chapter 2 - nucleic acids

• what elements are nucleic acids made up of?

  • carbon
  • hydrogen
  • oxygen
  • nitrogen
  • phosphorus
• what 3 components are nucleotides made up of?

  1. pentose sugar ( made up of 5 carbon atoms)
  2. phosphate group
  3. nitrogen-containing base
• how are the phosphate group, pentose group and base joined?

  through a condensation reaction to form a mononucleotide
• what is the name of the bond formed of 2 mononucleotides between the deoxyribose sugar and the phosphate group?

  phosphodiester bond
• structure of DNA:

  • double helix with 2 polynucleotide chains held together by hydrogen bonds between complimentary chains
  • deoxyribose sugar
  • nitrogen-containing bases -> adenine, thymine, cytosine and guanine
  • large molecule
• what bonds attach the bases together?

  hydrogen bonds
• what are base pairings?

  • adenine always pairs with thymine via 2 hydrogen bonds
  • cytosine always pairs with guanine via 3 hydrogen bonds

  this is known as complementary base pairing
• purines:

  • larger bases that contain 2 carbon ring structures -> adenine and guanine

  pyramidines:

  smaller bases that contain 1 carbon ring structure -> cytosine and thymine
• what are the features of DNA which allows genetic information to be passed along?

  • sugar-phosphate backbone -> protects coding bases on inside of helix
  • double-stranded -> allows strands to act as templates in DNA replication
  • large molecule -> makes molecule compact
  • weak hydrogen bonds -> allows strands to separate in DNA replication
• what is RNA?

  a type of nucleic acid that uses information from DNA to synthesise proteins
• what 3 components is RNA made up of?

  1. ribose(pentose sugar)
  2. adenine, uracil, guanine & cytosine
  3. phosphate group
• how many polynucleotide strands is RNA made of?

  a single stranded molecule made up of 1 polynucleotide strand
• DNA:

  • holds genetic information
  • pentose sugar -> deoxyribose
  • bases -> A,T,C&G
  • size -> long
  • number of strands -> 2

  RNA:

  • uses genetic information from DNA to synthesise proteins
  • pentose sugar -> ribose
  • bases -> A, U, G & C
  • size -> short
  • number of strands -> 1
• what are the complementary base pairings in RNA?

  • adenine always pairs with uracil
  • cytosine always pairs with guanine
• what must happen before cells divide by mitosis or meiosis?

  all the DNA must replicate to provide a copy for the new cell
• what is the process in which DNA replicates?

  semi - conservative replication
• what is semi-conservative replication?

  DNA replication where each new DNA molecule consists of one original strand and one newly synthesized strand.
• why are the complementary base pairings important in DNA replication?

  it enables identical copies of DNA to be created & reduces copying errors which would result in mutation
• what happens in step 2 of semi-conservative replication?

  each of the separated parental DNA strands acts as a template - free floating DNA nucleotides within the nucleus are attracted to the complementary base pairings on the template strands
• what occurs in step 3 of semi-conservative replication?

  the adjacent nucleotides are joined together(to form the phosphodiester bond) by a condensation reaction. DNA polymerase catalyses the joining together of adjacent nucleotides
• what are the 2 key enzymes involved in semi-conservative replication?

  1. DNA helicase
  2. DNA polymerase
• what is the function of DNA helicase?

  breaks the hydrogen bonds between the complementary base pairs
• who discovered the structure of DNA In 1953

  watson & craik with the help of franklin's research
• what were the 2 hypotheses about how DNA must replicate?

  1. semi-conservative - each replicated DNA molecule contains one of the original DNA and one newly synthesised strand
  2. conservative - the original DNA remains intact & the two newly synthesised strands of DNA join together
• what were the 3 facts that watson & craik based their investigation on?

  1. all the bases contain nitrogen
  2. nitrogen contains 2 isotopes - 14N (lighter) & 15N (heavier)
  3. bacteria take in the nitrogen isotopes to make new DNA nucleotides : bacteria grown in a medium containing only 14N will have DNA which only has this isotope and will be lighter, and bacteria grown in a medium containing only 15N will have DNA which only has this isotope and will be heavier
• What was the first step in Watson and Crick's investigation?

  They grew bacteria in a medium with 15N
• What was the result of spinning the bacteria in a centrifuge?

  A single band showed at the bottom
• What does the single band at the bottom indicate?

  It indicates a heavy isotope
• What did Watson and Crick do after growing bacteria in 15N?

  Placed it in a medium with 14N
• What happened after allowing the bacteria to divide once in the 14N medium?

  The band shifted slightly higher
• Why did the band shift after centrifugation?

  Due to strands containing 15N and 14N
• What did Watson and Crick do after the first division in 14N medium?

  They repeated the process and allowed it to divide again
• What was the result after the second division of DNA strands?

  Two bands appeared: 50% 15N and 14N, 50% 14N and 14N
• What did they do after observing the two bands?

  They repeated the process again
• What was the composition of DNA after the third division?

  75% DNA is 14N and 25% is 15N and 14N
• How does the percentage of 15N change with each division?

  It decreases with each division
• what replication did watson & craik declare?

  semi-conservative replication because each time they spun the test tube in a centrifuge, each DNA replication contained a conservative strand and a newly synthesised strand
• what does ATP stand for?

  adenosine triphosphate
• what is ATP?

  a nucleotide derivative which is formed from a molecule of ribose (pentose sugar), a molecule of adenine (nitrogen-containing base) & 3 inorganic phosphate groups
• why are the 3 phosphate groups inorganic?

  because they contain no carbon atoms