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CellsC8: Cells arise from pre-existing cells, and cell division leads to an increase in cell numberLearning Objective Understand that cell division is the basis of growth, reproduction and repair Explain why chromosomes must be replicated by semi-conservative replication before a cell divides to ensure that daughter cells are identical to the parent cell Describe how prokaryotes divide by binary fission Describe the cell cycle Illustrate the process of mitosis in eukaryotic cells. Why do Cells Divide? Cell division is the basis of: Growth, Repair, Reproduction The ratio of cell membrane Surface Area: Volume Ratio is a factor that limits cell size When a cell reaches its OPTIMUM size the NUCLEUS initiates cell divisioin Why DNA replicates in a cell before division All cells contain Deoxyribose Nucleic Acid (DNA) In prokaryotes the DNA is found in the cytoplasm in the form of a circular chromosome In eukaryotes the DNA is found in chromosomes located in the nucleus as linear DNA. When a cell divides to form 2 identical daughter cells, then the amount of DNA in a cell must be doubled before the cell divides to maker sure that both daughter cells receive a copy of the DNA. This process is called semi-conservative replication of DNA What are chromosomes? Chromosomes around found with the nucleus of cells When NOT dividing chromosomes are in the form of chromatin granules When the cell is preparing to divide these thread like chromatin granules become shorter and fatter and we refer to them as chromosomes. Chromosomes contain GENES. Genes determine an individuals characteristics such as eye colour, nose shape etc.
What are Genes? Genes are arranged along chromosomes like a string of beads on a necklace. Each gene controls a specific characteristic (sometimes several characteristics are controlled by several genes working together) These genes are made of DNA (A,T,C,G) DNA carries the code to synthesis all the proteins required to carry out life processes as well as codes to replicate, It is very important that chromosomes are replicated prior to cell division This process enables daughter cells to be an identical replica of the parent cell To enable the chromosomes to be divided evenly between daughter cells the chromosomes behave in an ORDERLY way to prepare for cell division along the CELL CYCLE
The Cell CycleThe cell cycle has two main parts; Growth and preparation (Interphase) Cell Division (MITOSIS) Cell division is split further into two stages MITOSIS (Nuclear Division) Cytokinesis (Cytoplasm Division) MitosisMitosis, the division of the nucleus is divided into four stages (IPMAT) Prophase Metaphase Anaphase Telophase Interphase Cells spend the majority of their life in Interphase Interphase is occurs between divisions and is characterised by GROWTH and DEVELOPMENT Interphase was once thought to be a 'resting period' because when viewed through a microscope at this time of the cell cycle there are no clear indicators or anything occuring Now it is known that Interphase is the longest part of the cell cycle, in which the cell is busy producing materials for growth and synthesising DNA 'semi-conservative replication' in readiness for division Interphase is divided into THREE parts: G1 or Gap 1: The first growth and development period following division S or Synthesis: The second stage of Interphase where the cell replicates its chromosomes and commits to division (see checkpoint information C9) G2 or Gap 2: The final growth stage, organelle synthesis MitosisProphase Chromosome become shorter and fatter The Nuclear Membrane Disappears Centrioles start to move to opposite poles of the cell and spindle fibres start to form MetaphaseChromosomes line up on the spindle fibres across the equator of the cell (one chromosome per spindle fibre)AnaphaseThe centromeres split moving to opposite poles of each cell, carrying an identical chromosomeTelophase Daughter chromosomes reach opposite ends of the cell and nuclear membranes reform around the two separate chromosome bundles Cytokinesis occurs and the cell splits into two identical daughter cells
Mitosis Animation: https://highered.mcgraw-hill.com/sites/0072495855/student_view0/chapter2/animation__mitosis_and_cyto...
