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Chapter 13: The Cell Cycle Leaving Certificate Biology Higher Level Cell Continuity • Cells that arise from cells of the same type – Living organisms maintain cell continuity by mitosis and cell division Chromatin and Chromosomes • Chromatin is elongated DNA • Chromosomes are composed of DNA and protein • Chromosomes are arranged into homologous pairs – pairs of chromosomes that contain genes that control the same characteristics Haploid and diploid • Haploid means one set of chromosomes • Diploid means two sets of chromosomes – In humans the haploid number is 23 – The diploid number is 46 – arranged into 23 pairs Cell cycle – The cell cycle occurs under three distinct stages: 1. Interphase 2. Mitosis 3. Cell division Interphase • Interphase is the stage where a cell is not dividing and a cell spends the vast majority of its time in this stage • Chromosomes are elongated and the cell goes about its daily functions • Towards the end of interphase the cell organelles and DNA are replicated End of interphase The DNA is replicated and condenses into duplicated chromosomes which are held together by centromeres Centromere DNA replication 4n 2n Nucleus Mitosis • Mitosis is nuclear division that leads to the formation of two identical daughter cells • Stages: – Prophase – Metaphase – Anaphase – Telophase Prophase 2. The nuclear membrane begins to disappear and spindle fibres begin to appear: Nucleus Metaphase • The replicated chromosomes line up along the equator of the cell held in place by spindle fibres: Nuclear membrane has completely disappeared Spindle fibre Anaphase • The spindle fibres begin to contract pulling one chromosome from each pair to each pole: Centromeres split at anaphase Single chromosome pulled to pole Telophase • The chromosomes at each pole begin to unravel and lengthen; spindle fibres disappear; and the nuclear membranes reform at each pole: Chromatin and nuclear membranes reform Cell Division • Cell division occurs immediately after mitosis and involves formation of cleavage furrow (in animals cells) or cell plate (in plant cells) prior to cell splitting Cleavage furrow (animals); cell plate (plants) Cell Division – Mitosis v Meiosis 2n 2n Mitosis Meiosis 2n 2n n n n n Cancer • Cancer is a disease whereby a cell loses its ability to control both the rate of mitosis and cell division • Mitosis is normally carefully controlled but when it is out of control it can result in cancer • Cancer can be either benign or malignant – Benign cancer involves cells that divide out of control for a limited period of time and do not spread – Malignant cancers involve cells that divide rapidly uncontrollably and spread and invade other tissues – destroying that tissue in the process Cancer • Anything that has the potential to cause cancer is called a carcinogen – which damage DNA • Two carcinogens: – UV light – damages DNA of skin cells – Cigarette smoke – damages the DNA of cells lining the mouth, airways and lungs