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Cell Cycle and Mitosis Background • All cells come from pre-existing cells • Humans begin life with a single cell from which 600 billion trillion cells are made (6x1023) • Each second, millions of cells die. They must be replaced to ensure the proper functioning of our bodies DNA Basics • DNA or Deoxyribonucleic Acid is a large molecule of the sugar deoxyribose and phosphate linked to four different bases (A,T,C,G) • DNA contains the genetic information that specify the proteins that allow life. It is the “blueprint” of the cell DNA (con’t) • DNA is organized into specific informationcontaining sections called genes • The genes specify which proteins are made and the proteins then determine observable traits (eg. Eye colour) • Mitosis is designed to ensure exact copies of the DNA in chromosomes are passed on to daughter cells Cell Cycle • The cell cycle is an ordered set of events, culminating in cell growth and division into two daughter cells • Non dividing cells are not considered to be in cell cycle • The cell cycle is divided into two distinct phases – Interphase and Mitosis Cell Cycle • Interphase consist of three phases: • - gap 1 (G1) – cell growth - S (Synthesis) – replication of DNA - gap 2 (G2) – cell growth • Mitosis (M) consists of four phases which will be studied in detail Interphase • The cell is engaged in metabolic activity and performing its preparations for mitosis • Chromosomes are not clearly discerned Chromosomes • Carry cell’s DNA along with some proteins • Found in pairs that are similar but not identical. Known as homologues or is said to be homologous • For most of the cell cycle, chromosomal material organized into long, thin strands of thread-like material known as chromatin Chromosomes (con’t) • During mitosis, chromatin coils and thickens to form the short, thick chromosomes Chromosome Structure • Before a cell divides, each chromosome duplicates itself • A duplicated chromosome is made up of two identical chromatids (sister chromatids) held together by a centromere Mitosis • Mitosis is divided into four distinct stages 1. 2. 3. 4. Prophase Metaphase Anaphase Telophase Prophase 1. 2. 3. 4. 5. Chromatin begins to shorten and thicken Nucleolus and nuclear membrane break down Spindle fibres begin to form near the poles. These fibres are microtubules and they form a microskeleton Centrosomes are made up of 2 centrioles and they migrate to opposite poles Asters begin to form Metaphase 1. Chromatids are shortened and thickened into chromosomes 2. Sister chromatids align along the middle or equatorial plate Anaphase 1. Centromeres divide and chromatids separate 2. Spindle fibres pull separated chromosomes to opposite centrosome 3. Nuclear membranes assemble around chromosome clusters 4. Cytokinesis usually begins Telophase 1. Daughter chromosomes arrive at opposite poles 2. Chromosomes begin to uncoil into chromatin 3. Nuclear membrane begins to form around the chromatin 4. Nucleolus reappears and the spindle fibres dissappear Cytokinesis • Once the two nuclei have been formed, a division of the cytoplasm, called cytokinesis occurs • Begins in anaphase and continues throughout telophase • In plants – cell plate forms followed by a the formation of the cell wall Cytokinesis (con’t) • In animals – a cleavage furrow forms around the middle of the parent cell pinching the cell into two equal halves • Each daughter cell now begins a new cell cycle starting with the first growth stage Cancer • Carcinogenic agents may disrupt the normal mitotic process • Cells reproduce at a rate far in excess of normal cell division and lose control of growth, differentiation and replication