Mitosis
The Mitotic cell Cycle You should be able to: describe the
structure of chromosomes describe the cell cycle the cycle of
events by which body cells grow to a certain size and thendivide
into two explain how a nucleus divides into two genetically
identical nuclei by mitosis prepare and observe a root tip squash
in order to see stages of mitosis with a light microscope explain
the significance of mitosis explain the significance of telomeres
explain the significance of stem cells outline how uncontrolled
cell division can lead to cancer
Telomeres and enzyme TelomeraseTelomeres are protective
sequences of nucleotides found at the ends of chromosomes, which
become shorter every time a cell divides.A gradual degeneration of
the organism occurs, resulting in ageing.Some cells are able to
replenish their telomeres using the enzyme telomerase. It is
thought that cancer cells can do this and so remain immortal. It
may therefore be possible to prevent the ageing of normal cells by
keeping the enzyme telomerase active.
Chromosomes
In the nucleus of each cell, the DNA molecule is packaged into
thread-like structures called chromosomes. Each chromosome is made
up of DNA tightly coiled many times around proteins called histones
that support its structure.
Chromosomes are not visible in the cells nucleusnot even under a
microscopewhen the cell is not dividing. However, the DNA that
makes up chromosomes becomes more tightly packed during cell
division and is then visible under a microscope.
They were originally termed chromosomes because chromo means
colored and somes means bodies.The number of chromosomes is
characteristic of the species. For example, in human cells there
are 46Chromosomes, and in fruit fly cells there are only 8.
The structure of chromosomesIt is made of two identical
structures called chromatids, joined together.Each chromatid
contains one of these DNA copies, and the two chromatids are held
together by a narrow region called the centromere, forming a
chromosome.
The centromere can be found anywhere along the length of the
chromosome, but the position is characteristic for a particular
chromosome.
Each chromatid contains one DNA molecule. DNA is the molecule of
inheritance and is made up of a series of genes.
Each gene is one unit of inheritance, coding for one polypeptide
that is involved in a specific aspect of the functioning of the
organism.When cells divide, one chromatid goes into one daughter
cell and one goes into the other daughter cell, making the daughter
cells genetically identical.
The DNA is wound around the outside of these protein molecules.
The combination of DNA and proteins is called chromatin.
Chromosomes are made of chromatin. Chemically speaking, most of
the proteins are and are of a type known as histones. Because they
are alkaline, they can interact easily with DNA, which is
acidic.
NucleosomeA nucleosome is a basic unit of DNA packaging in
eukaryotes, consisting of a segment of DNA wound in sequence around
eight histone protein cores. This structure is often compared to
thread wrapped around a spool.
The nucleosome is cylindrical in shape, about 11 nm wide by 6 nm
long.It is made up of eight histone molecules.The DNA is wrapped
around the outside of the cylinder, making 1 turns (equivalent to
147 base pairs) before linking to the next nucleosome.
EuchromatinThe packing is not tight. Euchromatins are loosely
coiled regions.Euchromatin contains less DNA.The genes in the
Euchromatin are active.
HeterochromatinHeterochromatins thus have tighter DNA
packaging.Heterochromatins are compactly coiled regions.
Heterochromatin contains more DNA.The genes in the Heterochromatin
are mostly inactive.
The DNA between the nucleosomes is also held in place by a
histone molecule. Nucleosomes line up like a string of beads to
form a fiber 10 nm wide.This string can be further coiled and
supercoiled, involving some non-histone proteins.Chromosomes seen
just before nuclear division represent the most tightly coiled
(condensed) form of DNA.
MitosisThe cell cycle is the regular sequence of events that
takes place between one cell division and the next. It has three
phases, namely interphase, nuclear division and cell division.