Prokaryotic Cells Divide by Binary Fission (bacteria etc.) Bacteria reproduce by binary fission which literally means 'dividing in half' The DNA in a bacteria replicates prior to cell division and then the plasma membrane and the cell wall grow inward to constrict the cell and divide it into two (cytokinesis) Each daughter cell is genetically identical to the parent cell This process takes 20 minutes and thus bacteria can multiply exponentially If environmental conditions change and one bacteria dies, all are likely to die as they are identical, unless there has been a mutation in their DNA
C9: Division may be regulated by internal and external factorsLearning Objective Know that cells produce gene products (proteins) that regulate the cell cycle Understand that hormones may regulate cell division Understand that carcinogens upset the normal controls of cell division by causing MUTATIONS Cell division may be regulated by internal and external factors In multicellular organisms the division of cells MUST be carefully controlled and cannot go 'unchecked' The effects of 'uncontrolled cell division' = MUTATIONS, such as CANCER The CELL CYCLE controls the process of division Differentiation Cells that divide are called STEM CELLS When stem cells divide, one daughter cell remains a stem cell while the other DIFFERENTIATES A differentiated cell is one that has become 'SPECIALISED' in structure and function These cells tend not to divide or at least divide at a greatly reduced rate. Differentiated cells cannot revert back to their former state (stem cells) A cell that has differentiated is said to be in G0. Its division apparatus is disabled and if it is ever stimulated to divide again it will occur very slowly Specific gene products inside cells regulate the cell cycle
G1 - A cell goes through growth phase, if specialisation occurs the cell goes into G0 phaseS - DNA is replicatedG2 - Further growth phase and preparation for mitosisM - Mitosis where cell division occurs PMAT
The cell cycle is dependent on whether the conditions are correct at three different checkpointsCheckpoint 1: Restriction Point (R)Towards the end of G1 phase - the cell cycle will only proceed into S phase if :-intracellular (eg cell size, large) and extracellular conditions (temperature and growth factors) are met.Checkpoint 2At the end of G2 - the cell cycle only enters mitosis if :- DNA replication was successful Environmental conditions are met MPF (Mitosis Promoting Factor) levels must rise - made up of an enzyme, Cdk and a protein, cyclin Checkpoint 3: (M)Metaphase checkpoint - the cell cycle only continues when Chromosomes are aligned MPF levels must fall Checkpoint genes exist with the sole purpose of arresting development until the cell can be made ready for progression by independent genes.Control of the Cell CycleResearch has shown that there are important factors which influence the time and rate of division.Triggers of cell division
1. SIZEWhen cells are fully grown their SA/V ratio is smaller and therefore can be a reason to divide
2. SIGNALS FROM THE ENVIRONMENT nutrient dependence: if certain nutrients are not present in the fluid surrounding a cell it will not divide anchorage dependence: some cells will only divide if they are attached to a certain substrate density dependence: closely packed cells will usually not divide, especially if their membranes are touching
* The regulation of cell division is achieved by a set of proteins that interact at the CHECKPOINTS to allow the cell to proceed to the NEXT PHASE.MPF's = mitosis promoting factors trigger a cell to proceed from interphase to mitosisCheckpoint 1 - Divide?The role of growth factos Growth Factors (GF) are protein molecules that regulate cell division (prompted by hormones) If cells are deprived of GF's they WILL NOT DIVIDE = G0 phase Growth Factors travel in animal blood and plant sap (in the extracellular environment). They attach to specific receptor molecules on the cell membrane of Target cells The target cell relays a message across the membrane to a series of 'relay proteins', which activate the process of transcription to produce proteins that stimulate cell division.
PDGF (platelet derived growth factors) control the division of fibroblasts and are important in the clotting of blood. The weedicide 'round-up' contains a plant GF 2, 4, 5, T and glycol which stimulates plant cells to divide too rapidly causing the plant to die There are over 50 known proteins that act as growth factors Checkpoint 2 - Continue? The basis of regulation at the second checkpoint rests with two types of proteins One is an enzyme (kinase) and the other a regulatory protein called cyclin which activates the enzyme When the two molecules combine they form a complex called the mitosis promoting factor (MPF) ENZYME (kinase) + CYCLIN = MPF An increase in cyclin stimulates the start of MITOSIS ANAPHASE cannot commence at Checkpoint 3 until there is a decrease in MPF - so the MPF self destructs! Understand that carcinogens upset the normal controls of cell division by causing mutations Cancer = the uncontrolled division of cells Begins with a single abnormal cel Caused by a mutation or a series of mutations to genes in body cells These mutations interfere with genes which control and regulate cell division This mutation then leads to an abnormal growth of cells which invades other tissues and may move to other parts of the body If change was not passed onto the next generation of cells, new cells would receive UNALTERED DNA and be NORMAL! CANCER CEKKS have abnormal cell cycles In some cases checkpoints are not functioning and no GF's are required to control movement to the next phase of the cycle In other cases, the cancer cells manufacture their own GF's and proceed through the cell cycle without restriction! Chemotherapy works o interrupt the cell cycle as do antimitotic drugs Mutations A mutation = a spontaneours or induced change in a cells DNA There is a strong correlation between CARCINOGENESIS (the onset of cancer) and MUTAGENESIS (a change in DNA sequence) 3 Main Causes/Agents=> CHEMICAL CARCINOGENS: Change the nucleotide sequence in DNA=> RADIATION: produce chromosome breaks=> VIRUSES: Ass foreign DNA sequences to the host DNAUsually a single exposure to these agents will not cause cancer - an accumulation or several exposures is needed. Mutagens increase the rate of cancer, especially carcinogens which include: asbestos, cigarette smoke, radiation and excessive heat. New chemicals are now tested for mutagenicity by exposing them to microbes growing on agar plates before they are available for general use.
