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Cellular Reproduction http://embryology.med.unsw.edu.au Cells must reproduce for several reasons: Growth and Development – as an organism grows, it needs more cells (baby vs adult) since the size of a single cell is limited Repair and Replacement – injuries or replacement of dead cells (cut on skin – more skin cells needed to repair the cut) Asexual Reproduction – single celled organisms must make more of themselves ▪ Binary fission – cell reproduces an identical copy of itself ▪ Includes 2 stages 1. DNA replicates, 2. cell divides Cells have a size limitation. Cells grow until they reach their size limits, then they either stop growing or divide. Cells that divide can then each grow to the size limitation for that type of cell, allowing the organism to increase in size. Most cells are smaller than a period (.) typed on a paper. The key factor that keeps cells small is the ratio of the cells surface to area volume. Surface area of the cell – area covered by the plasma membrane (structure through which all nutrients and waste products must pass) Volume – space taken up by inner contents of cell, includes organelles in the cytoplasm and nucleus As a cell grows the ratio of surface area to volume decreases Cells that are too BIG have difficulty supplying nutrients and expelling enough waste products – too far to travel Small cells have a higher ratio of surface to volume – more efficient! Small cells are more efficient. Transport of substances – substances move by diffusion or by motor proteins along the cytoskeleton – shorter distance = faster Cellular communication – signaling proteins work more efficiently because they travel a shorter distance ▪ For example: The signals that trigger protein synthesis might not reach the ribosome fast enough for protein synthesis to occur to sustain the cell Cytoskeleton used for transporting within a cell Cell division prevents the cell from becoming too large The way the cell reproduces allowing growth and healing Cells reproduce by a cycle of growing and dividing called the cell cycle Each time a cell goes through one complete cycle, it becomes two cells. Interphase – the stage during which the cell grows carries out cellular functions (is alive and doing cell stuff) and replicates DNA 3 substages Mitosis – divided into 4 substages Cell’s nucleus and nuclear material divide Cytokinesis – occurs near the end of mitosis Cytoplasm is divided resulting in 2 daughter cells How long the cell cycle takes depends on the cell that is dividing. In Eukaryotic cells, the length of the cell cycle varies from 8 minutes to one year. For most normal animal cells – 12 to 24 hours Most of the cell’s life is spent in Interphase Three substages: G1, S, G2 G1 (Gap 1) – cell grows and performs normal functions ▪ Period immediately after the cell divides ▪ Preparations are made to replicate DNA ▪ Some cells such as muscle and nerve cells, exit the cell cycle and do not divide again S (synthesis) – cell copies DNA Replication of DNA occurs at this time (Chromosomal Replication) Key Terms Chromosomes – structures that contain the genetic material Chromatin – relaxed form of DNA is the cell’s nucleus – looks like spaghetti ▪ Chromatin makes the nucleus appear dark and dense at this stage. Rat liver cells – the specks are relaxed chromatin G2 (Gap 2) – preparation for division The cell synthesizes a protein that makes microtubules for cell division The cell takes inventory and makes sure it’s ready to continue with mitosis ▪ (Performs a double check to ensure the new cells have what they need to function properly) Mitosis is the division of the cell’s nuclear material. Four substages: 1. Prophase 2. Metaphase 3. Anaphase 4. Telophase First stage of mitosis is prophase The cell’s chromatin tightens into chromosomes, which are shaped like an X Each half of the chromosome “X” is called a sister chromatid Sister chromatids are attached at the center by a centromere and contain identical copies of DNA What is the location of the sister chromatids? As prophase continues,… The nucleolus and nuclear membrane disappear Spindle fibers (microtubules) form in the cytoplasm In animal cells and most protists, another pair of microtubule structures called centrioles migrate to the ends, or poles, or the cell Aster fibers begin coming out of the centrioles Spindle fibers attach to the sister chromatids on both sides of the centromere and then to opposite sides of the cell – ensures each new cell receives one complete copy of DNA The second stage of mitosis – metaphase “meta” means middle, one of the shortest stages of mitosis Sister chromatids are pulled by motor proteins along the spindle apparatus (spindle fibers, centrioles and aster fibers) to the center of the cells Chromatids line up along the equator of the cell Ensures that the new cells have accurate copies of the chromosomes The third stage of mitosis – anaphase Chromatids are pulled apart Microtubules of the spindle apparatus begin to shorten causing the two sister chromatids to separate into two identical chromosomes Separation happens simultaneously At the end the chromosomes have moved with the help of the