Interphase Interphase is often included in discussions of
mitosis, but interphase is technically not part of mitosis, but
rather encompasses stages G1, S, and G2 of the cell cycle.The stage
of the cell cycle when a cell is preparing itself to duplicate is
called interphase. During interphase, the cell obtains nutrients,
and duplicates its chromosomes.During interphase, the chromosomes
are found arranged in the nucleus and appear as a network of long,
thin threads, called chromatin.
S phase. (S stands for synthesis of DNA).In S phase, the cell
synthesizes a complete copy of the DNA in its nucleus. It also
duplicates a microtubule-organizing structure called the
centrosome. The centrosomes help separate DNA during M phase.
G1 phase
Interphase begins with G1 (G stands for gap) phase. During this
phase, the cell makes a variety of proteins that are needed for DNA
replication.The cell grows physically larger, copies organelles,
and makes the molecular building blocks it will need in later
steps.
G2 Phase During the second gap phase the cell grows more, makes
proteins and organelles, and begins to reorganize its contents in
preparation for mitosis.During G2 the cell continues to grow and
new DNA is checked and any errors are usually repaired.
Preparations are also made to begin the process of division. For
example, there is a sharp increase in production of the protein
tubulin which is needed to make microtubules for the mitotic
spindle.Interphase therefore consists of G1, S and G2.
M phaseNuclear division follows interphase. This may be referred
to as the M phasecell division involves constriction of the
cytoplasm between the two new nuclei, a process called cytokinesis.
In plant cells, it involves the formation of a new cell wall
between the two new nuclei.
Mitosis is divided into four phases. Each phase is characterized
by specific processes involving different structures.
ProphaseMetaphaseAnaphaseTelophase
Early prophase
centrosomesThey are only found inside of eukaryotic cells.
Centrosomes are comprised of two centrioles that are essentially
just rings of microtubules.The purpose of the centrosome is to help
organize microtubules to be utilized during cell division.
Late prophase
Metaphase
Anaphase
Telophase
Centromere
a centromere is responsible for the movement of the replicated
chromosomes into the two daughter cells during mitosis and
meiosis.Centromere joins the sister chromatids. The two copies of a
replicated chromosome are called sister chromatids, and they must
stay joined together until it is time for them to be physically
pulled into the two future daughter cells.
kinetochores are plate like structures composed of several
layers. Multiple microtubules appear to insert into the
kinetochore, which is positioned on the side of the chromosome
facing the spindle pole.
Centrosome Centrosomes are comprised of two centrioles that are
essentially just rings of microtubules.The purpose of the
centrosome is to help organize microtubules to be utilized during
cell division. The centrosome is a microtubule organizing center
and consists of two centrioles that are held perpendicular to one
another.
Biological significance of mitosis It is an equational division
through which identical daughter cells are produced having the same
amount and type of genetic constitution as that of the parent
cell.It is responsible for growth and development of multi-cellular
organisms from a single-celled zygote.
The number of chromosomes remains the same in all the cells
produced by this division. Thus, the daughter cells retain the same
characters as those of the parent cell.Mitosis helps in restoring
wear and tear in body tissues, replacement of damaged or lost part,
healing of wounds and regeneration of detached parts (as in tail of
a lizards).
It is a method of multiplication in unicellular organisms.
Asexual reproduction.
Immune response - The cloning of B- and T-lymphocytes during the
immune response is dependent on mitosis.
The significance of telomeres
Telomere (tel-uh-meer) from the Greek telos (end) and meros
(part)
The ends of chromosomes are sealed by structures called
telomeres.These are made of DNA with short base sequences that are
repeated many times.In telomeres, one strand of the DNA is rich in
the base guanine (G) and the other strand is rich in the
complementary base cytosine (C).
Their main function is to ensure that when DNA is replicated,
the ends of the molecule are included in the replication and not
left out.It has been shown that some cells do not top up their
telomeres at each division. These tend to be fully differentiated
cells. With each division, their telomeres get a little shorter
until the vital DNA is no longer protected and the cell dies.This
could be one of the mechanisms of ageing, by which we grow old and
die.
