Download Chapter 12 – The Cell Cycle – Pages 215

Chapter 12 – The Cell Cycle – Pages 215-229
The Key Roles of Cell Division
1. Explain how cell division functions in reproduction, growth, and repair.
Because of cell division organisms can carry on from one generation to the
next. Cells divide to make new cells for growth and development. Old cells
wear out and must be replaced or repaired and cell division is critical in this
situation. Cells reproduce by cell division.
2. Describe the structural organization of the genome.
The genome is all of the genetic information of a cell, which is found in each
cell in multicellular organisms. The structure is a mass of DNA called
chromatin. The length varies with the species, but in humans if unwound
would be about 3 m. Chromosomes are the genetic information of the
genome and are in pairs. (1 set from each parent) These duplicate
chromosomes are called sister chromatids. A protein keeps them together
which is called a centromere.
3. Describe the major events of cell division that enable the genome of one
cell to be passed on to the daughter cells.
Mitosis is the division of the nucleus and cytokinesis is the division of the
cytoplasm. The chromosomes replicate or duplicate so that when they divide
the same number of chromosomes is maintained in the two new cells as the
original.
4. Describe how the chromosome number changes throughout the human life
cycle.
During meiosis the chromosome number decreases by ½ so that when
fertilization occurs the chromosome number will be reestablished. Meiosis
produces only sperm and egg and does not divide again.
the Mitotic Cell Cycle
5. List the phases of the cell cycle and describe the sequence of events
that occurs during each phase.
The cell cycle consists of Interphase- Which includes G1-first gap-growth
and preparation for the S phase. S phase is synthesis- This is where DNA
replicates – the third phase of interpose is called G2 – growth, formation of
new organelles
Mitosis is nuclear divison
Prophase – Nuclear membrane begins to break down as does the nucleolus –
chromatin shortens and thickens and is more visible as individual
chromosomes. Spindle fibers and centrioles starting to be visible
Prometaphase- a few fragments of nuclear membrane present- Centrioles
have moved to opposite poles. Kinetochores and nonkinetochores
microtubules present
Metaphase – sister chromatids connected at the center by a centromere
which has a kinetochore associated with it- spindle fibers present
metaphase plate in place but may not be visible.
Anaphase – when the sister chromatids split(now chromosomes) move to
opposite poles with the help of the kinetochores and nonkinetochores. (these
also make the cell longer)
Telophase – the nuclear membrane and nucleolus reappear – chromatin starts
to form
Cytokinesis- the division of the cytoplasm should be immediate and two new
daughter cells are formed
6. List the phases of mitosis and describe the events characteristic of each
phase.
Prophase mitotic spindle begins to form in the cytoplasm –this is made
from microtubules in the cytoskeleton and other proteins specifically
tubulin is one of these proteins.
The rest as in #5
7. Recognize the phases of mitosis from diagrams and micrographs.
(there will be diagrams on this test)
8. Draw or describe the spindle apparatus, including centrosomes,
kinetochores microtubules, nonkinetochore microtubules, asters, and
centrioles (in animal cells)Centrosome-doesn’t have a membrane but helps
with the organization of the cell’s microtubules(also called microtubuleorganizing center0 Centrosomes replicate in interphase and will be called
spindle poles at the end of prometaphase and moved to opposite poles.
Centrioles are only found in animal cells-they are located in the center of
the centrosome(not necessary for cell division) obviously plants don’t have
them
Sister chromatids visible in prophase are attached by a centromere which
also has a kinetochore(proteins and specific sections of chromosomal DNA)
Spindles attach to the kinetochores at the end of prometaphase. Each end
of chromosme pulls and so it is a tug-of-war. Since not all of the
microtubules are attached and they have an in interaction with
nonkinetochore microtubules from the opposite poles. These overlap during
metaphase and the centromeres line up in metaphase called the metaphase
plate.
Asters –microtubules extend from the centrosomes in the shape of a star,
hence the name which means star.
9. Describe what characteristic changes occur in the spindle apparatus
during each phase of mitosis.
Starts in Prophase- The formation starts with the centrosome which is
associated with the centrioles in animal cells but not plant cells. This area is
the organization area, called the "microtubule-organizing center"
Centrosomes(during interphase) replicate and when they move to opposite
poles of the cell as they start at the nucleus, the spindle fibers grow from
the centrosomes. The name of centrosomes changes here to spindle poles.
