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Cell Growth and Reproduction The Cell Cycle
Questions you will be able to answer? • What problems does growth cause for cells? • When an organism becomes larger what happens to the cells? • How do organisms grow? • Why do cells stay so small?
Cells Limitations • Cells come in a variety of sizes • Red blood cell diameter of 8 micrometers • Nerve Cells Length up to 1 meter • Ostrich egg yolk diameter of 8 cm • With so many varieties of cells, why can’t we be made of just once Giant Cell?
Most cells are between 2µm and 200µm • A micrometer is 1 millionth of a meter! • Too small to be seen with naked eye
Cell Limitations • Diffusion • Surface area to volume ratio • DNA
Limits • Diffusion limits cell size • Movement from higher concentration to lower concentration • Larger the distance, slower the diffusion rate • A cell 20 cm would require months for nutrients to get to the center
Diffusion limits Cell Size • Diffusion lets a variety of materials through the membrane such as glucose and oxygen. (in and out) • Diffusion works efficiently over small areas but when the area gets larger it becomes slow and inefficient • Cells and organelles would die before any nutrients could reach them.
Surface Area to Volume Ratio Limiting Factor • When size increases the volume increases faster than the surface area. • In your book on page 243. • The volume increases drastically while the surface area increases in small increments.
Surface Area to Volume Explanation • If cell size doubled the cell would require 8X more nutrients and have 8X more waste to excrete. • The surface area would only increase by a factor of 4. • This means the membrane would not have enough surface area to through which O2, nutrients, and wastes can diffuse. • The Cell will either starve or poison itself.
DNA limits Cell Size • DNA is in the Nucleus and is dependent on certain proteins to perform critical cell functions. • There is a limit on how quickly that the proteins can be copied in the nucleus and made into proteins in the cytoplasm. • The cell will die if there is not enough DNA to support proteins needs of the cell.
DNA limits cell size • larger cells need more DNA. Needs more of everything • Most cells have just one nucleus
So.. What happens when they get too big? • Cells Divide before they become too large to function properly. • Cell division has a lot of benefits.
Why do cells divide? • Replacement • Repair • Growth
The Cell Cycle • Cell Theory says all cells come from preexisting cells. • Once a cell reaches its optimum size it has to do one of 2 things • Divide • Stop Growing • Most cells divide • Dividing allows the cell the to • Reproduce • Heal injuries
Cell are reproducing by the second. Right now in your intestine there are millions of cells being reproduced by the second. While on the palm of your hand and the bottom of your feet you are shedding and replacing cells New Cells
Cell Cycle • All cells have life cycles. • They grow and develop. • All organisms grow and change • Worn out tissues are repaired or are replaced by newly produced cells.
Important definitions: • Chromosomes: • Carriers of the genetic material that is copied and passed from generation to generation of cells. • Before they become chromosomes they are CHROMATIN. • Long strands of DNA wrapped around proteins. • Looks like tangled up spaghetti • Must organize before division.
The Cell Cycle • Many things in life follow patterns. • Cell are no different. • The Cell Cycle is the growth and division of a cell. • Goes through 2 periods • Growth • Division • There are 3 main Stages of the Cell Cycle
Majority of Cells Life are in the 1st Phase • Interphase • Cell grows in size • Carries out cellular functions • Replicates DNA • Prepares for Division • Divided into 3 substages
Mitosis • Stage of the Cell Cycle during which the cell’s nucleus and nuclear material divide. • 4 Substages • Towards the end cytokinesis begins. • The method which a cell’s cytoplasm divides creating a new cell.
Depends on the cell can take minutes to a year. For most normal cells, it takes 12-24 hours Time it takes to Divide
Stages of Interphases • The cell grows • Develops into a mature, functioning cell • Duplicates DNA • Prepares for Division • Three Stages • G1, S, and G2 • Gap 1, Synthesis, and Gap 2
G1 or 1st Stage of Interphase • Stage immediately after a cell divides. • Functions • Growing and carrying out normal cell functions • Preparing to replicate DNA
2nd Stage or Synthesis Interphase • A cell copies DNA to prepare for division • This where the chromosomes and chromatin start to organize themselves for division.
