Chapter 5 - The Mitotic Cell Division (updated) PDF

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Chapter 5: The Mitotic Cell Cycle Chromosomes They are structures made of DNA and histones. Found in the nucleus of eukaryotic cells. The number of chromosomes present is characteristic of the species (e.g. Human cells have 46 chromosomes while fruit fly cells have only 4 chromosomes). The structure of chromosomes Chromosomes are made up of 3 materials which are: 1. Proteins (70%). These include histones, scaffold proteins, and polymerases. 2. DNA (15%) 3. RNA (10%) Chromosomes exist in two forms. One in non-dividing cells and the other in cells that are dividing. In non-dividing cells, chromosomes are extremely long, thin, threadlike and dispersed throughout the nucleus and exist as single-armed structure. Individual chromosome could not be seen and are in this form, they are known chromatin. During the synthesis phase of interphase (the phase before cell division occurs), the chromosomes appear as a double structure where it is made up of 2 chromatids that are identical to each other. Each chromatid has one copy of the DNA. The reason why there are 2 chromatids because during the synthesis phase, the DNA undergoes replication forming a new strand of DNA that eventually becomes one of the chromatids. During cell division, chromatin condense, shorten, and thicken making individual chromosomes more visible when they are stain appropriately. The chromatids are joined together by a region called centromere which does not contain any gene.The position of the centromere is characteristic of a particular chromosome. At the end of the chromosomes are structures called telomeres which are repeated sequences of bases that hold no Made by Ang Yi Han

important genetic information. The significance of telomeres will be detailed later in this chapter.

The total length of all of the DNA in the 46 chromosomes of human cells is 1.8m and all of this has to be fit inside a nucleus which is only 6 micrometre in diameter. In order for the whole length of the DNA to fit inside the nucleus, it has to undergo a series of precise complex coilings.. The protein molecules used for coiling the DNA are known as histones. Histones are proteins that allow DNA to wrap around it to make it condense from chromatin to chromosome. The basic unit structure for the coiling within the chromatin is called nucleosome. The DNA is wound around a cylindrical nucleosome (made up of 4 histone proteins and it is about 6nm long and 11nm wide), making 1 turns (equivalent to 147 base pairs) before linking to the next nucleosome. The DNA found between nucleosomes are called linker DNA.

The enzyme, telomerase is responsible for adding extra DNA to the telomeres ensuring that the cells containing this enzyme can continue to replicate their DNA successfully and undergo more cell divisions (e.g. stem cells and cancer cells) Certain cells such as specialised cells do not have telomerase and so they cannot carry out cell divisions indefinitely. The shortening of the telomeres play a role in cellular aging and this partially explains why all living organisms age and die.

The amount of coiling is different in the different stages of the cell cycle. It is the most tightly coiled during metaphase and this is the reason why chromatin can be easily seen during cell division using an appropriate stain. Telomeres and their significance Mitosis Telomeres are short base sequences of DNA that are reIt is the division of a nucleus into 2 so that the 2 daughter peated multiple times and are found at the ends of chromocells have exactly the same number and type of chromosomes. somes as the parent cell. Their main function is to prevent the ends of chromosomes It is a part of the cell cycle. to deteriorate or fuse with neighbouring chromosomes. It is needed to maintain the intergrity and stability of linear euThe cell cycle It is the sequence of events that takes place between on cell karyotic genomes. division and the next. During DNA replication, the enzymes that are responsible for replicating DNA cannot go all the way to the end of the chroIt consists of 3 main phases which are: mosome and they stop short of the end of the chromosome. 1. Interphase 2. Nuclear division (Mitosis); M phase This means that every DNA replication would cause the 3. Cell division (Cytokinesis) shortening of the strands of DNA that are made. Some genetic information will be lost as a result and this causes the death of the cell. The presence of telomeres prevent this from happening and allows the enzymes to copy the whole length of the DNA without losing any essential genes. Made by Ang Yi Han

M phase (stands for mitosis)

Interphase Interphase can be broken down into 3 smaller phases which are: 1. G1 phase (stands for gap 1) - The cells make RNA, enzymes, and other proteins needed to induce growth when they receive requisite signals required to iniatiate this process. If the cells are not going to divide again, they remain in this phase. 2. S phase (stands for synthesis) - When the cells are commited to dividing, they carry out this phase where DNA replication takes place causing the number of DNA molecules to double in the cells (from 2n to 4n). Structures known as kinetochores are also formed in this phase, they are attached to the centromeres. Chromosomes are only visible during prophase. 3. G2 phase (stands for gap 2) - The cells continue to grow and this is when the newly replicated DNA is being proofread for errors. When errors are detected, they are corrected. This prepares the cells for the M phase.

To make it easier to decribe the whole process of mitosis, the process is split into 4 different phases which are: 1. Prophase Chromatin coils up, becoming shorter and thicker (it condenses) making them visible when they are stained. The 2 chromatids are visible and are held by the centromere. Centrosome duplicates to form 2 daughter centrosomes. The daughter centrosomes then proceed to move towards opposite ends of the cell where they form the poles of the spindle. Centrosomes organise the production of microtubules which form the spindle fibres. At late prophase, the nuclear envelope disintegrates to form part of several chromosomes and the nucleolus disappears (breaks up into small vesicles).

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2. Metaphase Centrosomes are located at the opposite poles of the cell. The spindle fibres of centriole that are produced are attached to the kinetochores at the centromeres of the chromosomes. Chromosomes move and line up along the equatorial plane of the cell (the mid-line of the cell) and are right angles to the axis formed by the centrosomes.