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Testing for Malignancy A biopsy is used to test cells to see if they are malignant There are 3 main types of cancer: => Carcinomas: originate in the external or internal coverings of the body such as intestines, the skin or in glands such as the breasts => Sacromas: originate in connective tissue in the bosy => Leukaemias and lymphomas: originate in tissues which from white blood cells such as the spleen, lymph and bone marrowCancer is treated with surgery, radiotherapy and chemotherapy which are all aimed at interfering with the process of cell division, but they have significant side effectsEarly detection is still the best treatment!
C10: Existing cells are the products of evolutionLearning Objective Understand that there is evidence the prokaryotic cells existed before eukaryotic cells Explain how the ancestry of most existing eukaryotic cells probably involved endosymbiotic events First Life on Earth Fossil evidence suggest the earth was formed 4.5 billion years ago (BP) The conditions on earth were very different to those found today: The atmosphere The earliest form of life one earth is about 3.5 billion years old. Current thinking = first cells were prokaryotes that anaerobically respired (without oxygen) This Suggests life began in the ocean in a primordial soup of organic molecules Primordial Definition: of a cell, part, or tissue at the earliest stage of development OR existing at or from the beginning of time OR basic and fundamentalSupport for Simple Cellular Formation In 1953 chemist - STANLEY MILLER mixed primordial gases and subjected them to similar conditions as early as Earth. He was able to form organic molecules including amino acids, nucleotides and ATP! He showed that molecules spontaneously polymerise (join together) to from simple proteins and nucleic acids Since 1953 other studies have shown that these molecules polymerise in the presence of lipids - when this occurs simple membranes form! These discoveries support the idea that cells could have developed billions of years ago Formation of Prokaryotic Autotrophs The earliest form of prokaryotes are 3 billion years old, called stromatolites These simple bacteria could photosynthesis and changed the composition of the atmosphere (produced oxygen) to enable other organisms to evolve Endosymbiosis The term SYMBIOSIS describes a pattern of living in which two organisms help each other for mutual benefit. ENDO means inside, therefore ENDOSYMBIOSIS = 'living inside for mutual benefit' It is though that the evolution from prokaryotes to eukaryotes occurred through an ENDOSYMBIOTIC EVENT
INFOLDING OF MEMBRANES The first step thought to have led to the evolution of eukaryotic cells from prokaryotic cells was the infolding of the cell membrane of protists (bacteria) This is thought to have formed the endoplasmic reticulum, nucleus and golgi apparatus
ENDOSYMBIOSIS The second step was endosymbiosis of small prokaryotic cells by larger cells producing mitochondria and chloroplasts within the larger cell Simply a large cell engulfed a smaller cell when the gave benefit to both cells, the larger cell protected the smaller cell and the smaller cell provided energy of some form to the larger cell
Evidence for EndosymbiosisCHLOROPLASTS & MITOCHONDRIA BOTH have their own DNA. Separate from the DNA in the nucleus of the eukaryotic cell. Their DNA resembles prokaryotic DNA = circular BOTH contain their own ribosomes that closely resemble bacterial ribosomes (prokaryotic) BOTH can self replicate independently of the cell. This replication resembles binary fission (division of prokaryotic cells) BOTH have two membranes. The outer is similar to the 'host' cell. The inner contains enzymes and transport proteins found in the membranes of bacteria!