3rd and final stage of Interphase Cell prepares for the division of nucleus Proteins are also synthesized in this stage The cell takes inventory and makes sure it is ready for mitosis. http://www.cellsalive.com/cell_cycle.htm Gap 2 or G2 Interphase
Questions for Homework • Relate cell size to cell functions, explain why cell size is limited • Hypothesize what the result would be if a large cell managed to divide, despite the fact that it had grown beyond an optimum size. • Summarize the Primary stages of Cell Cycle • Describe what happens to DNA during the S stage of interphase. • Make a Diagram of the stages of Interphase and describe what happens in each.
Game • http://nobelprize.org/educational_games/medicine/2001/
Two Mitotic Phases of Eukaryotic Cell Division • Mitosis – the nucleus & its content, including chromosomes, divide & are evenly distributed to form 2 daughter nuclei. • Cytokinesis – the cytoplasm is divided in two.
Prophase • Chromatin fibers become more tightly coiled & folded • Nucleoli disappear • Sister chromatids joined at the centromere • Mitotic spindle begins to form; microtubules grow from centrosomes which are moving apart • Nuclear envelope breaks in to fragments
Metaphase • Chromosomes on metaphase plate • The microtubules attach to a particular chromatid all from one pole of spindle; those attached to sister chromatids from the other pole
Anaphase • Two centromeres of each chromosome come apart separating sister chromatids • Motor proteins, powered by ATP, “walk” daughter chromosomes centromere toward opposite poles • Spindle microtubules attached to kinetochores shorten • Spindle microtubules not attached lengthen elongating the cell
Telophase • Reverse of prophase • Cell elongation continues • Nuclear envelopes form • Chromatin uncoils • Mitotic spindle disappear • Nucleoli reappear
Cytokinesis • Division of cytoplasm • In animal cells, 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 new membrane & cell walls between the cells.
Anchorage Dependence • Most animal & plant cells do not divide unless they are in contact with a solid surface. • Most cells of the animal body are normally anchored to an extracellular matrix or to other cells of same tissue.
Cell Cycle Control System • Cyclically operating set of proteins in the cell that both trigger & coordinates the major events in the cell cycle. • 3 key checkpoints: G1, G2 & M phase
Importance of G1 Checkpoint • If a cell receives a go ahead signal (for example, from a growth factor) at the G1 checkpoint, it will usually go through its cycle & divide. • If it does not receive the go ahead signal at G1, it will switch into a non-dividing mode. • Our non-dividing nerve cells & muscle cells are permanently arrested at the G1 checkpoint.
Density-Dependent Inhibition & Its Relationship to Growth Factor • Cells usually stop dividing when they touch one another. • Growth factor is a protein secreted by certain body cells that stimulates cells in the vicinity to divide • Most cells require growth factors to begin dividing & they stop dividing when they run out of these substances.
Cancer • Cancer, which currently claims the lives of 1 out of every 5 people in the United States & other developed nations is a disease of the cell cycle. • Cancer cells do not have a properly functioning cell cycle control system; They divide excessively & can invade other tissues of the body. • This excessive growth can result in an abnormal mass of cells called a tumor.
Terms related to Cancer • Tumor – abnormal mass of cells • Benign tumor – an abnormal mass of essentially normal cells always remain at their original site. • Malignant tumor – a mass of cancer cells which are capable of spreading into neighboring tissues & often to other parts of the body. • Metastasis – the spread of cancer cells beyond their original site. • Carcinomas – Cancers which originate in the external or internal coverings of the body, such as skin or the lining of the intestine. • Sarcomas – cancers which arise in tissues that support the body, such as bone & muscle • Leukemias & Lymphomas – cancers of blood forming tissues, such as bone marrow, spleen & lymph nodes.
Cancer Cells Differ from Normal Cells • Cancer cells are largely unrestrained by the system that normally controls cell division. • They are not usually affected by density-dependent inhibition. • They continue to divide even in high densities. • Many cancer cells have cell cycle control systems that proceed past checkpoints even in the absense of growth factors. • Cancer cells have reduced need for anchorage & often grow without be attached to a surface.
Treatment for Cancer • Chemotherapy • Radiation • Both attempt to halt the spread of cancer by stopping them from dividing.