1B - More Biological Molecules

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DNA & RNA Carry Important Information

  • DNA (deoxyribonucleic acid) used to store genetic information 
  • RNA (ribonucleic acid) similar to structure of DNA
  • One of main functions of RNA is to transfer genetic info from DNA to ribosomes
  • Ribosomes are body's 'protein synthesis' - read RNA to make polypeptides in process called translation 
  • Ribosomes made from RNA & proteins 
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DNA & RNA are Polymers of Nucleotides

  • Nucleotide is a type of biological molecule 
  • Made from:
    • pentose sugar
    • nitrogen-containing organic base 
    • phosphate group 
  • Nucleotide really important 
  • They're monomers make up RNA & DNA 
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The Sugar in DNA is Called Deoxyribose

  • Pentose sugar in DNA nucleotide is called deoxyribose
  • Each DNA nucleotide has same sugar & phosphate group 
  • Base on each nucleotide can vary 
  • 4 possible bases - Adenine, Thymine, Cytosine, Guanine
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Sugar in RNA is Called Ribose

  • RNA contains nucleotides with ribose sugar
  • RNA nucleotide contains phosphate group & 1 of 4 bases
  • In RNA - uracil replaces thymine as base 
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Nucleotides Join Together to Form Polynucleotides

  • Polynucleotide is polymer of nucleotides 
  • Both DNA & RNA nucleotides form polynucleotides
  • Nucleotides join via condensation reaction between phosphate group of 1 nucleotide & sugar of another 
  • Forms phosphodiester bond - phosphate group & 2 ester bonds 
  • Chain of sugars & phosphates known as sugar-phosphate backbone
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DNA is Made of Two Polynuclotide Chains in Double-

  • 2 DNA polynucleotide strands join together by hydrogen bonding between bases
  • Each base can only join with 1 particular partner - complementary base pairing 
  • Adenine always pairs with thymine & cytosine always pair with guanine 
  • Always equal amounts of adenine & thymine in DNA molecule and equal amounts of cytosine and guanine 
  • 2 hydrogen bonds form between A & T, 3 bonds between C & G
  • 2 antiparallel polynucleotide strands twist to form DNA double-helix 
  • DNA first observed in 1800s but lots of scientists at time doubted that it could carry genetic code because it has relatively simple chemical composition 
  • Some argued that genetic information must be carried by proteins - much more chemically varied
  • By1953, experiments had shown DNA was carrier of genetic code
  • This was also year in which double-helix structure was determined by Watson & Crick  
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RNA is a Relatively Short Polynucleotide Chain

  • RNA is made from single polynucleotide chain 
  • Much shorter than most DNA polynucleotides 
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DNA Replicates by Semi-Conservative Replication

  • DNA copies itself before cell division so that each new cell has full amount of DNA
  • The method is called semi-conservative replication because half of strands in each new DNA molecule are from original 
  • Means that there's genetic continuity between generations of cells 
  • 1. DNA helicase breaks hydrogen bonds between bases on 2 polynucleotide DNA strands 
    • Makes helix unwind to form 2 single strands 
  • 2. Each original single strand acts as template for new strand 
    • Complementary base pairing means that free-floating DNA nucleotides are attracted to complementary exposed bases ojn each original template strand 
  • 3. Condensation reactions join nucleotides of new strands together - catalysed by enzyme DNA polymerase 
    • Hydrogen bonds form between bases on original & new strands 
  • 4. Each new DNA molecule contains one strand from original DNA molecule & one new strand 
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DNA Polymerase Moves in Opposite Ways Along Antipa

  • Each end of DNA strand is slightly different in structure
  • One end called 3 prime end & one end called 5 prime end 
  • In DNA helix, strands run in opposite directions - antiparallel 
  • Active site of DNA polymerase is only complementary to 3 prime end of newly forming DNA strand - so enzyme can only add nucleotides to new strand to 3 prime end 
  • This means new strand is made in 5 prime to 3 prime direction & that DNA polymerase moves down template strand in a 3 prime to 5 prime direction 
  • Because strands in double-helix are antiparallel, DNA polymerase working on one of template strands moves in opposite direction to DNA polymerase working on other template strand 
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Meselson & Stahl Provided Evidence for Semi-Conser