spindle apparatus to each of the poles of the cell The fourth and last stage of mitosis – telophase The chromosomes arrive at the poles and begin to relax, or decondense Two new nuclear membranes begin to form and the nucleoli reappear Spindle apparatus disassembles and some of the microtubules are recycled by the cell to build various parts of the cytoskeleton The nuclear material is divided ending mitosis The final process of the cell cycle is cytokinesis. Cyto – Greek word kytos, meaning hollow vessel – refers to cell Kinesis – Greek word kinetikos, meaning putting in motion During this stage, the cytoplasm is divided. Cytokinesis results in two cells, each with identical nuclei Animal cells – cytokinesis occurs due to microfilaments constricting, or pinching, the cytoplasm (the pinched area is called a furrow) Plant cells have a rigid cell wall covering the cytoplasm so instead of pinching, a structure called a cell plate forms between the two daughter nuclei Cell walls then form on either side of the cell plate - the outcome is two genetically identical cells Prokaryotic cells divide by binary fission and finish cell division a different way When prokaryotic DNA is duplicated, both copies attach to the plasma membrane As the plasma membrane grows, the attached DNA molecules are pulled apart The cell complete fission, producing two new prokaryotic cells The timing and rate of cell division are important to the health of an organism varies depending on the type of cell a mechanism involving proteins and enzymes controls the cell cycle Proteins called cylins bind to enzymes (CDK) in the stages of interphase and mitosis to start the various activities Different cyclin/enzyme combinations signal activities such as… Start of cell cycle, DNA replication, protein synthesis, and nuclear division Built-in check points monitor the cell cycle and can stop the process if an error is detected G1 check point includes a check for DNA damage and can stop the cycle before entering the S stage of interphase (before the damaged DNA is replicated) When control of the cell cycle fails, cancer results Cancer is the uncontrolled growth and division of cells Grow and divide quickly as long as they receive nutrients Cancer cells crowd normal cells and can kill an organism Cancer can occur in a healthy, active, young organism not just a weak one Mutations – changes in the segments of DNA that code for the production of proteins Cells will often fix mutations in DNA but if the repair system fails, cancer can result Environmental factors – increase the risk of cancer Carcinogens – cancer causing agents like tobacco, tobacco smoke, alcohol, some viruses, and radiation from the Sun or X-rays Avoid carcinogens All types of tobacco are dangerous and should be avoided – even smokeless tobacco and secondhand smoke have been proven to cause cancer FDA (Food and Drug Administration) – government agency that works to ensure that food and drink are safe Labels and warnings are required for products that might be carcinogens Radiation – amount of exposure should be limited (Example: ultraviolet radiation from the Sun and X-rays) Sunscreen is recommended for everyone who is exposed to the Sun Lead aprons are used when X-rays of teeth are taken More than one change in DNA is required to change an abnormal cell into a cancer cell Over time, changes occur May explain why the risk of cancer increases with age An individual who inherits one or more change from a parent is at a higher risk for cancer Apoptosis – a programmed cell death Not all cells are intended to survive Cells undergoing apoptosis typically shrink and shrivel All animal cells have a “death program” that can be activated Normal Cell Dying Cell Examples of apoptosis Plants – lose leaves in autumn – cells attached to the tree undergo apoptosis The development of the human hand and foot Cells occupy the spaces between the fingers and toes giving a webbed appearance These cells undergo apoptosis so that the webbing will not be present in the mature organism Cells that are damaged beyond repair, including cells with DNA damage that could lead to cancer will also undergo apoptosis Apoptosis can help protect organisms from developing dangerous cancerous growth Cancer cell undergoing apoptosis Most cells in an organism are designed for specialized function Skin, heart, muscle, blood In 1998, scientists discovered a way to isolate a unique type of cell in humans called the stem cell Stem cells – unspecialized cells that can develop into specialized cells under the right conditions Stem cells can remain in an organism for many years, continually dividing There are two types: Embryonic and Adult Embryonic – mass of cells that are the result of a fertilized egg (100-150 cells) – controversial due to ethical concerns to use in research Adult – found in various tissues in the body (adults and infants) and might be used to maintain and repair the same kind of tissue in which they have been found ▪ Adult does not refer to age, even newborns have adult stem cells Harvard scientists have been able to use adult stem cells to repair bone and brain tissue in mice Less controversial because the donors can give consent