Stem cells
Stem cells have the remarkable potential to develop into many
different cell types in the body during early life and growth.A
stem cell is a cell that can divide an unlimited number of times
(by mitosis). When it divides, each new cell has the potential to
remain a stem cell or to develop (differentiate) into a specialized
cell such as a blood cell or muscle cell.
TotipotencyCell potency is a cell's ability to differentiate
into other cell types.The more cell types a cell can differentiate
into, the greater its potency.Totipotency is the ability of a
single cell to divide and produce all of the differentiated cells
in an organism. Spores and zygotes are examples of totipotent
cells
Pluripotent stem cells Pluripotent stem cells are master
cells.These cells have the potential of taking on many fates in the
body, including all of the more than 200 different cell
types.Embryonic stem cells come from pluripotent cells, which exist
only at the earliest stages of embryonic development. In humans,
these cells no longer exist after about five days of
development.
However, for growth and repair it is essential that small
populations of stem cells remain which can produce new cells. Adult
stem cells have already lost some of the potency associated with
embryonic stem cells and are no longer pluripotent. They are only
able to produce a few types of cell and may be described as
multipotent. For example, the stem cells found in bone marrow are
of this type. They can replicate any number of times, but can
produce only blood cells, such as red blood cells, monocytes,
neutrophils and lymphocytes. Mature blood cells have a relatively
short life span, so the existence of these stem cells is
essential.
Stem cell therapyStem-cell therapy is the use of stem cells to
treat or prevent a disease or condition.Bone marrow transplantation
is the only form of this therapy that has progressed beyond the
experimental stage into routine medical practice, but in the future
it is hoped to be able to treat conditions like diabetes, muscle
and nerve damage, and brain disorders such as Parkinsons and
Huntingtons diseases.Experiments with growing new tissues, or even
organs, from isolated stem cells in the laboratory have also been
conducted.
CancerA disease in which abnormal cells divide uncontrollably
and destroy body tissue.Cancer develops when the bodys normal
control mechanism stops working. Old cells do not die and cells
grow out of control, forming new, abnormal cells. These extra cells
may form a mass of tissue, called a tumor.
Cancers show us the importance of controlling cell division
precisely, because cancers are a result of uncontrolled
mitosis.Cancerous cells divide repeatedly and form a tumor, which
is an irregular mass of cells.
Cancers are thought to start when changes occur in the genes
that control cell division. A change in any gene is called a
Mutation. The particular term for a mutated gene that causes cancer
is an oncogene, after the Greek word onkos, meaning bulk or
mass.
. Mutations are not unusual events, and most of the time they
dont lead tocancer. Most mutated cells are either affected in some
way that results in their early death or are destroyed by the bodys
immune system. Since most cells can be replaced, mutation usually
has no harmful effect on thebody.
Cancerous cells, however, manage to escape both possible fates,
so, although the mutation may originally occur in only one cell, it
is passed on to all that cells descendants. By the time it is
detected, a typical tumor usually contains about a thousand million
cancerous cells.Any agent that causes cancer is called a carcinogen
and is described as carcinogenic.
Benign tumors
Malignant tumorsThey spread through the body, invade other
tissues and destroy them that cause cancer, and these are known as
Malignant tumors. Malignant tumors interfere with the normal
functioning of the area where they have started to grow. They may
block the intestines, lungs or blood vessels. Cells can break off
and spread through the blood and lymphatic system to other parts of
the body to form secondary growths.
MetastasisMetastasis is the spread of a cancer from one organ or
part of the body to another without being directly connected with
it. In metastasis, cancer cells break away from the original
(primary) tumor, travel through the blood or lymph system, and form
a new tumor in other organs or tissues of the body. The new,
metastatic tumor is the same type of cancer as the primary tumor.
For example, if breast cancer spreads to the lung, the cancer cells
in the lung are breast cancer cells, not lung cancer cells. The
plural form of metastasis is metastases.