Download Mitosis and the Cell Cycle

Mitosis and the Cell Cycle - 1
Growth and reproduction are two of the characteristics of life. The cell theory
states "All cells come from preexisting cells by a process of cell
reproduction, or cell division". Cell division is the process by which all the cells
of a multicellular organism are formed during growth and development. Cell
division is used for repair and replacement of cells and tissues during one's
lifetime. Asexual reproduction, a means of making more individuals for many
groups of organisms, is also accomplished by cell division. In our discussion of cell
reproduction, we shall focus on the processes of cell reproduction in eukaryotic
organisms.
We know that all cells of an individual have exactly the same DNA, and their DNA
is found in structures called chromosomes. Each species has a fixed
chromosome number, a number that does not change from generation to
generation. The total of a cell's DNA is called its genome.
The DNA must also stay the same from cell to cell within an organism, so that
when cells divide, new cells formed will have exactly the same DNA as the original
cell. To ensure that chromosomes and DNA remain the same in new cells (the
genome remains constant) when cells divide, it is crucial to have a mechanism that
exactly replicates (duplicates) the DNA of the original cell and distributes the
copied DNA equally to the new cells. To form new cells, we must also separate the
cytoplasm and critical organelles, such as mitochondria and chloroplasts, of the
original cell into the new cells formed.
We will look at the mechanism by which cells duplicate DNA in a later unit. At this
time we shall focus on mitosis, the process by which the duplicated chromosomes
are equally distributed to new nuclei in eukaryotic organisms. The distribution of
the cytoplasm of the original cell into new cells is called cytokinesis.
Before discussing how cells divide, it's probably useful to discuss the
structure of chromosomes and chromosome terms (of which there are
a sufficiency).
Structure of Chromosomes
Chromosomes are composed of DNA and protein, a mixture called chromatin.
DNA, as we know, is a double chain of nucleotides. It's estimated that our human
DNA consists of 140 million nucleotides per cell. Each of our chromosomes is
about 5cm in length. Chromosomes fit into the nucleus because they are coiled.
Every 200 nucleotides coil around a complex of 8 histone protein molecules
forming a nucleosome. Histones are positively charged so that they are
attracted to the phosphate groups of the nucleotides. Nucleosomes further coil
into supercoils.
Mitosis and the Cell Cycle - 2
Some DNA gets highly condensed and is known as heterochromatin.
Heterochromatin may stay so tightly condensed that its DNA cannot be read (or
expressed). The DNA that can be expressed is called euchromatin. How DNA gets
expressed is for a later chapter.
Chromosomes vary in size and shape. The display of chromosomes of any given
individual is known as the karyotype. Karyotypes are made of chromosomes
during the metaphase state of mitosis, when they are particularly distinctive.
Karyotype analysis can be useful for finding chromosome abnormalities, and will be
discussed later.
Human Male karyotype
Mitosis and the Cell Cycle - 3
Chromosome Terms before and after Duplication
• An unduplicated chromosome is one chromosome. A chromosome more or
less consists of two arms that extend from a narrow "centralized" region called
the centromere. The centromere region is often visible when chromosomes
duplicate.
• When a chromosome duplicates, it becomes one duplicated chromosome,
and the two copies remain attached to each other. Note it is still o n e
chromosome.
• The two exact copies of the duplicated chromosome, which remain attached at
the centromere region, are called "sister" chromatids. They are identical to
each other.(Remember this; it is essential!)
• At the centromere region of the duplicated chromosome, there are structures
(made of protein and DNA) called kinetochores. The kinetochores attach to
microtubules of the spindle during mitosis. The kinetochores of the two sister
chromatids face in opposite directions.
•
After the identical sister chromatids are separated during mitosis, each (called
a "daughter" chromosome now) becomes a single unduplicated chromosome
again.
Rule to remember: A chromatid must be attached to its identical chromatid
and the two sister chromatids comprise one duplicated chromosome. "Sister"
chromatids are not two chromosomes. They are one duplicated
chromosome that consists of two identical chromatids. The only time you can
use the word chromatid is when you have the two identical chromatids attached
to each other.