C11: Human beings culture cells for a variety of purposesLearning Objective Understand that to culture cells specific conditions must be maintained Animals cells are cultured for a variety of medical uses and are also used to test cosmetics instead of live animals Plant cells are cultured for propagation Propagation Definition: the process of spreader to a larger area or greater numberHumans Culture Cells Humans culture cells for a variety of purposes Biologists and medical scientists have developed methods to grow cells in a laboratory These are called 'cell culture' techniques The techniques to culture plant and animal cells is slightly different WHY MIGHT THIS BE?General Cell Culture Techniques
DISSECTION1. The tissue of interest is carefully dissected to expose cells of the same type
SUPSENSION2. Add proteolytic (protein digesting) enzymes to the dissected tissue to digest the intracellular matrix of connective tissue and release the cell.
CELL CULTURE3. Cells are placed in a culture medium which contains the necessary sugars and salts. In some cases small amounts of antibiotics are added to prevent bacterial growth4. Culture is kept in a flat flask at the optimum temperature (human cells 37°C)
The cells quickly cover the bottom of the flask from where they can be removed for experimental work The Hayflick Limit: Most cultured cells will usually grow 30-40 generations Laboratory or (in vitro) techniques enable tissue cultures to be grown in a lab and safely stored for many years Plant Tissue CultureBiologists can culture cells using the following techniqueTechniques for plant tissue culture: Select healthy cells from actively growing tissue (eg buds, leaf tips) Wash in a sterilising solution (hypochlorite) to remove any contaminating bacteria or fungi Place on sterile growing media (usually an agar gel). The growing media should contain all the materials required for growth growth hormones and nutrients such as minerals Incubate in a stable warm light environment The tissue will develop into a thallus and eventually leaves and roots will develop. The tissue can then be further divided or planted out Thallus Definition: a plant body that lacks differentiation into distinct parts (as stem, leaves, and roots)Application: Plant tissue culture is used in agriculture to produce large numbers of genetically identical plants to improve the consistency of plants. Depending on the concentration and type of hormones used the tissue can be made to divide/differentiate into roots or shoots or both. This technique is used to clone valuable plants Animal Tissue CultureBiologists can culture cells using the following technique:Techniques for animal tissue culture: Select healthy growing cells and sterilise the surface of the tissue sample Separate the cells by adding enzymes. This overcomes any contact inhibition Place in a sterile nutrient solution (containing glucose, amino acids, minerals and growth hormones) and incubate at body temperature At intervals it will be necessary to remove cells to allow the remaining cells to divide Cultured cells are kept at optimal temperatures and will grown and divide for 30-40 generations. Application: This technique is used to grow skin for burn victims and one day may be used to grow new organs for transplant Used widely in medicine to provide skin tissue and reserve cells for organs Used instead of live animals for testing cosmetics and drugs Difficulty: Unlike bacteria and plant cells, animals cells are unable to synthesise most of their requirements from simple compounds in the culture medium. All vitamins and other necessary organic compounds must be supplied and the osmotic balance and temperature carefully monitored. The medium must be aerated unless grown as a monolayer. Bioethics is the study of what controls should be placed on the use of cell cultures, such as growing human embryos without a womb
C12: Chemical can interfere with cell metabolismLearning Objective Discuss possible benefits and/or harmful effects of chemicals that human being use Understand the bioaccumulative potential of synthetic chemicals Bioaccumulation Definition: (of substances, esp. toxins) to build up within the tissue of organismsHuman Chemical Legacy The major task facing scientists this millennium will be to clean up the chemicals that pollute our air and water supplies and to ensure that our planet is sustainable for future generations There are no specific examples you need to know at least 3 different chemicals that have both beneficial and harmful effects (from at least 3 different categories) Chemical Wrap in a NutshellAntibiotics Kill bacteria that cause disease Also kill beneficial organisms Can trigger allergic reactions Can encourage bacterial resistance Insecticides like DDT Kill harmful insects and have increase crop yield and quantity Can kill helpful insects like bees and other pollinators Can accumulate in the food chain and affect other organisms (e.g. prevent calcium deposition in birds eggs leading to falling birth rates) Inorganic Fertilisers Can increase plant growth, productivity and so profitability There may be damage to the environment in extraction (e.g. phosphate mining on pacific islands) There may be damage due to run off causing eutrophication (blooms of blue-green algae that can kill organisms in fresh water environments due to reduced oxygen levels)