(end of prometaphase) Attached to the center of the sister chromatids are
centromeres which also has a kinetochore also made of proteins and some
choromsomal DNA that seem to regulate the pulling of chromosomes. There
are kinetochores that aren't associated with the centromeres and they are
called nonkinetochores. First the kineochores play a "tug of war" until they
reach the center. The nonkinetochores are also attached to microtubules
but these tend to overlap at the midpoint which is called the metaphase
plate.(now we are metaphase) As soon as anaphase begins the proteins which
were holding the sister chromatids together are now inactive.
10. Explain the current models for poleward chromosomal movement and
elongation of the cell's polar axis.
Kinetochores have motor proteins that "walk" a chromosome along a
microtubule(like a tight rope, or when you have a rope to guide you along a
path) The microtubules also shorten up depolymerizing(break down into
smaller subunits.)
Nonkinetochore microtubules actually move past each other during anaphase
and they lengthen by adding the protein tubulin.
11. Compare cytokinesis in animals and plants.
In animal cells a cleavage furrow forms and is a pinching in of the two
prospective daughter cells. This starts near the metaphase plate and
proteins actin and myosin work together as a drawstring to pull the two cells
together and then separating into two new cells. In plant cells a cell plate
forms and this is derived from the Golgi Apparatus. The Golgi produce
vesicles which move to towards the center along microtubules and this is the
beginning of the formation of a cell plate. This plate grows and then fuses
with the cell membrane and then two daughter cells are formed.
12. Describe the process of binary fission in bacteria and how this process
may have evolved in eukaryotic mitosis. Regulation of the Cell Cycle.
Originally in the 1960's it was thought that after the circular DNA
replicated that the cell divided and this was binary fission. Now it is
thought that there is an origin of replication and as soon as the DNA
replicates it begins to divide and this goes on quickly. Sort of like what
happens in eukaryotic cells which may show an evolutionary connection to
bacteria and the evolution of eukaryotic cells. Two intermediate examples
of cell division are in dinoflagellates, where the nuclear envelope remains
intact and the replicated chromosomes are attached and then separate as
the cell elongates before cell division. The second example is diatoms which
has an intact nuclear envelope and a spindle within the nucleus separates the
chromosomes.
Evolution, Unity, and Diversity
13. Describe the roles of checkpoints, cyclin, CDk, and MPF in the cell cycle
control system.
CDk's are cyclin-dependent kinases
MPF Maturation-promoting factor or mitosis promoting factor
What causes a cell to divide? The question is starting to be answered as
different molecules are identified and tracked. So now we think that there
are checkpoints in the cycle that either stop or send the go ahead for the
cell to undergo division. The signal to stop can be overridden if all the
necessary events have occurred up to that point otherwise a cell stays in GO
indefinitely.
Other cells that do divide have a cyclic fluctuations. In a normal cell there
is a checkpoint at G1 and G2. Different proteins called protein kinases
fluctuating concentrations of protein, called cyclin-dependent kinases
because the kinase must have a cyclin to be active. MPF is a cyclindependent kinase. There is a high concentration of MPF in G1, S and G2 and
then it falls during mitosis
MPF causes the nuclear envelope to fragment. There are 3 CDk proteins at
the G1 checkpoint.
14. Describe the internal and external factors that influence the cell cycle
control system.
Internal factors are in the cell and until the factor is met the cell will not
proceed in cell division for example there is the APC(anaphase-promoting
complex” Only when all of the kinetochores are attached to the spindle does
this “wait” signal cease. “
External factors can be chemical of physical. If a nutrient is missing a cell
may fail to divide. A growth factor must be present.
PDGF(platelet-derived growth factor) is an example. These promote cell
growth /division when there is an injury. There are growth factors in many
types of cells, growth factors are proteins that stimulate cells to divide.
This brings up the idea of density – dependent inhibiton so the number of
cells will stop the cell from dividing. Anchorage dependence means the cells
need to be attached to divide.
Science as a Process
15. Explain how the abnormal cell division of cancerous cells differs from
normal cell division.
In cancerous cells the checkpoints are not working properly, out of control.
Since cancer is uncontrolled cell growth the large number of cells will
interfere with cells that are ok and eventually cause death if left
unchecked. The external factors and internal factors are not working
properly So cells that are cancerous form a tumor, can grow and enter the
circulatory system and invade other parts of the body. Since there are so
many different checkpoints it is difficult to treat all cancers alike. As
scientists are learning more they are more successful in controlling cancer
because they find the particular way each cancer works.