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The Cell Cycle.pdf

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McMaster University
Lovaye Kajiura

Biology 1A03: Cell Structure & Function THE CELL CYCLE All cells arise from pre-existing cells by the process of cell division. Fig. 11.1 Two Types of Nuclear Division Occur in Many Species MITOSIS MEIOSIS - Occurs in somatic cells - Occurs only in reproductive cells - Produces gametes – sperms or eggs - Daughter cells are genetically identical - Daughter cells (ova, sperm) are genetically to parent cell (contains the same hereditary information as the different from the parent cells (contains half of the parent cell). hereditary information) IMPORTANCE OF MITOSIS Mitosis - is the division of a somatic parent cell into two genetically identical daughter cells. - accompanied with cytokinesis- the division of the cytoplasm into two daughter cells. - ensures that each of the daughter cells contain a complete set of chromosomes. Mitosis and Cytokinesis is responsible for three major events in multi-cellular organisms: 1. Growth – the body grows by cells continually dividing. 2. Repair- when you cut yourself, cells repairs the skin and heals by dividing through mitosis and cytokinesis. 3. Reproduction- mitosis followed by cytokinesis is the basis for reproduction by asexually reproducing species. 11.1 MITOSIS AND THE CELL CYCLE Chromosomes- DNA  Threadlike structures contained in the nucleus  Consists of a long DNA double helix wrapped around proteins. DNA encodes the cell’s hereditary information or genetic material.  Genes are segments of the DNA that codes for a particular protein or ribonucleic acid found in cells.  By staining the chromosomes with dyes and observing the chromosomes under a light microscope, biologists could follow the distribution of this genetic material to daughter cells in mitosis.  Humans have 46 chromosomes in each cell.  uncondensed, uncoiled, long and thin chromosomes Chromatin  found in interphase  consist of DNA wrapped around globular proteins called, histones  compact “X” shaped form of chromatin Chromosomes  found in prophase  replicated DNA  has two sister chromotids held together in the centre by a centromere.   Single strand of chromosomes. Chromatids  Replicated DNA  The two chromatids in a chromosome is called sister chromatids. When they are pulled apart in mitosis, they become chromosomes.  Sister chromatids represent exact copies of the same genetic information (will get separated into each daughter cell in mitosis).  Each chromatid contains one long DNA double helix. ANIMAL & PLANTS COMPLETE THE CELL CYCLE TO DIVIDE: 1) INTERPHASE - non-dividing phase and it represents a very large portion of the Cell Cycle. - the cell spends most of its time in the interphase. - chromosomes are uncondensed, uncoiled and are long and thin called chromatin. - even when dyed, the chromosomes are not visible under a light microscope in this phase. - requires a lot of energy. Interphase consists of three phases: 1. G1 Phase-the first gap phase  First growth and development.  First cell growth to ensure that the daughter cells are identical and growing of organelles, such as mitochondria. 2. S Phase- Synthesis Phase  Chromosomes duplicates- DNA synthesis  Replicates organelles and manufacture additional cytoplasm.  Centrioles replicate 3. G2 Phase- the second gap phase  Further growth and development.  Second cell growth to prepare parent cell for mitosis and cytokinesis. Why do the GAP PHASES exist?  Before mitosis occurs, the parent cell must be large enough and must duplicate organelles and DNA to ensure that the daughter cells are the exact same. What is GAP 0 PHASE?  Gap 0 Phase is the NON-DIVIDING STATE or the post-mitotic phase.  Cells in G 0hase do not go through mitosis. These cells will not divide.  Examples: Nerve and muscle cells enter G0 phase once they have fully matured. Fully matured nerve and muscle cells do not undergo mitosis- will not divide and increase more. Therefore, when these cells are damaged- it can never be replaced.  However, many cells may also trigger its phase from G phase to G phase when more cells are needed. When liver 0 1 cells are critically damaged (cirrhosis of the liver) from drinking alcohol or other incidients, the liver cells are triggered back to G 1hase to regenerate itself and replace the damaged cells. When this is complete, it goes back to being in G 0 phase. 