Mitosis is one part of the cell cycle. The cell cycle includes all activities of a
cell from the time it is formed until (and if) it divides or dies.
Mitosis and the Cell Cycle - 4
To summarize - When cells divide:
• We must form two new cells from the original cell.
• The new cells formed must have all of the genetic material for the organism,
so we need a mechanism that exactly replicates (duplicates) the DNA of
the original cell and distributes the copied DNA equally to the new cells.
Mitosis is the process by which the duplicated chromosomes are equally
distributed to new nuclei. We will see how DNA duplication is accomplished
later.
• We must also separate the cytoplasm, and critical organelles, such as
mitochondria and chloroplasts, of the original cell into the new cells formed
so that the new cells can survive, grow and function. The distribution of the
cytoplasm of the original cell into new cells is called cytokinesis.
Mitosis and the Eukaryotic Cell Cycle.
As mentioned, the processes, or events, of cell division can be related to the
normal lifetime of a cell. For our convenience, these events are divided into
"stages" of a cell's cell cycle. The cell cycle starts when a cell is formed and
continues until it divides. Some cells never divide, others are specialized to divide
(especially in plants, where virtually all cell division occurs in specialized tissue
called meristem. Cell division is a brief part of the life cycle; most of the life of a
cell is spent in normal activities of growth and maintenance, a stage called
interphase.
The events of interphase and cell division are:
Interphase
Growth (called G1 or First Gap in the cell cycle)
DNA Duplication (called S for synthesis in the cell cycle)
Preparation for Division (called G 2 )
Note: If a cell never divides, following G1 it will be in a “permanent” state of G0 (or
non-dividing state).
Cell Division (or cell reproduction)
Mitosis, with
Prophase
Prometaphase
Metaphase
Anaphase
Telophase
Cytokinesis
Mitosis and the Cell Cycle - 5
Interphase is divided into the respective activities:
Growth (called G1 or Gap in the cell cycle)
• Normal growth and cell activities
• The chromosomes are stretched out and grainy in appearance. They stain
well, and early on this material was called chromatin.
DNA Duplication (called S for synthesis in the cell cycle)
• DNA duplication forming the duplicated chromosomes
• This event is triggered in the G1 phase. Once started this process can not be
reversed; the cell is committed to divide.
Preparation for Division (called G 2 )
• Condensation of chromosomes is initiated during the G2 phase of the cell
cycle, although condensed chromosomes are not visible until well into
prophase of mitosis.
• Synthesis of materials, such as tubulin, needed for mitosis and cytokinesis
also takes place during G2, as does the
• Duplication of centrioles in those organisms that produce centrioles occurs,
and the microtubule organizing center is initiated.
Interphase
The Stages of Mitosis: Mitosis is a continuum. Humans have decided to
separate the process into stages for the convenience of our discussions. Some
humans even separate the stages into sub-stages and intermediate stages. For
our purposes, cell division in eukaryotes involves three events:
DNA Duplication
• Process of duplicating the genetic material of the nucleus
• This occurs during Interphase, when growth and/or normal metabolic activities
take place.
Mitosis
• Process of distributing the duplicated DNA equally to the two new nuclei.
Cytokinesis
• Process of separating the cytoplasm contents
Mitosis and the Cell Cycle - 6
Prophase
Chromosome Condensation
The duplicated chromosomes continue condensation and become visible as
threadlike structures. Chromosomes continue condensing and thickening as
prophase progresses. Special motor proteins are involved in the
condensation of chromosomes. The duplicated chromosomes are firmly
attached at their centromeres throughout this condensation and coiling.
Microtubule Organization and the Mitotic Spindle
• Microtubules and associated proteins form the spindle apparatus during
prophase and determine the poles of the cell. Some of the cell's
cytoskeleton will disassemble to provide spindle microtubules. The spindle
also synthesizes additional microtubules from the protein, tubulin,
synthesized during interphase.
