NUCLEIC ACIDS

• DNA stands for

  deoxyribonucleic acids
• bases in the DNA

  A = adenine

  T = thymine

  C = cytosine

  G = guanine
• reactions that form mono-nucleotides:

  2 condensation reactions

  base + sugar = glycosidic bond

  phosphate + sugar = phosphodiester bond
• DNA structure

  • two polynucleotides
  • joint together by hydrogen bonds between bases
  • coil up to form double helix
  • each base can only join to complementary base
• DNA structure

  • A -> T (joint by 2 hydrogen bonds)
  • C -> G (joint by 3 hydrogen bonds)
  • strands run anti-parallel

  
• RNA

  ribonucleic acid
• bases in RNA

  1. adenine
  2. cytosine
  3. guanine
  4. uracil
• differences in RNA

  1. has uracil instead of thymine
  2. sugar is ribose instead of deoxyribose
  3. nucleotide strand is single not double
  4. there are 2 different types of RNA (mRNA and tRNA) whereas only one type of DNA
• DNA makes up

  genes
• genes are sections of DNA

  found on chromosomes
• genes are instructions for

  proteins
• proteins are made of

  amino acids
• order of the amino acid determines

  the protein
• triplet code (codon)

  makes an amino acid
• different codons for different

  amino acids
• in a gene the order of bases on DNA determines

  the amino acid that is made
• sequence of codons on DNA acts as template for

  the protein
• purines

  double ringed structure (A , G)
• pyrimidines

  single ringed structure (C,U,T)
• nucleotide structure

  A) phosphate

  B) pentose sugar

  C) nitrogenous base

  
• DNA has sugar-phosphate backbone

  protects coding bases on inside of helix
• DNA is double stranded

  allows strands to act as templates in DNA replication
• DNA is a large molecule

  so can store a lot of information
• DNA is a double helix

  so is compact
• DNA has complementary base paring

  so DNA replication can be accurate
• DNA has weak hydrogen bonds between bases

  allows strands to easily separate during replication
• DNA replication

  semi-conservative replication
• for semi-conductive replication to take place

  1. 4 nucleotides must be present
  2. both strands of DNA must be copied so can act as template
  3. the enzyme, DNA polymerase, must be present
  4. a source of energy (ATP) required to drive the process
• 1. DNA replication

  • DNA Helicase
  • breaks the hydrogen bonds between bases of the two strands of DNA
  • (not completely separate just unzips bit by bit)
• 2. DNA replication

  • DNA Helicase completes splitting a part
  • free nucleotides are attracted to their complementary bases on the templates
  • nucleotides cannot join unless energy is present
• 3. DNA replication

  • once all nucleotides are in position
  • phosphodiester bonds are formed creating the backbone
  • this is done by DNA polymerase
• 4. DNA replication

  • two identical strands have now been formed
  • we call it semi-conservative replication as half of the original molecule is conserved and the other half is newly synthesised
• exam question on semi-conservative replication of DNA

  1. the enzyme DNA helicase breaks the hydrogen bonds between the bases of two strands
  2. the double helix separates into two strands
  3. each exposed strand now acts as a template
  4. complementary free nucleotides are attracted to the bases on the strands
  5. energy is required to activate the free nucleotides
  6. once nucleotides are in place, their backbone is joined together by DNA polymerase
  7. two new strands are produced
  8. each of these contain half of the original strand and half of the new strand
• evidence for semi-conservative replication

  1953

  meselson + stahl
• meselhson and stahl proved they were right by

  discrediting the other ways of replication
• conservative replication

  the original DNA would remain intact and a separate new DNA molecule would be made from scratch
• semi-conservative replication

  the original DNA would split in half then act as a template to be filled with free nucleotides
• dispersive replication

  chunks taken from DNA to make new DNA

  (idiotic)
• 1958 - experiment to prove semi-conservative replication

  1. all bases in DNA contain nitrogen
  2. nitrogen has 2 isotopes N14 and N15
  3. putting nitrogen into a bacterium's growth medium means it will incorporate it into DNA
• 1. they grew a large population of E.coli on a N15 growth medium

  • all DNA in this bacteria is 'heavy DNA' full of N15
  • they extracted this DNA and put it into an ultracentrifuge
  • the 'heavy DNA' appears as a band low down
  • did this for both mediums