Chapter 8 1 Interphase can divided into three subphases the G1 phase first gap the S phase synthesis of DNA also known as DNA replication0 and the G2 phase second gap During all three subphases the cell grows The chromosomes are duplicated during S phase At the beginning of the S phase each chromosomes is single At the end of this subphase after DNA replication the chromosomes are doubled each consisting of two sister chromatids joined along their lengths During the G2 phase the cell grows more as it completes preparation for cell division The mictotic phase is divided into two overlapping stages called mitosis and cytokinesis In the Prophase stage of mitosis during prophase changes occur in both the nucleus and the cytoplasm In the Prometaphase stage the nuclear envelope breaks into fragments and disappears In the Metaphase stage the mitotic spindle is fully formed with its poles at opposite ends of the cell In the Anaphase stage it begins when the two centromeres of each chromosomes come apart separating the sister chromatids In the Telephase stage the cell elongation that started in anaphase continues In the Cytokinesis stage the division of the cytoplasm usually occurs simultaneously with telephase with two daughter cells completely separating soon after the end of mitosis 2 In animals cytokinesis involves a cleavage furrow in which contracting microfilaments pinch the cell in two In plants it involves formation of a cell plate a fusion of vesicles that forms a new plasma membranes and cell walls between the cells 3 Physical factors that influence cell division are density and dependent inhibition and anchorage dependence Chemical factors can include the rate of cell growth 4 For many cells the G1 checkpoint seems to be the most important in cell division If a cell receives a go ahead signal for example form a growth factor at the G1 checkpoint it will usually enter the S phase eventually going on to complete its cycle and divide If such signal never arrives the cell wall switches to permanently non dividing state called the G0 phase Many cells in the human body such as mature nerve cells and muscle cells are in the G0 phase 5 A Malignant tumor can spread into neighboring tissues and invade other parts of the body displacing normal tissue and interrupting organ function as it grows If the abnormal cells remain at their original site the lump is called a Benign tumor The spread of cancer cells beyond their original site is called metastasis 6 The two chromosomes of such a matching pair are homologous chromosomes because each chromosome carries genes controlling the same inherited characteristics A cell with a single chromosome set is called haploid Chapter 8 Most animals and plants are said to be diploid organisms because all somatic cells contain pairs of homologous chromosomes Haploid is one set of chromosomes Diploid is two set of chromosomes 7 The stages of meiosis Prophase I The nuclear membrane dissolves Chromatin tightly coils up Homologous chromosomes each composed of two sister chromatids come together as pairs in a process called synapsis During synapsis chromatids of homologous chromosomes exchange segments in a process called process crossing over The chromosomes tetrads move toward the center of the cell Metaphase I Tetrads align at the cell equator Anaphase I Homologous pairs separate and move toward opposite poles of the cell Unlike mitosis the sister chromatids making up each doubled chromosomes remain attached Telophase I Duplicated chromosomes have reached the poles Usually cytokinesis occurs along with telophase Meiosis II follows meiosis I without chromosomes duplicated Each of the two haploid products enter meiosis Meiosis II events middle of the cell are in mitosis Prophase II A spindle forms and moves chromosomes toward the Metaphase II Duplicated chromosomes align at the cell equator like they Meiosis II Anaphase Sister chromatids separate Individual chromosomes move toward opposite poles Telophase II Chromosomes have reached the poles of the cell A nuclear envelope forms around each set of chromosomes With cytokinesis four haploid cells are produced 8 Mitosis and meiosis both begin with diploid parent cells that have chromosomes duplicated during the previous interphase However the end products differ Mitosis produces two genetically identical diploid somatic daughter cells Meiosis produces four genetically unique haploid somatic daughter cells 9