• The spindle apparatus originates from a microtubule organizing
center, also called the centrosome. The centrosome is self-duplicating
and duplicates during interphase. In animal cells, centrioles are found
within the centrosome. Microtubules radiating from the centrosomes are
called asters. Centrioles are not essential to this process. Cells which
lack centrioles still form the spindle apparatus.
Prophase
Prometaphase (Late Prophase)
Nuclear membrane
• The nuclear membrane degrades in prometaphase into small vesicles, used
to synthesize new nuclear membrane material in the new cells.
Spindle and Duplicated Chromosomes
• The spindle apparatus will extend from the poles of the cell through the
center of the cell to the opposite pole of the cell. In animals, as the
spindle formation is completed, the centrosomes will move to the
respective poles of the cell and are called the spindle poles.
Mitosis and the Cell Cycle - 7
•
Some microtubules from each pole of the cell attach to the
kinetochores of each of the sister chromatids. The organization of the
spindle microtubules is such that each duplicated chromosome has
microtubules from one pole attaching to one of the sister chromatids, and
microtubules from the opposite pole attaching to the other sister
chromatid. If there are mistakes in attachment, chromatids are not
separated correctly during mitosis.
• The spindle microtubules pull on the kinetochore regions of the sister
chromatids in a "tug-of-war" which eventually leads to a migration of the
duplicated chromosomes to the equatorial plane of the cell. Microtubules
not attached to kinetochores pass through the equatorial region as they
extend from pole to pole of the cell.
Prometaphase
Metaphase
• The spindle apparatus has moved the chromosomes to the equator of the
cell, aligning the centromeres of each duplicated chromosome along the
equator of the cell.
• Centromeres of each sister chromatid are aligned with each other and each
sister chromatid is connected at its kinetochore to a microtubule.
• This alignment of chromosomes along the equatorial plane of the cell is often
called the metaphase plate.
Metaphase
Mitosis and the Cell Cycle - 8
Anaphase
• Centromeres of each duplicated chromosome separate to start anaphase. You
can't see this; the separating chromosomes are the first visual sign of
anaphase. By definition, each sister chromatid is now a single unduplicated
chromosome.
• Kinetochore microtubules from each pole pull the chromosomes away from each
other and toward the respective poles of the cell. They do so by decomposing
at the kinetochore end, hence shortening themselves. The microtubules attach
at the centromere region of the chromosomes so that chromosomes are pulled
centromere first towards the poles of the cell.
• Motor proteins move the kinetochores of the chromosomes along the
shortening kinetochore microtubules.
•
Polar microtubules lengthen, moving the poles of the cell further apart, and,
in animal cells elongating the cell.
Overlapping Polar Microtubules in Metaphase and Anaphase
•
Since sister chromatids are identical, each of the two clusters of
chromosomes being pulled to the two poles of the cell has one copy of each
original chromosome. As the chromosomes are pulled to the poles, they
begin to lengthen out.
Anaphase
Mitosis and the Cell Cycle - 9
Telophase
• The cell is elongated more by continued lengthening of the non-kinetochore
microtubules (decreasing the overlap).
• Membrane vesicles and membrane fragments form new nuclear membranes
around each group of chromosomes (at the two poles)
• Chromosomes uncoil and become indistinct as chromatin
• The spindle microtubules disperse and the spindle apparatus disappears
• New nucleoli form
Telophase
Mitosis in Blood Lily
Mitosis and the Cell Cycle - 10
Mitosis in Newt Lung
Mitosis in Whitefish Blastula
Mitosis and the Cell Cycle - 11
Cytokinesis: Separation of the Cytoplasmic Contents
Speaking precisely, mitosis describes events of chromosomes and nuclei. Most
cells accompany mitosis with cytokinesis, the separation of the cytoplasm of the
original cell into two new cells. This is not always the case. Some organisms
(including many fungi and algae) are "multinucleate"; they just have one cell body
with many nuclei. Some animal tissues are also multinucleate.