  • Meselson & Stahl validated Watson & Crick's theory 
  • Before that, people were unsure whether DNA replication was semi-conservative or conservative 
  • If method was conservative, original DNA strands would stay together & new DNA molecules would contain 2 new strands
  • Meselson & Stahl showed that DNA is replicated using semi-conservative method 
  • Used 2 isotopes of nitrogen - heavy nitrogen & light nitrogen 
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Meselson & Stahl's Experiment

  • 2 samples of bacteria were grown - one in nutrient broth containing light nitrogen, & one in broth with heavy nitrogen 
  • As bacteria reproduced they took up nitrogen from broth to help make nucleotides for new DNA 
  • So nitrogen gradually became part of bacteria's DNA 
  • A sample of DNA was taken from each batch of bacteria & spun in centrifuge 
  • DNA from heavy nitrogen bacteria settled lower down centrifuge tube than DNA from light nitrogen bacteria - because it is heavier 
  • Then bacteria grown in heavy nitrogen broth were taken out & put in broth containing only light nitrogen 
  • Bacteria were left for 1 round of DNA replication & then another DNA sample was taken out & spun on centrifuge 
  • If replication was conservative, the original heavy DNA, which would still be together would settle at bottom & new light DNA would settle at top 
  • If it was semi-conservative - new bacterial molecules would contain 1 strand of old DNA containing heavy nitrogen & 1 strand of new DNA containing light nitrogen 
  • So DNA would settle between where light & heavy DNA settled
  • Turned out - DNA settled out in middle showing mixture of light & heavy nitrogen 
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Water is Vital to Living Organisms

  • Warer is metabolite in loads of important metabolic reactions - condensation & hydrolysis 
  • Water is a solvent - some substances dissolve in it 
    • Most metabolic reactions take place in solution so water is pretty essential 
  • Water helps with temperature control - high latent heat of vaporisation & high specific heat capacity 
  • Water molecules are very cohesive which helps water transport in plants as well as transport in other organisms 
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Water Molecules Have a Simple Structure

  • A molecule of water is one atom of oxygen joined to two atoms of hydrogen by shared electrons 
  • Because shared negative hydrogen electrons are pulled towards oxygen atom, the other side of each hydrogen atom is left with slight positive charge 
  • Unshared negative electrons on oxygen atom give it slight negative charge 
  • This makes water a polar molecule - has a partial negative charge on one side & partial positive charge on other 
  • Slightly negatively-charged oxygen atoms attract the slight positively-charged hydrogen atoms of other water molecules 
  • This attraction is called hydrogen bonding & it gives water some of it useful properties 
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Water is an Important Metabolite

  • Many metabolic reactions involve a condensation or hydrolysis reaction 
  • Hydrolysis reaction requires molecule of water to break bond 
  • Condensation reaction releases molecule of water as new bond is formed
  • E.g. amino acids are joined together to make polypeptides by condensation reaction
  • Energy from ATP is released through hydrolysis reaction 
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Water Has High Latent Heat of Vaporisation

  • It takes a lot of heat energy to break hydrogen bonds between water molecules 
  • So water has a high latent heat of vaporisation - a lot of energy is used up when water evaporates 
  • Useful for living organisms because it means they can use water loss through evaporation to cool down without loosing too much water 
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Water Can Buffer Changes in Temperature

  • Hydrogen bonds between water molecules can absorb a lot of energy 
  • So water has high specific heat capacity - takes a lot of energy to heat up 
  • Useful for living organisms because it means that water does not experience rapid temperature changes 
  • Makes water good habitat because temperature under water is likely to be more stable than on land 
  • Water inside organisms also remains at fairly stable temperature - helping them maintain constant internal body temperature 
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Water is a Good Solvent

