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CELL GROWTH AND DIVISION Chapter 10 CELL REPRODUCTION Section 1 Introduction • Cell reproduction: new cells made that are exact copies of the cells they replace • Our bodies make over 2 trillion cells a day Why Cells Reproduce • Cell Size • A cell grows larger by producing more cell products • They take in nutrients, process them, and get rid of wastes • Substances travel farther to reach where they are needed Why Cells Reproduce • Cell Maintenance • Work of cells is done by proteins • More proteins are needed as cells get larger • Cell proteins made from DNA Why Cells Reproduce • Making New Cells • Each new cell made has a higher surface area-to-volume ratio than its parent • Cells divide when they grow to a certain size because it’s hard to maintain large cells Why Cells Reproduce • Cell exchange is limited by: • Surface area-to-volume ratio • Cell’s DNA Chromosomes • Cell activity is directed by DNA • Genes: hereditary pieces that make up DNA • Simple organisms have thousands of genes • Chromosomes: made up of DNA and proteins Chromosomes • Prokaryotic Chromosome • Single circular molecule of DNA • Contains thousands of genes • Condensed through twisting Chromosomes • Eukaryotic Chromosome • Contain several chromosomes in each nucleus • Eukaryotic DNA is packaged into highly condensed chromosome structures with the help of many proteins • Chromatin: what chromosomes are made of Chromosomes • Forms of Chromatin • Histone: protein around which DNA is wound • 8 histones come together to form a histone core • DNA winds around histones to form nucleosomes • Nucleosomes: histone core and DNA around it Chromosomes • Packaging During Cell Division • During the life of a cell, its chromosomes exist as coiled or uncoiled nucleosomes • As the cell gets ready to divide, chromosomes condense even further • Keeps DNA from getting tangled Chromosomes • Chromosome Structure • Chromatid: one half of a chromosome • Very thick • Two chromatids are held together at a centromere • During cell division, sister chromatids divide at the centromere and one ends up in each daughter cell Preparing for Cell Division • All new cells are produced from dividing preexisting cells • Before a cell divides, a copy of DNA is made for each daughter cell Preparing for Cell Division • Prokaryotes • Circular DNA molecule is attached to the inner cell membrane • Cytoplasm divides when a new cell membrane forms between the two DNA copies • Cell wall forms around the new membrane, pushing inward until it pinches off Prokaryotes Preparing for Cell Division • Eukaryotes • More complex than prokaryotes • Have many organelles that have to be copied • DNA has to be copied, sorted, and separated MITOSIS Section 2 Eukaryotic Cell Cycle • Cell cycle: a repeating sequence of cellular growth and division during the life of a cell • The life of a eukaryotic cell cycles through phases of growth, DNA replication, preparation for cell division, and division of the nucleus and the cytoplasm • Made up of 5 phases: • First three phases known as interphase • Other two phases make up cell division Cell Cycle Eukaryotic Cell Cycle • Interphase • During interphase, the cell is not dividing • It’s growing and preparing to divide • Certain cells spend less time in interphase • Three phases: • G1 • S • G2 Eukaryotic Cell Cycle • G1: first gap phase • Cell grows rapidly • Cell builds more organelles • Cells not dividing remain in G1 phase • S: synthesis phase • Cell’s DNA is copied • Twice as much DNA as when the phase starts Eukaryotic Cell Cycle • G2: second gap phase • Cell continues to grow • Cell prepares to divide • Forms special structures that help the cell divide • Ex: microtubules Eukaryotic Cell Cycle • Cell Division • Mitosis: nucleus divides to form two nuclei • Each nucleus contains a complete set of the cell’s chromosomes • Nuclear membrane breaks down briefly • Sister chromatids are pulled to opposite sides of the cell • Cytokinesis • Cytoplasm divides • Two daughter cells are physically separated Stages of Mitosis • Although mitosis is continuous, it is divided into four stages: • Prophase • Metaphase • Anaphase • Telophase Stage 1: Prophase • Chromosomes begin to condense • Nuclear envelope breaks down • Spindle forms • Centrosomes (made up of a pair of centrioles) move to opposite sides of the cell Stage 2: Metaphase • Chromosomes are compacted to their most condensed form • Nuclear membrane fully dissolved • Chromosomes line up at equator (middle) of cell • Spindle fibers form between chromosome’s centromere and the poles of the cell Stage 3: Anaphase • Spindle fibers shorten • Causes sister chromatids to move to opposite ends of the cell • Each pole now has a full set of chromosomes Stage 4: Telophase • Nuclear envelope forms around the chromosomes at each pole • Chromosomes uncoil and change back to chromatin • Spindle dissolves • Mitosis is complete Cytokinesis • As mitosis ends, cytokinesis begins • Cytoplasm is separated • Two cells are formed • During cytokinesis, the cell membrane grows at the center of the cell and divides it into two daughter cells of equal size • Each daughter cell has about half of the parent’s cytoplasm and organelles Cytokinesis • Separating the Cytoplasm • In cells that lack a cell wall, the cell is pinched in half by a belt of protein threads • Cell membrane moves in and pinches off • In cells that have a cell wall, vesicles line up across the middle of the cell and form a cell plate • Separates the cell into two new cells Cytokinesis • Continuing the Cell Cycle • After cytokinesis is complete, each cell enters the G1 stage of interphase • Cell cycle starts all over again REGULATION Section 3 Controls • Scientists study cell cycle by observing cells in a petri dish • A few cells in a dish of nutrients divide rapidly • When they touch one another or the sides of the petri dish, they stop dividing • Happens just like in your body • When you cut your skin or break bones, your cells divide rapidly to heal the wound • When they start touching each other, the wound is closed and they stop dividing Controls • Cell division is highly controlled • Cell growth and division depend on protein signals and other environmental factors • Controls of the cell cycle: • Proteins from within the cell • Environment • Signals from other cells Checkpoints • Cell undergoes an inspection process to ensure the cell is ready for the next phase in the cell cycle • Feedback signals at key checkpoints in the cell cycle can delay or trigger the next phase of the cell cycle • Three main checkpoints: • G1 • G2 • Mitosis Checkpoints • G1 Checkpoint • Before the cell copies its DNA, it checks its surroundings • If conditions are favorable: the cell moves on • If conditions are unfavorable: the cell goes through a resting period Checkpoints • G2 Checkpoint • Before mitosis begins, the cell checks for any mistakes in the copied DNA • If mistakes are found: enzymes correct mistakes that are found • If no mistakes are found: cell may begin to divide Checkpoints • Mitosis Checkpoint • When chromosomes line up at the equator in metaphase, the cell checks that the chromosomes are properly attached to the spindle fibers • Ensures that genetic material is distributed equally between the daughter cells Cancer • Cancer: a group of severe and sometimes fatal diseases that are caused by uncontrolled cell growth • Uncontrolled cell growth and division can result in masses of cells that invade and destroy healthy tissues Cancer • Loss of Control • Damage to a cell’s DNA can cause the cell to respond improperly or to stop responding • The cell cycle can no longer be controlled • Eventually, defective cells lead to more defective cells, which leads to a tumor Cancer • Development • Benign tumor: does not spread to other parts of the body • Can be removed by surgery • Malignant tumor: invades and destroys healthy tissues and organs • Metastasis: when malignant tumors break free and move about the body Cancer • Treatment • Some cancers can be treated by using drugs that kill the fastgrowing cancer cells • Called chemotherapy (chemo) • Some cancers can be treated by surgery • Prevention • Best preventative measure is avoid things that cause cancer • UV radiation • Chemicals in cigarette smoke
