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.