. Also during prophase, giemsa staining can be applied to elicit G-banding in chromosomes.
This even alignment is due to the counterbalance of the pulling powers generated by the opposing kinetochore microtubules,[2] analogous to a tug-of-war between equally strong people, ending with the destruction of B cyclin.One of the cell cycle checkpoints occurs during prometaphase and metaphase. Only after all chromosomes have become aligned at the metaphase plate, when every kinetochore is properly attached to a bundle of microtubules, does the cell enter anaphase. It is thought that unattached or improperly attached kinetochores generate a signal to prevent premature progression to anaphase, even if most of the kinetochores have been attached and most of the chromosomes have been aligned. Such a signal creates the mitotic spindle checkpoint. This would be accomplished by regulation of the anaphase-promoting complex, securin, and separase.
Movement is mediated by the kinetochoremicrotubles, which push and pull on the chromosomes to align them into what is called the metaphase plate. Chromosomes on the metaphase plate are held there tightly by pushing and pulling forces from the microtubules.To achieve proper segregation, the twokinetochores on the sister chromatids must be attached to opposite spindle poles (bipolar orientation). Only this pattern of attachment will ensure that each daughter cell receives one copy of the chromosome.
If the spindle is misoriented in anaphase, a surveillance mechanism named the spindle position checkpoint (SPOC) comes into play to delay mitotic exit until the spindle resumes the correct orientation. The SPOC imposes this delay by inactivating the mitotic exit network