  • Lot of important substances in metabolic reactions are ionic 
  • Means they are made from 1 positively charged atom or molecule & 1 negatively charged atom or molecule 
  • Because water is polar - positive end of water molecule will be attracted to negative ion & the negative end of water molecule will be attracted to positive ion 
  • This means ions will get totally surrounded by water molecules - in other words they'll dissolve 
  • So water's polarity make it a useful solvent 
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There's Strong Cohesion Between Water Molecules

  • Cohesion is attraction between molecules of same type 
  • Water molecules are very cohesive because they are polar 
  • Strong cohesion helps water to flow making it great for transporting substances 
  • E.g. it is how water travels in columns up the xylem in plants 
  • Strong cohesion also means that water has a high surface tension when it comes in to contact with air 
  • This is the reason why sweat forms droplets which evaporate from skin to cool an organism down 
  • Also reason that pond skaters & some other insects can 'walk' on surface of pond 
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ATP is the Immediate Source of Energy in a Cell

  • Plant & animal cells release energy from glucose - respiration 
  • A cell can't get its energy directly from glucose 
  • In respiration - energy released from glucose is used to make ATP
  • ATP is made from nucleotide base adenine, combined with ribose sugar & 3 phosphate groups 
  • Known as nucleotide derivative because it is a modified form of a nucleotide 
  • Once made, ATP diffuses to part of cell that needs energy 
  • Energy in ATP is stored in high energy bonds between phosphate groups 
  • Released via hydrolysis reactions 
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ATP is Quickly Made & Used

  • When energy is needed by cell, ATP is broken down into ADP & inorganic phosphate
  • This is hydrolysis reaction 
  • Phosphate bond is broken & energy is released 
  • Reaction is catalysed by enzyme ATP hydrolase 
  • ATP hydrolysis can be 'coupled' to other energy-requiring reaction in cell - means energy released can be used directly to make coupled reaction happen, rather than being lost as heat
  • The released inorganic phosphate can also be put to use - can be added to another compound (phosphorylation) which often makes compound more reactive 
  • ATP can be re-synthesised in condensation reaction between ADP & inorganic phosphate 
  • This happens during both respiration & photosynthesis & is catalysed by enzyme ATP synthase 
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Inorganic Ions Have an Electrical Charge

  • An ion is an atom that has an electrical charge 
  • An ion with a positive charge is called a cation 
  • An ion with a negative charge is called an anion 
  • An inorganic ion is one which does not contain carbon (few exceptions)
  • There are inorganic ions, in solution, in cytoplasm of cells & in bodily fluids of organisms 
  • Each ion has specific role depending on properties 
  • Ions role determines the concentrations 
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Iron Ions Are an Important Part of Haemoglobin

  •  Haemoglobin - large protein carries oxygen around body in red blood cells 
  • 4 different polypeptide chains - each with iron ion Fe2+ in centre
  • Fe2+ binds to oxygen in haemoglobin 
  • When oxygen is bound - becomes Fe3+ temporarily until oxygen released
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Hydrogen Ions Determine pH

  • pH calculated based on concentration of hydrogen ions in environment 
  • More H+ ions present the lower the pH the more acidic the environment 
  • Enzyme-controlled reactions are all affected by pH
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Sodium Ions Help Transport Glucose and Amino Acid

  • Glucose & amino acids need help crossing membranes 
  • Molecule of glucose or amino acid can be transported in to a cell alongside sodium ions
  • Known as co transport 
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Phosphate Ions are an Essential Component of ATP a

  • When phosphate ion attached to another molecule - phosphate group 
  • DNA, RNA & ATP all contain phosphate groups 
  • Bonds between phosphate groups that store energy in ATP 
  • Phosphate groups in DNA & RNA allow nucleotides to join & form polynucleotides
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