Cytokinesis coincides with the events of telophase or occurs immediately after, so
that at the completion of mitosis, the original cell is separated into two cells, each
with a nucleus and DNA identical to that of the original cell. Although the end
result of cytokinesis is always two new cells, the mechanism of separation is
different in plants and animals, so we shall discuss them separately.
Cytokinesis in Animal Cells
The cells of animals lack cell walls. Cytokinesis in animal cells is started with the
formation of a cleavage furrow, a depression or pinching in of the plasma
membrane.
This is caused by a ring of microfilaments, the contractile ring, composed of the
protein, actin, associated with myosin, which forms across the middle of the cell
after the chromatids are separated in anaphase. This ring contracts, pinching the
membrane toward the center of the cell, which eventually pinches the cell in two.
The additional membrane surface needed is supplied by membrane material
synthesized during interphase.
Cytokinesis in Animal Cells
Cytokinesis in Plant Cells
Cytokinesis in Plant Cells
Each cell of a plant is surrounded by a cell wall, which is quite rigid. Plant cells can
not form a cleavage furrow. Instead, plant cells are separated by the cell plate
formation.
Cell plate formation involves making a cross wall at the equator of the original cell.
Golgi vesicles containing wall material fuse along microtubules forming a disk-like
structure that is called the phragmoplast or cell plate. As cellulose and other
fibers are deposited, the cell plate is formed creating a boundary and new cell wall
between the two new cells. Membrane material from the original cell fuses to each
side of the cell plate forming new cell membranes on the dividing sides of the
original cell.
Mitosis and the Cell Cycle - 12
Variations in Mitosis in Eukaryotes
The process of mitosis in most eukaryotes is remarkably the same. However,
some protists, such as the dinoflagellates and some diatoms, do not degrade the
nuclear membrane during mitosis. Spindles may not (the dinoflagellates) or may
(the diatoms) form within the nucleus.
Binary Fission in Prokaryotes
The process of cell division in prokaryotes, called binary fission, parallels that of
eukaryotes. Bacteria have just one circular molecule of DNA and the absence of a
nucleus in the prokaryotic cell account for a number of differences in the
"mechanics" of the process. The single DNA molecule is attached to the plasma
membrane prior to duplication and cell division. After duplication (which is very
similar to that of eukaryotes), the duplicated molecule is also attached to the
plasma membrane. The cell elongates separating the two DNA molecules. After a
period of elongation, in which the original cell about doubles its length, plasma
membrane grows inward, pinching off the two halves of the original cell. New cell
wall material is synthesized.
Mitosis and the Cell Cycle - 13
When and Where does Mitosis Occur?
Growth
All growth (increase in numbers of cells) in individual organisms takes place by
mitosis, from the fertilized egg (zygote) to death.
Replacement
Many cells are routinely replaced in organisms. This replacement of cells is done
by mitosis. For examples, we replace the cells which line our digestive tract every
1 - 3 days.
Repair and Maintenance
Mitosis is used for repair and replacement of damaged cells or tissues, whenever
possible. This includes regeneration of lost parts for some organisms.
Non-Sexual (Asexual) Reproduction
Mitosis is used for all asexual reproduction or propagation. This is especially
common in plants, fungi and protists. Asexual reproduction produces offspring
genetically identical to the original parent. Animals rarely reproduce non-sexually.
When this does occur, the offspring are genetically identical to the parent, as
would be expected of any mitosis.
Cloning, a method of producing genetically identical offspring, uses mitosis,
precisely because mitosis duplicates the DNA exactly. Cloning is quite easy to do
with many plants, since they are easily propagated non-sexually anyway. We find
huge groves of Aspen trees, all of which are root clones. Many agricultural
products, such as navel oranges, originate from cloned individuals.
Tissue culture is also a popular way of cloning plants. Most cells of plants retain
the ability to "dedifferentiate" and become embryonic-like. Most animal cells, once
specialized (or differentiated), can not do so.
