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Cell Size Limitations • Cells come in a wide variety of sizes and shapes. • Considering this wide range of cells sizes, why then can’t most organisms be just one giant cell? Website upload 2014 Diffusion limits cell size • Although diffusion is a fast and efficient process over short distances, it becomes slow and inefficient as the distances become larger. • Because of the slow rate of diffusion, organisms can’t be just one giant-sized cell. Website upload 2014 DNA limits cell size • The cell cannot survive unless there is enough DNA to support the protein needs of the cell. Website upload 2014 Surface area-to-volume ratio 1 mm 4 mm 2 mm 1 mm 2 mm 1 mm SA = 6 mm2 Vol = 1 mm3 Ratio of 6:1 4 mm 2 mm SA = 24 mm2 Vol = 8 mm3 SA = 96 mm2 4 mm Vol = 64 mm3 Ratio of 1.5:1 Ratio of 3:1 • As a cell’s size increases, its volume increases much faster than its surface area (SA). • If cell size doubled, the cell would require eight times more nutrients and would have eight times more waste to excrete. Website upload 2014 Surface area-to-volume ratio 1 mm 4 mm 2 mm 1 mm 2 mm 1 mm SA = 6 mm2 Vol = 1 mm3 Ratio of 6:1 4 mm 2 mm SA = 24 mm2 Vol = 8 mm3 SA = 96 mm2 4 mm Vol = 64 mm3 Ratio of 1.5:1 Ratio of 3:1 • The surface area (SA), however, would increase by a factor of only four. • The cell would either starve to death or be poisoned from the buildup of waste products. Website upload 2014 Cell Division • Cell division is the process by which new cells are produced from one cell. • Cell division results in two cells that are identical to the original, parent cell. Website upload 2014 Chromosomes • Structures, which contain DNA and become darkly colored when stained, are called chromosomes. • Chromosomes are the carriers of the genetic material that is copied and passed from generation to generation of cells. • Accurate transmission of chromosomes during cell Website upload 2014 division is critical. The large, complex chromosomes of eukaryotes duplicate with each cell division • Chromosomes contain Sister chromatids a very long DNA molecule with thousands of genes Centromere – Individual chromosomes are only visible during cell division – They are packaged as chromatin when visible Website upload 2014 Centromere The large, complex chromosomes of eukaryotes duplicate with each cell division • Before a cell starts dividing, the chromosomes are duplicated – This process produces sister chromatids Sister chromatids Centromere Centromere Website upload 2014 double-stranded human chromosomes ready for mitosis Website upload 2014 The Cell Cycle • The cell cycle is the sequence of growth and division of a cell. • The majority of a cell’s life is spent in the growth period known as interphase. Interphase Website upload 2014 The Cell Cycle • Following interphase, a cell enters its period of nuclear division called mitosis. • Following mitosis, the cytoplasm divides, separating the two daughter cells. Mitosis Website upload 2014 Interphase: A Busy Time • Interphase, the busiest phase of the cell cycle, is divided into three parts: G1,S, & G2 Interphase DNA synthesis and replication (S) Rapid growth and metabolic activity (G1) Cell prepares for division (G2) Website upload 2014 Interphase: A Busy Time • During the first part of interphase, (G1),the cell grows and protein production is high. Interphase (G1) Rapid growth and metabolic activity (G1) Website upload 2014 Interphase: A Busy Time • In the second part of interphase, (S),the cell copies its chromosomes. Interphase (S) DNA synthesis and replication (S) Website upload 2014 Interphase: A Busy Time • After the chromosomes have been duplicated, the cell enters another shorter growth period, (G2),in which mitochondria and other organelles are manufactured and cell parts needed for cell division are assembled. Interphase (G2) Cell prepares for division (G2) Website upload 2014 Biology is the only subject in which multiplication is the same thing as division… Website upload 2014 MITOSIS: Making New Cells Website upload 2014 Where it all began… You started as a cell smaller than a period at the end of a sentence… . Website upload 2014 Getting from there to here… • Going from egg to fetus…. the original fertilized ova (egg) has to divide… and divide… and divide… and divide… Website upload 2014 Why do cells divide… Single-celled organisms for reproduction asexual reproduction (identical clones) amoeba starfish Website upload 2014 Why do cells divide… Multi-celled organisms for growth & development from fertilized egg to adult for repair & replacement replace cells that die from normal wear & tear or from injury amoeba Website upload 2014 starfish The Phases of Mitosis • The four phases of mitosis are Prophase, Metaphase, Anaphase, and Telophase. •PMAT Website upload 2014 Prophase: The first phase of mitosis • During prophase, the chromosomes coil to form visible chromatins. Spindle fibers Disappearing nuclear envelope Doubled chromosome Website upload 2014 Prophase: The st 1 phase of mitosis • The two halves of the doubled structure are called sister chromatids. • Sister chromatids are held together by a structure called a centromere, which plays a role in chromosome movement during mitosis. Sister chromatids Centromere Website upload 2014 Metaphase: The nd 2 stage of mitosis • During metaphase, the chromatids move to the equator (middle) of the spindle. Website upload 2014 Anaphase: The 3rd phase of mitosis • During anaphase, the centromeres split and the sister chromatids are pulled apart to opposite poles of the cell. Website upload 2014 Telophase: The 4th phase of mitosis • During telophase, two distinct daughter cells are formed. • The cells separate as the cell cycle proceeds into the next interphase. Nuclear envelope reappears Two daughter cells are formed Website upload 2014 DNA must be duplicated… chromosomes in cell DNA in chromosomes cell 4 single-stranded chromosomes duplicated chromosomes nucleus duplicated chromosomes 4 double-stranded chromosomes cell Website upload 2014 nucleus