3. Anaphase The spindle fibres contract causing the centromeres to divide and break. The sister chromatids are now seperated. The sister chromatids are being pulled apart by the spindle fibres and move towards opposite poles with the centromere being pulled first (they are now refered as daughter chromosomes).

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4. Telophase Nucleoli reform inside the two nuclear envelopes. The chromosomes uncoil and become less dense to form chromatin. Spindle fibres break down and disappear. The nuclear envelope reforms around the chromosomes that are grouped at both ends of the cell.

Cytokinesis It is process where the cytoplasm of a single cell is divided to form 2 daughter cells. Usually initiates during telophase (the end of mitosis). This is needed to ensure that the number of chromosomes is maintained from one generation to another.

The cleavage furrow continues to move inwards until the process of abscission splits the cell into 2. Cytokinesis in plants The presence of cell wall and the absense of centrioles in plants causes the process to be different from the cytokinesis in animals. Instead of forming a contractile ring in the middle of the cell, a cell plate is formed instead. Golgi apparatus in the cell releases vesicles containing the materials required to construct a cell wall. These vesicles fuse at the equatorial plane of the cell forming the cell plate. The cell plate will then develop to form a cell wall.

Biological significance of mitosis 1. The cells that are produced from mitosis are genetically identical. This is important for growth in multicellular organisms. It also helps to retain cell function and genetic stability. 2. The process of mitosis followed by cell division produces new identical cells that can replace cells that are constantly dying. Thus it is important in the replacement of cells and repair of tissues.

Cytokinesis in animals A contractile ring is formed in the middle of the cell. The contractile ring contracts the cell surface membrane causing the formation of a cleavage furrow. Through the process of hydrolysis, the cleavage furrow moves inwards. Made by Ang Yi Han

3. Mitosis is the basis of asexual reproduction where genetically identical offspring(s) is produced from a single parent. 4. Mitosis plays an important role in immune response (chapter 11). Stem Cells Stem cells are cells which can undergo cell division an unlimited number of times via mitosis. Every time a stem cell divides, it has the potential to either remain as a stem cell or differentiate into a specialised cell (e.g. blood cell, muscle cell, neuron, and etc). The potency of a stem cell is the measure of the extent of the power a stem cell to produce different types of cell. Stem cells that can form any type of cell are totipotent (e.g. zygote including cells up to the 16-cell stage of development). Stem cells that can form any type of cell in the body of an organism are pluripotent (e.g. embryonic stem cells) Stem cells that can only turn into a few types of celll in a specific tissue are multipotent (e.g. stem cells in the bone marrow where they produce only blood cells). iPS cells (induced pluripotent stem cells) have been artificially created by scientists. The properties of stem cells open up few possible uses such as: 1. Stem cell therapy where adult stem cells are transplanted into damage tissue in order to treat injury or disease. 2. Stem cells may be used for the substances they produce which encourage the body to repair itself. Cancer Cancer cells are cells that show the following characteristics: 1. Uncontrolled growth (as a result of uncontrolled mitosis) 2. The ability to invade on and destroy adjacent tissues. 3. Metastasis where the cells can spread to other locations in the body through lymph or blood. Made by Ang Yi Han

Cancer cells have the ability to divide repeatedly and excessively to form an abnormal mass of cells called tumour (consists of millions of cells). Cancer cells are formed as a result of mutation (where there is a change in the structure of the DNA). However, not all mutations lead to cancer. Most mutations are usually dealt with by causing early death (apoptosis) in the cells that have mutationsor are detected and destroyed by the immune system. Cancerous cells manage to escape both of these possible fates. Cancer can start when there is a change in one base. The healthy cell becomes genetically damaged and starts to multiply out of control. One of the bases has been copied and if it doesn t undergo apoptosis, it will divide again with a different gene other than normal. There are two important genes when mutated, can lead to cancer: 1. Proto-oncogene - Genes that stimulate cell division. Mutation causes this gene to be changed to oncogene which stimulates excessive cell division. 2. Tumour suppressor genes - Genes that are responsible for accurate DNA replication and inhibit cell division if anything goes wrong during DNA replication. Mutation switches off the functions of tumour suppressor genes. Carcinogens are any agents that can cause cancer. Examples include: 1. Ionising radiations such as x-rays, gamma rays, and particles released from the decay of radioactive elements. 2. Chemicals such as tar in tobacco smoke. Other factors that can increase the likelyhood of cancer can include virus infections (virus can cause genetic changes in affected cells) and hereditary predisposition (can be caused by the inheritance of a single faulty gene). There are two different types of tumour which are: 1. Benign tumours - Tumours that do not spread from their site of origin (e.g. ovarian cysts, warts, brain tu-

mours, and etc.). This type of tumour does not cause cancer. 2. Malignant tumours - Tumours that can spread from their site of orgin to the rest of the body via the blood/lymph (metastasis), invade other tissues and eventually destroy them. This type of tumour causes cancer. They interfere with the normal functioning of the affected area and may also block instestines, lungs or blood vessels. Practical: Investigating mitosis using a root tip squash Samples of the meristem (growth region) taken from the root tip of plants can be used to study mitosis. The root tips of garlic, onion, broad bean, and sunflower can be used. Procedure: 1. The bulbs or seeds are grown suspended by a pin over water for a period of a week or two. 2. The tips of the roots are then removed and are placed in a suitable stain (e.g. warm, acidified acetic orcein. This stains the chromosomes with a deep purple). 3. Mount the stained root tips on a glass slide and gently squash it with a blunt instrument (e.g. the end of the mounting needle). 4. Place the slide under the light microscope. Observe and draw the cells seen under the light microscope.

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