What passes for cloning in animal cells is a different process (to be discussed
some other time). We can make genetically identical organisms by removing nuclei
from non-fertilized egg cells and replacing the egg cell nuclei with nuclei from
diploid somatic cells. This has been done with many organisms, including sheep and
cattle. The resulting organisms will have the DNA of the diploid donor and will be
genetically identical. This successful "cloning" of mammals has resulted in a flurry
of research, and speculation about the ability and desire to clone humans. This is
one of the biological issues which has serious ethical consequences. One of the
reasons each of us should learn as much as possible about biology is to make
informed decisions about the ethical applications of research.
Mitosis and the Cell Cycle - 14
Regulating the Cell Cycle
The control of cell division is one of the most active areas of biological research if
only because cancers are diseases of cell division out of control. In humans, some
cells routinely divide; taste bud cells and cells lining the digestive tract divide about
every three days, skin cells monthly; red blood cells by the millions are produced
each day. Other cells in mature humans never divide. Nerve cells and muscle cells
are two examples. Virtually all brain development occurs by the time we are two.
Liver cells divide only if damage occurs.
In plants, only cells in specialized areas of the plant body, called meristems,
normally divide. In fact meristems are specialized for cell division. However, there
are numerous exceptions in plants, including some that are a part of normal
development. And, as mentioned previously, many plant cells have a remarkable
ability to dedifferentiate and become "embryonic", something that does not happen
in animals.
But what tells a cell to divide - or not to divide? Such activities are part of the
normal cell regulation and checks and balances. In cell fusion studies during the
1970's, researchers learned that there are definite cell-cycle controls which
direct and coordinate the events of the cell cycle.
If one looks at the animal cell cycle, there are a number of "points" where controls
keep the cell cycle in the stage it is at until they are over-ridden by chemical
signals. One of these is in G1 , another in G2 just prior to mitosis and a third is
midpoint in mitosis, (where proteolytic enzymes control the separation of
centromeres at the start of anaphase). Cells will stay in G1 until they receive a
signal to proceed with DNA duplication. Cells that never leave G1 are said to be in a
non-dividing cycle called G0.
It should not surprise us that the signals are protein kinase molecules, in this case
a class of protein kinases called cyclin-dependent kinases (Cdk), because they
are activated by protein called cyclin, whose concentration is cyclical (hence
cyclin).
In cell division in animals, cyclin concentration rises during interphase and
complexes with the cyclin-dependent kinases forming a complex (cyclin-CdK).
The first such complex discovered was called the "maturation (or mitosis)
promoting factor" (or MPF). This complex triggers mitosis at the end of G2 .
Cyclin - Cyclin-dependent Kinase Complex (MPF)
Mitosis and the Cell Cycle - 15
The cyclin-dependent kinases phosphorylate a number of proteins needed to do
mitosis. They also phosphorylate, and through serine and threonine kinase relay
pathways, promote other proteins to phosphorylate the nuclear membrane lamina
layer so that it degrades. As mitosis is nearing completion, the CdK complex
promotes its own degradation by activating proteolytic enzymes that destroy
cyclin.
Control of Mitosis by the Cyclin-CdK complex
Although the general role of cyclin-dependent kinases is known, specific ways in
which they work and their substrates are still mostly mystery. Signals that
activate the cyclin-dependent kinases can be internal or external. One signal
pathway that is known is the anaphase checkpoint.
Mitosis Kinetochore Checkpoint – Anaphase Control
The mitosis anaphase checkpoint is regulated by the kinetochores of the sister
chromatids. They delay anaphase until all kinetochores are attached to spindle
fibers (microtubules). Proteins associated with the kinetochores have a signal
pathway that blocks the cyclin breakdown and the associated breakdown of the
proteins that hold the sister chromatids together. When all kinetochores are
attached, the anaphase promoting complex stops its block, the proteins holding the
sister chromatids together are degraded and cyclin associated with the mitosis
checkpoint is broken down, permitting mitosis to continue. This control ensures
that each cell formed will have the correct complement of chromosomes; no stray
chromosome can avoid the metaphase plate.
