DNA + RNA

• Role of DNA


  holds genetic information
• Role of RNA


  transfers genetic information from DNA to ribosomes
• Structure of DNA


  - Nucleotides contain a pentose sugar, a nitrogenous base, and a phosphate group

  - Nucleotides (monomers) make DNA or RNA which are nucleic acids (polymers)

  - Nucleotides are joined together by condensation reactions forming phosphodiester bonds

  - 2 strands are joined in anti-parallel, held together by Hydrogen bonds between specific complementary base pairs, twisting into a double helix
• Structure of RNA


  Single RNA polynucleotide strand
• Differences between DNA and RNA NUCLEOTIDES


  - DNA nucleotides have the pentose sugar deoxyribose, whereas RNA nucleotides have the pentose sugar ribose

  - DNA nucleotides can have the base thymine, whereas RNA nucleotides have uracil instead
• Differences between DNA and RNA MOLECULES


  - DNA molecules are double stranded (double helix),

  whereas RNA molecules are single stranded

  - DNA is longer whereas RNA is shorter
• Structure of DNA related to its functions

  Double stranded --> both strands can act as templates for semi-conservative replication

  Weak hydrogen bonds between bases --> can be unzipped for replication

  Complementary base pairing --> accurate replication

  Many hydrogen bonds between bases --> stable / strong molecule

  Double helix with sugar phosphate backbone --> protects bases / H bonds

  Long molecule --> store lots of (genetic) information (that codes for polypeptides)

  Double helix (coiled) --> compact
• Process of DNA replication


  - DNA Helicase breaks hydrogen bonds between bases, unwinds double helix

  - = two strands which both act as templates

  - Free floating DNA nucleotides attracted to exposed bases via specific complementary base

  pairing, hydrogen bonds form (adenine-thymine; guanine-cytosine)

  - DNA polymerase joins adjacent nucleotides on new strand by condensation, forming

  phosphodiester bonds (= sugar phosphate backbone)

  - Replication is semi-conservative - each new strand formed contains one original / template strand

  and one new strand

  - Ensures genetic continuity between generations of cells

  - H bonds reform
• Process of DNA polymerase


  - DNA has antiparallel strands

  - so the shape of the nucleotides are different

  - DNA polymerase is an enzyme with a specificshaped active site which can only bind to substrate with a complementary shape

  - Can only bind to and add nucleotides to the phosphate 3' end of the developing strand (so works in a 5' to 3' direction)
• Meselson and Stahl


  - bacteria grown ina nutrient solution containing heavy nitrogen (15N)

  - Nitrogen incorporated into bacterial DNA bases

  - Bacteria is then transferred to a nutrient solution containing light nitrogen (14N) and allowed to grow and divide twice

  - During this process, DNA from different samples of bacteria was extracted, suspended in a solution in seperate tubes and spun in a centrifuge
• Outcome of Meselson - Stahl Experiment


  Sample 1:

  DNA rom bacteria grown for several generations in a nutient solution containing (15N)

  -> DNA molecules contain 2 heavy strands

  Sample 2:

  DNA from bacteria grown originally in a nutrient solution containing (15N), then transferred for one division to a solution containing (14N)

  -> DNA molecules contain 1 original 'heavy' strand and 1 new 'light' strand

  Sample 3:

  DNA from bacteria grown originally in nutrient solution (15N), then transferred for two divisions to a solution containing (14N)

  -> 50% DNA molecules contain 1 original 'heavy' strand and 1 new 'light' strand, 50% contain both 'light' strands