MITOSISMitosis is normal cell division,

which goes on throughout life in all parts of the body.

• Cells divide to make more cells.

• While all the other organelles can be randomly separated into the daughter cells, the chromosomes must be precisely divided so that each daughter cell gets exactly the same DNA.

• Mitosis is a process of cell division which results in the production of two daughter cells from a single parent cell.

• The daughter cells are identical to one another and to the original parent cell.

Machinery of Mitosis• The chromosomes are pulled

apart by the spindle, which is made of microtubules.

• The spindle fibers are attached to each centromere(which is part of the chromosome), and anchored on the other end to a centrosome( centriole).

• There are 2 centrosomes, one at each end of the spindle.

• The Chromosomes are lined up between the poles of the spindle fibers.

• When the spindle fibers contract, the Chromosomes are pulled to the opposing poles.

• The cell then divides to separate the two poles.

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(like mitosis).

• Interphase is considered to be the 'living' phase of the cell, in which the cell obtains nutrients, grows, reads its DNA, and conducts other "normal" cell functions.

• Chromosomes are not clearly distinguished in the nucleus, although a dark spot called the nucleolus may be visible.

• The cell may contain a pair of centrioles .• Stages of interphase includes G1, S, G2 phases

Interphase

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Interphase

               

stages

In a typical animal cell, mitosis can be divided into four principals stages. They are:

1. Prophase

2. Metaphase

3. Anaphase

4. Telophase

Prophase

• When mitosis begins, the chromatin of the chromosome becomes gradually more and more coiled so that the chromosome becomes recognizable ( condensation) as a thread-like structure that eventually acquires a rod-like appearance.

• Towards the end of prophase, the two chromatids constituting the chromosome become distinct and the chromosome now has a typical structure.

• While these changes are occurring in the chromosomes, a number of other events are also taking place.

• The nucleolus breaks down and

• The two centrioles separate and move to opposite poles of the cell.

• They produce a number of microtubules that pass from one centriole to the other and form a spindle.

• At the end of prophase, the nuclear envelope breaks down into vesicles.

• Centromeres captures spindle fibers

Prophase

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Prophase

               

Metaphase

• With the formation of the spindle, chromosomes move to a position midway between the two centrioles ( Equator)

Metaphase

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Metaphase

               

Anaphase

• The centromere of each chromosome splits longitudinally into two so that the chromatids now become independent chromosomes.

• At this stage, the cell can be said to contain 46 chromosomes.

• One chromosome of each such pair now moves along the spindle to opposite poles of the mitotic spindle.

Anaphase

• At this point, each individual chromosome goes from:– 1 chromosome with 2 chromatids to:1 chromosome with 2 chromatids to:– 2 chromosomes with one chromatid each.2 chromosomes with one chromatid each.

Anaphase

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Anaphase

               

Telophase

• The two daughter nuclei are formed by appearance of nuclear membranes.

• Chromosomes gradually elongate and become indistinct.

• Nucleoli reappear.

• The centriole is duplicated at this stage or in early interphase

Telophase

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Telophase

               

Cytokinesis• Cytokinesis is the process when the cytoplasm of a

single eukaryotic cell is divided to form two daughter cells.

• It is usually initiated during the late stages of mitosis, and sometimes meiosis, splitting a binucleate cell in two, to ensure that chromosome number is maintained from one generation to the next.

• The cytoplasm divides, the cell membrane pinches inward ultimately producing two daughter cells .

Cytokinesis

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Cytokinesis

               

Summary of Mitosis• Prophase: Prophase:

• Chromosomes condense• Nuclear envelope disappears• centrosomes move to opposite sides of the cell• Spindle forms and attaches to centromeres on the

chromosomes

• MetaphaseMetaphase• Chromosomes lined up on equator of spindle• centrosomes at opposite ends of cell

• AnaphaseAnaphase• Centromeres divide: each 2-chromatid

chromosome becomes two 1-chromatid chromosomes

• Chromosomes pulled to opposite poles by the spindle

• TelophaseTelophase• Chromosomes de-condense• Nuclear envelope reappears• Cytokinesis: the cytoplasm is divided into 2 cells

Cancer• Cancer is a disease of uncontrolled cell division. • It starts with a single cell that loses its control mechanisms due to a

genetic mutation. • That cell starts dividing without limit, and eventually kills the host.• Normal cells are controlled by several factors.

– Normal cells stay in the G1 stage of the cell cycle until they are given a specific signal to enter the S phase, in which the DNA replicates and the cell prepares for division. Cancer cells enter the S phase without waiting for a signal.

– Normal cells are mortal. This means that they can divide about 50 times and then they lose the ability to divide, and eventually die. This “clock” gets re-set during the formation of the gametes. Cancer cells escape this process of mortality: they are immortal and can divide endlessly.

– Normal cells that suffer significant chromosome damage destroy themselves due to the action of a gene called “p53”. Cancer cells either lose the p53 gene or ignore its message and fail to kill themselves.

Cancer Progression• There are many different forms of cancer,

affecting different cell types and working in different ways. All start out with mutations in specific genes called “oncogenesoncogenes”.

• The normal, unmutated versions of the oncogenes provide the control mechanisms for the cell. The mutations are caused by radiation, certain chemicals (carcinogens), and various random events during DNA replication.

• Once a single cell starts growing uncontrollably, it forms a tumor, a small mass of cells. No further progress can occur unless the cancerous mass gets its own blood supply. “Angiogenesis” is the process of developing a system of small arteries and veins to supply the tumor. Most tumors don’t reach this stage.

• A tumor with a blood supply will grow into a large mass. Eventually some of the cancer cells will break loose and move through the blood supply to other parts of the body, where they start to multiply. This process is called metastasis.metastasis. It occurs because the tumor cells lose the proteins on their surface that hold them to other cells.

Cancer Treatment• Two basic treatmentsTwo basic treatments: surgery to remove the tumor, and

radiation or chemicals to kill actively dividing cells.• It is hard to remove all the tumor cells. Tumors often lack sharp

boundaries for easy removal, and metastatic tumors can be very small and anywhere in the body.

• Radiation and chemotherapy are aimed at killing actively dividing cells, but killing all dividing cells is lethal: you must make new blood cells, skin cells, etc. So treatment must be carefully balanced to avoid killing the patient.

• Chemotherapy also has the problem of natural selection within the tumor.

• If any of the tumor cells are resistant to the chemical, they will survive and multiply. The cancer seems to have disappeared, but it comes back a few years later in a form that is resistant to chemotherapy. Using multiple drugs can decrease the risk of relapse: it’s hard for a cell to develop resistance to several drugs at the same time.