Mitosis and the Cell Cycle - 16
External Signals and Cell Division Control
It has long been known that cell division, like many other cell activities, cannot
occur if essential nutrients are not available. Growth factors, a class of
proteins that are involved with signal transduction pathways, are important in
promoting and in arresting cell division, so that cells divide when needed and do not
divide inappropriately (except when cancers happen).
It takes a certain concentration of growth factor to continue to trigger cell
division. It also takes a certain amount of nutrients so that growth factors and
nutrient supply are both important in regulating the rate of cell division. Absence
of appropriate growth factor is one means of keeping a cell in G0 .
In plant tissue culture, a cell will remain undivided until and unless an appropriate
mixture of nutrients and growth regulators (plant hormones) is provided. Early
plant tissue culture succeeded when researchers added coconut endosperm
(coconut milk) to the culture medium. The endosperm is rich in plant hormones
because its normal job is providing nutrients to the developing embryo.
In animal cell
and exceeds
divide. As in
as a density
inhibition.
cultures, cell division is halted when the cell population gets too dense
the nutrient and growth factor concentration needed to continue to
population biology and ecology, such regulation of cell division is known
dependent activity or in this case a density dependent
Animal cell division in cultures is also arrested when cells do not have an
appropriate substrate or the extracellular matrix of cells to attach to during
division. Normally, membrane proteins and components of the cytoskeleton trigger
anchoring signal pathways that regulate cell division. Such cell division control is
known as anchorage dependence.
Mitosis and the Cell Cycle - 17
Cancer and the Cell Cycle
Cancer is a disease of uncontrolled cell reproduction. The current statistics for
cancer say that one in four people in the United States today will get some form of
cancer. One in eight will get breast cancer. Almost any male who lives long enough
will have prostate cancer. No one knows why any one person gets cancer. Some
cancers are probably genetic. Some are related to the environment, especially the
smoker's environment. For others, substances in foods may be cancer causing,
cancer promoting or, conversely cancer preventing. We will discuss cancer later in
the context of mutation and gene alterations, although it is useful to mention
some of the features of cancer now, as related to cell division controls.
Cancer cells are not inhibited by density factors or the need to be anchored. They
freely divide and apparently "enjoy" being crowded. Cancer cells can migrate,
particularly through the lymph system from the tissues in which they arise to
other tissues and organs of the body. Unless controlled, cancers can and do cause
death to the organism.
Given adequate nutrients in culture media, cancer cells can divide indefinitely The
HeLa cell line has been cultured for about a half century, indicating that
telomerases (to be discussed with DNA duplication) are active in cancer cell lines.
Cancer cells do not just divide rapidly. They have chromosome abnormalities and
abnormal metabolism. Their cell surfaces are altered which promotes the
metastasis and migration to other areas of the body, where they can attach to
tissues and form more tumors (a mass of abnormal cells).
Research indicates that a number of things are at work in cancer cells. Some may
make their own growth factors (that promote cell division) and do not need
external signals. Some have abnormal cell signal pathways for the cell division
checkpoints, so checkpoints do not function properly. Two examples that affect
checkpoints are the p53 and Rb protein transcription factors that have mutations
in their genes in about 40 – 50% of all cancers. Other proteins involved in the
kinase relay pathways also show mutations in many cancers.
p53, for example, normally checks the integrity of DNA prior to duplication. If
there are errors in DNA, p53 will trigger the enzymes that correct errors. If
corrections cannot be made, p53 triggers the destruction of damaged cells.
Mutated p53 genes result in failure of p53 to function. Damaged DNA duplicates,
and can ultimately lead to cancer.
The Rb gene is a tumor suppressing gene that codes for proteins that block cyclin
from bonding to the Cdk complex in G1, blocking cell division and keeping cells in G0.
The Rb protein interferes with a number of regulatory proteins that would normally
promote the cyclin-Cdk complex.
Mitosis and the Cell Cycle - 18
Function of p53