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Transcript
MITOSIS AND CELL CYCLE Learning objectives: At the end of the lecture, students should be able to: • • • • • • • Identify steps of cell division:Mitosis( prophase, prometaphase, metaphase,anaphase,telophase) Explain the significance of mitosis. Define cell cycle. Explain various stages of cell cycle. Recall the events of somatic cell division. Discuss the significance of S phase of cell cycle. Integrate the knowledge of cell cycle to the basis of development of cancer. Lecture outline MITOSIS • Division of somatic cell into two identical daughter cells. • Mitosis is conventionally divided into five phases: – Prophase – Prometaphase – Metaphase – Anaphase – Telophase • Cytokinesis is well underway by late telophase Functions of cell division: The continuity of life is based upon the reproduction of cells, or cell division Necessary to overcome the limitations to Cell Growth like: DNA “Overload” – there is a limit to the number of proteins a DNA molecule can code for at one time. As the cell gets bigger, it needs more proteins, but, it may have to wait if there’s already too many proteins being made Exchange of Materials – there is a limit to the amount of materials a cell can move across it’s cell membrane. As the cell gets bigger, it needs more food/water to come in AND more waste to move out. Eventually it either starve or poison itself with wastes. Cell division is integral part of cell cycle Functions of Cell Division Life cycle of Cell The life cycle of a cell is the period from cell reproduction to the next cell reproduction. When mammalian cells are not inhibited and are reproducing as rapidly as they can, this life cycle may be as little as 10 to 30 hours. It is terminated by a series of distinct physical events called mitosis that cause division of the cell into two new daughter cells. Phases of the Cell Cycle The cell cycle consists of Mitotic (M) phase (mitosis and cytokinesis) Interphase (cell growth and copying of chromosomes in preparation for cell division) Cell Division ; M Phase Mitosis – nuclear division Prophase Metaphase Anaphase Telophase Cytokinesis – division of cytoplasm and organelles Results in 2 identical “daughter” cells Cell Division; Interphase (about 90% of cell cycle) can be divided into subphases: G1 phase (“first gap”) S phase (“synthesis”) G2 phase (“second gap”) Chromosome Structure Chromosome – condensed form of DNA that is visible during cell division Sister Chromatids – the 2 copies of DNA that make up the chromosome and are separated into the 2 daughter cells during cell division Centromere – where the 2 sister chromatids attach The degree of coiling can vary in different regions of the chromatin: Heterochromatin refers to highly coiled regions where genes aren’t expressed. Euchromatin refers to loosely coiled regions where genes can be expressed. Karyotype An ordered, visual representation of the chromosomes in a cell Chromosomes are photographed when they are highly condensed, then photos of the individual chromosomes are arranged in order of decreasing size: In humans each somatic cell has 46 chromosomes, made up of two sets, one set of chromosomes comes from each parent Prophase Chromatin condenses, this causes the chromosomes to begin to become visible Centrosomes separate, moving to opposite ends of the nucleus The centrosomes start to form a framework used to separate two sister chromatids called the mitotic spindle that is made of microtubules Nucleolus disappears Pro-metaphase Nuclear envelope fragments Chromosomes become more condensed A kinetochore is formed at the centromere, the point where the sister chromatids are attached Microtubules attach at the kinetochores Metaphase Shortest phase of mitosis Centrioles are at poles Chromosomes line up in the middle along equator Spindle fibers attach to chromosomes on the centromere Anaphase Each centromere splits making two chromatids free Each chromatid moves toward a pole Cell begins to elongate , caused by microtubules not associated with the kinetochore Telophase Formation of nuclear membrane and nucleolus Short and thick chromosomes begin to elongate to form long and thin chromatin Formation of the cleavage furrow - a shallow groove in the cell near the old metaphase plate Cytokinesis division of the cytoplasm In animal cells, cytokinesis occurs by a process known as cleavage, forming a cleavage furrow Summary; Mitotic Division of an Animal Cell Regulation of cell cycle External signals: growth factors fibronectin,etc Internal signals carcinogens Apoptosis Immune functions SIGNIFICANCE OF S PHASE OF CELL CYCLE • Synthesis Phase: • It is the period during which synthesis of DNA takes place. • DNA synthesis results in preparation of an exact replica of the genetic material. • Replication of centrioles also take place during this phase. • The S phase takes approx 8 hours to complete. Loss of Cell Cycle Control ; Cancer Cancer cells do not respond normally to the body’s control mechanisms Cancer cells form tumors, masses of abnormal cells within otherwise normal tissue If abnormal cells remain at the original site, the lump is called a benign tumor Malignant tumors invade surrounding tissues and can metastasize, exporting cancer cells to other parts of the body, where they may form secondary tumors Normal brain cell Cancer brain cell Reference: • Cell Division- Mitosis: Basic Histology by Junqueira • Cell Cycle: Basic Histology by Junqueira