DNA, RNA and Protein Synthesis

• DNA is made up from

  • pentose sugar
  • nitrogenous base
  • phosphate
• DNA includes the molecules

  • hydrogen
  • carbon
  • oxygen
  • nitrogen
  • phosphorus
• DNA = deoxyribose

  RNA = ribose
• DNA has the 4 bases: Adenine, Thymine, Guanine, Cytosine
• RNA has the 4 bases: Adenine, Uracil, Guanine, Cytosine
• Adenine and Guanine are double ring structures, known as purines

  Thymine and Cytosine are single ring structures, known as pyrimidines
• to join adjacent nucleotides the phosphate of one nucleotide and the hydroxyl group of carbon 3 on the pentose sugar on the other nucleotide chemically bond and form a phosphodiester bond.

  • condensation reaction where water is released
  • creates a dinucleotide
• DNA and RNA are examples of polynucleotides
• DNA consists of 2 polynucleotide stands with the sugar phosphate backbone on the outside and the bases in the centre. The 2 strands are held by hydrogen bonds forming between the bases
• A purine on one strand always pairs with a pyrimidine on the opposite strand so it means the distance between the sugar phosphate backbone is constant all the way down
• • strands are antiparallel
  • guanine and cytosine form 3 hydrogen bonds
  • adenine and thymine form 2 hydrogen bonds
  • strands twist and create a double helix
• DNA vs RNA

  • DNA contains deoxyribose whereas RNA contains ribose
  • DNA contains adenine, thymine, guanine and cytosine whereas RNA contains adenine, uracil, guanine and cytosine
  • DNA is found in the nucleus whereas RNA is found in the cytoplasm
  • DNA is extremely long whereas RNA is shorter
  • DNA is 2 polypeptides that twist and form a double helix whereas RNA is 1 polypeptide strand
• DNA replication

  1. DNA helicase 'unzips' the double helix and causes the hydrogen bonds to break between the complementary base pairs
  2. free nucleotides line up by complementary base pairing (adenine and thymine and guanine and cytosine) and are held by hydrogen bonds (no sugar phosphate backbone yet)
  3. DNA polymerase attaches to the molecule and forms phosphodiester bonds between the adjacent nucleotides and the sugar phosphate backbone is formed. (this is a condensation reaction)
• after DNA replication you end up with 2 copies of the DNA molecule. each with one original strand and one new strand (semi-conservative replication)

  • sometimes an incorrect base can be inserted into the polypeptide strand. this means the sequence has changed and is called a mutation. mutations are random and spontaneous
• all DNA contains nitrogen, there are 2 nitrogen isotopes

  • 14N - light nitrogen
  • 15N - heavy nitrogen
• 1. sample of bacteria grown in 14N
  2. extracted the DNA, spun in a centrifuge at a high speed
  3. formed a band at the top which scientists could detect (position of the band depends how heavy the DNA is)
  4. cultured the bacteria in a growth medium containing only 15N. after DNA had replicated all nitrogen atoms in DNA were 15N
  5. extracted, spun in a centrifuge and formed a band near the bottom
  6. took 15N bacteria and transferred to 14N and allowed it to replicate once
  7. extracted, spun in a centrifuge
  8. produced a band in the middle so told scientist DNA contained 1 strand of 14N and 1 strand of 15N
• DNA replication experiment showed scientists that DNA must have been replicated semi-conservatively
• the band position told scientists how heavy the DNA was

  • 14N was light DNA so band formed near the top
  • 15N was heavy DNA so band formed near the bottom
• Watson and Crick discovered that DNA is a double helix and theorised it would replicate semi-conservatively

  • Meselton and Stahl proved that DNA was replicated semi conservatively
• ATP

  cells need a way to transfer the energy from the molecule in smaller and more useful amounts. so to do this cells use the molecule ATP (adenosine triphosphate)
• structure of ATP

  • contains the base adenine bonded to the pentose sugar ribose (Adenosine)
  • on the other side of the ribose there are 3 phosphate groups meaning ATP is a nucleotide
  • it only takes a small amount of energy to break the covalent bond holding the last phosphate group in place, but when this bond is broken a great deal of energy is released and can be used by processes in the cell
  • breaking this bond requires a water molecule (hydrolysis). produce the molecule ADP (adenosine diphosphate), releasing the phosphate group and energy

  
• ATP is not a long term energy store

  • immediate energy source transferring energy from the sites of respiration to the parts of the cell that require energy
• processes within the cell which require energy (ATP)

  • active transport
  • muscle contraction
  • formation of molecules such as proteins
• the ADP and phosphate that are released are then recycled back to ATP

  • takes place during respiration during animal cells
  • takes place during respiration and photosynthesis in plant cells

  because during this you are adding a phosphate back onto the ADP this is a phosphorylated reaction, catalysed by the enzyme ATP synthase and because water is released this is a condensation reaction