Mitosis: Dividing DNA & cells • Stage 1: cell copies DNA • Making new DNA during S phase of interphase Copy DNA! DNA cell Copying of DNA occurs during interphase S nucleus Website upload 2014 (interphase S) Mitosis: Dividing DNA & cells • Stage 2: DNA winds into chromosomes – DNA is wound up into chromosomes to keep it organized. duplicated chromosomes Wind up! cell Organizing of DNA occurs during prophase nucleus Website upload 2014 (prophase) Mitosis: Dividing DNA & cells • Stage 3: Chromosomes line up – chromatids line up in middle – attached to protein “cables” that will help them move Line up! Lining of DNA along equator occurs during metaphase duplicated chromosomes line up in middle of cell Website upload 2014 (metaphase) Mitosis: Dividing DNA & cells • Stage 4: Chromosomes separate – chromatids divide, separating pairs – start moving to opposite ends Separate! Dividing & separating of DNA occurs during anaphase chromosomes divide & move to opposite ends Website upload 2014 (anaphase) Mitosis: Dividing DNA & cells • Stage 5: Cell starts to divide – cell start to divide – nucleus re-forms Divide! Cell division begins to occur during telophase Website upload 2014 (telophase) Mitosis: Dividing DNA & cells • Stage 6: DNA unwinds again – Identical ‘daughter’ cells separate – now they can do their every day jobs The End! New cells are formed during upload 2014 the process Website of cytokinesis. (cytokinesis) New “daughter” cells • Get 2 exact copies of original cells – same DNA – “clones” Website upload 2014 Cytokinesis • Following telophase, the cell’s cytoplasm divides in a process called cytokinesis. • Cytokinesis differs between plants and animals. • Toward the end of telophase in animal cells, the plasma membrane pinches in along the equator. Website upload 2014 Cytokinesis • Plant cells have a rigid cell wall, so the plasma membrane does not pinch in. • A structure known as the cell plate is laid down across the cell’s equator. • A cell membrane forms around each cell, and new cell walls form on each side of the cell plate until separation is complete. Website upload 2014 Results of Mitosis • When mitosis is complete, unicellular organisms remain as single cells. • In multicellular organisms, cell growth and reproduction result in groups of cells that work together as tissue to perform a specific function. • Tissues organize in various combinations to form organs that perform more complex roles within the organism. • Multiple organs that work together form an organ system. Website upload 2014 Mitosis in whitefish blastula Website upload 2014 Mitosis in plant cell Website upload 2014 onion root tip Website upload 2014 Overview of mitosis interphase I.P.M.A.T.C. prophase Please Make Another Two Cells cytokinesis metaphase anaphase Website upload 2014 telophase Mitosis & Cancer: When Making New Cells Goes Wrong! Website upload 2014 When is mitosis a good thing? • When you have to add or replace cells – growth & development – repair – replacement Website upload 2014 When is mitosis a BAD thing • When cells reproduce & they are not needed – these cells take over organs, but don’t do the right job – they just keep making copies – cancer • Uncontrolled cell growth • Damages organs Website upload 2014 Why would cells just make bad copies? • If DNA gets damaged, cells stop listening to correct instructions – Mutations (wrongly constructed DNA strands) • Some causes of mutations: UV radiation chemical exposure radiation exposure heat cigarette smoke pollution age genetics Website upload 2014 Normal Control of the Cell Cycle Proteins and enzymes control the cell cycle • The cell cycle is controlled by proteins called cyclins and a set of enzymes that attach to the cyclin and become activated. • Occasionally, cells lose control of the cell cycle. Website upload 2014 Normal Control of the Cell Cycle • This uncontrolled dividing of cells can result from the failure to produce certain enzymes, the overproduction of enzymes, or the production of other enzymes at the wrong time. • Cancer is a malignant growth resulting from uncontrolled cell division. Website upload 2014 Normal Control of the Cell Cycle • Enzyme production is directed by genes located on the chromosomes. • A gene is a segment of DNA that controls the production of a protein. Website upload 2014 Cancer: A mistake in the Cell Cycle • Currently, scientists consider cancer to be a result of changes in one or more of the genes that produce substances that are involved in controlling the cell cycle. • Cancerous cells form masses of tissue called tumors that deprive normal cells of nutrients. Website upload 2014 Tumors • Benign tumor – abnormal cells remain at original site as a lump – most do not cause serious problems & can be removed by surgery Website upload 2014 Tumors • Malignant tumor – cells leave original site • carried by circulatory systems to other tissues • start more tumors – damage functions of organs throughout body Website upload 2014 Cancer: A mistake in the Cell Cycle • In later stages, cancer cells enter the circulatory systems and spread throughout the body, a process called metastasis, forming new tumors that disrupt the function of organs, organ systems, and ultimately, the organism. Website upload 2014 The causes of cancer • The causes of cancer are difficult to pinpoint because both genetic and environmental factors are involved. Website upload 2014 The causes of cancer • Environmental factors, such as cigarette smoke, air and water pollution, and exposure to ultraviolet radiation from the sun, are all known to damage the genes that control the cell cycle. • Cancer may also be caused by viral infections that damage the genes. Website upload 2014 Cancer prevention • Physicians and dietary experts agree that diets low in fat and high in fiber content may reduce the risk of many kinds of cancer. • Vitamins and minerals may also help prevent cancer. • In addition to diet, other healthy choices such as daily exercise and not using tobacco also are known to reduce the risk of cancer. Website upload 2014