DNA and RNA

• DNA structure:

  DNA has a double helix structure, Antiparallel strands = DNA molecule is formed from two separate strands which wind around each other to form a spiral. The strands are polynucleotides (they're made up of a lot of nucleotides joined together in a long chain). They form the structural backbone of the DNA molecule.
• DNA molecules are long and coiled up very tightly so a lot of genetic information can fit into a small space in the cell nucleus.

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• DNA nucleotide structure:

  • A DNA nucleotide is made from a phosphate group, the pentose sugar deoxyribose and a nitrogen containing base. Four possible bases: Adenine [A], Thymine [T], Cytosine [C] and Guanine [G].
• Complementary base pairing

  • Two DNA polynucleotide strands join together by hydrogen bonding between the two bases
  • Adenine always pairs with thymine through two hydrogen bonds
  • Guanine always pairs with cytosine through three hydrogen bonds
  • This means there are always equal amounts if A + T in a DNA molecule and C + G.
• RNA structure:  

  • Sugar in a RNA nucleotide is a ribose sugar.
  • Uracil replaces thymine as a base, and always pairs with adenine
  • The nucleotides form a SINGLE polynucleotide strand
  • RNA strands are much shorter than most DNA polynucleotides.
• DNA molecule is adapted to carry out its function by 

  • Its two separate strands joined only with hydrogen bonds, allowing them to separate during DNA  replication and protein synthesis
  • Large molecules carrying an immense amount of genetic information.
  • Base pairing allowing DNA to be able to replicate and to transfer information as mRNA
• DNA replication:

  DNA copies itself before cell division, so each new cell has a full amount of DNA = semi conservative replication because half of the strands in each new DNA molecule are from the original DNA molecule = genetic continuity between generation of cells.
• How is DNA replicated:

  1. DNA helicase breaks the hydrogen bonds between bases on two polynucleotide DNA strands = helix unwinds to form two single strands
  2. Each original single strand acts as a template for a new strand, complementary base pairing means free floating DNA nucleotides bind to their complementary exposed bases on each original template strand.
  3. Nucleotides are joined together in a condensation reaction by DNA  polymerase, hydrogen bonds form between bases on the original and new strands.
  4. Each DNA molecule contains one strand from the original DNA molecule and one new strand.
• Evidence for semi conservative replication

  • Meselson and Stahl experiment: two isotopes of nitrogen- Heavy nitrogen 15N and light nitrogen 14N (STEP ONE AND TWO)

  1. 2 samples of bacteria were grown for many generations, one is a nutrient broth containing 14N and the other in a broth with 15N. As the bacteria reproduced they took up the nitrogen from the broth to help make nucleotides for new DNA. so nitrogen gradually became a part of bacteria's DNA.
  2. A sample of DNA was taken from each batch of bacteria and spun in a centrifuge. DNA from 15N settled lower than DNA from 14N as it is heavier
• Evidence for semi conservative replication STEP 3 AND 4

  1. Then the bacteria being grown in 15N (heavier) broth were taken out and put in broth containing only light nitrogen 14N. Bacteria was left for one round of DNA replication and then another DNA sample was taken out and spun in the centrifuge.
  2. If replication was semi conservative; the new bacterial DNA molecules would contain one strand of DNA containing heavy nitrogen (15N)  and one strand containing light nitrogen (14N). So the DNA would settle BETWEEN where the light nitrogen DNA settled and when the heavy DNA settled.
• If replication was conservative, the original heavy DNA would still be together and settle at the bottom and the new light DNA would settle at the top.