11.2 HOW DOES MITOSIS TAKE PLACE? 2) MITOTIC PHASE (M PHASE- dividing phase Mitotic phase consist of five phases PROPHASE 1. Chromosomes condenses and becomes visible 2. Mitotic Spindles, a structure that pulls the chromosomes into the daughter cells during mitosis and consists of microtubules, forms. -The microtubules (spindle fibres and mitotic spindles) originate from centrosomes- a pair of centrioles. 3. The mitotic spindles begin moving to opposite ends. PROMETAPHASE 1. Nucleolus and nuclear membrane/envelope disappears 2. Centromeres move to opposite ends. 3. Spindle Fibres, which are groups of microtubules, from each mitotic spindle, attaches to the sister chromatids at the kinetochore, which is located at the centromere. Each chromatid has two kinetochores. 4. Kinetochore microtubules are polymerized, randomly oriented at first, and then they become aligned parallel with microtubules. 5. Kinetochore microtubles begins moving the chromosomes to the center of the cell. METAPHASE 1. Centrioles are at the poles of the cell. (The Organizing Phase) 2. Chromosomes line up at the center of the cell (metaphase plate). 3. Formation of mitotic spindle is complete. ANAPHASE 1. Centromeres split, allowing sister chromatids to (Separating DNA copies) separate. 2. Sister chromatids are pulled apart by the spindle fibres to form individual chromosomes. 3. Kinetochore shorten- move the chromosomes to opposite ends of the cell. 4. Non-kinetochore microtubules elongate the cell. TELOPHASE 1. Nuclear envelope forms around each set of chromosomes. 2. Mitotic spindles and spindle fibres disintegrate. 3. Chromosomes uncondensed to become chromatin. KINETOCHORE MICROTUBULES  Extend from the poles to the kinetochores on the chromatids.  Attaches to the kinetchores.  They assist in the movement of chromosomes to the metaphase plate. NONKINETOCHORE MICROTUBULES  Radiate from each centrosome towards the metaphase plate without attaching to the kinetachore of the chromosomes.  May overlap with those from the opposite poles.  They assist in the elongating of the entire cell during anaphase.  They also form a cage like structure and network that facilitates the activities of the cell cycle components and for opening up the cell. CYTOKINESIS  Occurs immediately after mitosis.  Division of cytoplasm and organelles- The cytoplasm divides, forming to daughter cells that have its own nucleus and set of organelles.  IN ANIMALS: CLEAVAGE FURROW forms- the cell is pinched at the equator. The cleavage furrow forms because an actin filament ring forms inside the plasma membrane. The motor protein, myosin binds to the actin filament ring. When mysosin binds to ATP or ADP, the actin filament to slide, causing the ring to shrink and tighten. Because the ring is attached to the membrane, the membrane is pulled in, creating a cleavage furrow. The actin and mysosin filaments continue to slide pass each other until two daughter cells are formed.  IN PLANTS: CELL PLATE forms- a series of microtubules and other proteins define and organize the region where the new plasma membrane and cell wall will form. Vesicles from the Golgi apparatus, carry components of the plasma membrane and cell wall build up in the middle of the cell to create the cell plate. How do BACTERIA divide? (Box 11.2)  Bacteria divide into two identical daughter cells through BINARY FISSION.  Bacterial chromosomes are singular and circular, which are located in the nucleoid. Steps in BINARY FISSION 1. Chromosome is located in the middle of the cell. 2. Chromosomes replicate. 3. Chromosomes pull apart; ring of Ftsz protein forms. 4. FtsZ ring constricts, causing the membrane and cell wall to infold, forming two new daughter cells. HOW DO CHROMOSOMES MOVE DURING MITOSIS? How do the chromosomes move during the process of Mitosis? - the tubulin subunits of the kinetochore microtubules are depolymerized (lost) from the kinetochore ends - motor proteins, such as dynein attach and detach along the kinetochore microtubule’s length, this results in chromosome movement MITOTIC SPINDLE FORCES  Spindle fibres are composed of microtubules- which is composed of α-tubulin and β- tubulin dimers.  The length of the microtubule is determined by the number of tubulin dimers it contains.  Microtubules are asymmetric- meaning they
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