Key Concepts In eukaryotes most dividing cells go through a cycle that consists of four phases After chromosomes are copied during S phase they are moved to the middle of the cell during M phase mitosis One chromosome copy is distributed to each of two daughter cells Mitosis and cytokinesis produce two cells that are genetically identical to the parent cell 2011 Pearson Education Inc Key Concepts Progression through the cell cycle is carefully controlled In multicellular organisms uncontrolled cell division may lead to cancer Different types of cancer result from different types of defects in control over the cell cycle 2011 Pearson Education Inc Introduction to Cell Division Cells arise through the division of preexisting cells There are two types of cell division meiosis and mitosis Both forms of cell division are usually accompanied by cytokinesis in which the cytoplasm of the cell divides into two distinct daughter cells 2011 Pearson Education Inc Contrasting Mitosis and Meiosis Meiosis leads to the production of gametes eggs and sperm Daughter cells have half the amount of genetic material as the parent cell Mitosis leads to the production of all other cell types referred to as somatic cells Genetic material is copied and then divided equally Daughter cells are genetically identical to the parent cell 2011 Pearson Education Inc Functions of Mitosis Mitosis and cytokinesis are responsible for three key events in multicellular eukaryotes 1 Growth 2 Wound repair 3 Asexual reproduction 2011 Pearson Education Inc What Is a Chromosome Chromosomes contain a single long double helix of deoxyribonucleic acid DNA wrapped around proteins DNA encodes the cell s genetic information A gene is a section of DNA that encodes a specific protein or RNA Chromosomes can be stained with dyes and observed under the light microscope 2011 Pearson Education Inc 2011 Pearson Education Inc Chromosomes Change before and during Mitosis The purpose of mitosis is to distribute chromosomes to daughter cells during cell division To this end each chromosome is replicated prior to mitosis As mitosis starts the chromosomes condense from long thin filaments into compact structures that can be moved around the cell efficiently At the end of mitosis one of the chromosome copies is distributed to each of two daughter cells 2011 Pearson Education Inc Chromosome Replication Prior to mitosis each chromosome is replicated Each of the DNA copies in a replicated chromosome is called a chromatid Chromatids are joined together along their entire length as well as at a specialized region of the chromosome called the centromere Chromatids from the same chromosome are referred to as sister chromatids Even though a replicated chromosome consists of two chromatids it is still considered a single chromosome 2011 Pearson Education Inc 2011 Pearson Education Inc M Phase and Interphase Growing cells cycle between a dividing phase called the mitotic M phase and a nondividing phase called interphase 2011 Pearson Education Inc Interphase S Phase The cell cycle is the orderly sequence of events that occurs from the formation of a eukaryotic cell through the duplication of its chromosomes to the time it undergoes cell division A visualization technique called autoradiography allowed researchers to identify the part of the cell cycle during which DNA replication occurs Chromosome replication occurs only during interphase and not during M phase The stage in which DNA replication occurs is called the synthesis S phase 2011 Pearson Education Inc Interphase Gap Phases Interphase also includes two gap phases during which no DNA synthesis occurs The first gap G1 phase occurs before the S phase The second gap G2 phase occurs between S phase and mitosis During the gap phases organelles replicate and additional cytoplasm is made in preparation for cell division It takes a cell about 24 hours to complete one cell cycle G1 phase lasts 7 9 hours S phase lasts 6 8 hours G2 phase lasts 4 5 hours 2011 Pearson Education Inc The Cell Cycle There are a total of four phases in the cell cycle M phase and an interphase consisting of the G1 S and G2 phases Gap phases allow the cell to grow large enough and synthesize enough organelles to ensure the daughter cells will be normal in size and function 2011 Pearson Education Inc 2011 Pearson Education Inc Mitosis Overview Mitosis results in the division of replicated chromosomes and formation of two daughter nuclei with identical chromosomes and genes Mitosis is usually accompanied by cytokinesis Every species has a characteristic number of chromosomes Humans have 46 2011 Pearson Education Inc Chromosomes Change during the Cell Cycle Eukaryotic chromosomes consist of DNA associated with histone proteins In eukaryotes this DNA protein material is called chromatin During interphase most chromatin is relaxed or uncondensed forming long threadlike strands After replication during S phase each chromosome consists of two genetically identical sister chromatids attached at the centromere At the start of mitosis the replicated chromosomes condense 2011 Pearson Education Inc Events in Mitosis During mitosis the two sister chromatids separate to form independent chromosomes and one copy of each chromosome goes to each of the two daughter cells As a result each daughter cell receives a copy of the genetic information that is contained in each chromosome Mitosis M phase is a continuous process with five subphases based on specific events Prophase Prometaphase Metaphase Anaphase Telophase 2011 Pearson Education Inc 2011 Pearson Education Inc Prophase During prophase chromosomes condense and first become visible in the light microscope The mitotic spindle if made up of microtubules called spindle fibers forms from a microtubule organizing center Polar microtubules push the poles of the cell away from each other during mitosis Kinetochore microtubules pull chromosomes to the poles of the cell during mitosis In animal cells the microtubule organizing center is a centrosome a structure that contains a pair of centrioles 2011 Pearson Education Inc Prometaphase During prometaphase The nuclear envelope breaks down The nucleolus disappears Kinetochore microtubules from each mitotic spindle attach to one of the sister chromatids of each chromosome Attachment occurs in the centromere region at the kinetochore 2011 Pearson Education Inc Metaphase During metaphase the formation of the mitotic spindle is completed Motor proteins on the