DNA and RNA

• DNA stands for deoxyribonucleic acid.
• The function of DNA is to carry genetic information.
• 4 base varieties for DNA:

  • Adenine
  • Thymine
  • Cytosine
  • Guanine
• Monomers of nucleic acids are called nucelotides.
• Double stranded DNA forms via hydrogen bonds between complimentary bases:

  • Adenine and Thymine
  • Cytosine and Guanine
• To join adjacent nucleotides, the Carbon 3 of one nucleotide undergoes a condensation reaction with the phosphate group of the adjacent nucleotide. This forms a phosphodiester bond.
• DNA is antiparallel. This is due to one strand running in the 3 prime to 5 prime direction whilst the other runs in the 5 prime to 3 prime direction.
• RNA transfers genetic codes to ribosomes in order to make proteins from DNA.
• 4 base varieties for RNA:

  • Adenine
  • Guanine
  • Cytosine
  • Uracil
• RNA is single stranded.
• 3 types of RNA:

  • Messenger RNA (mRNA)
  • Transfer RNA (tRNA)
  • Ribosomal RNA (rRNA)
• DNA Replication:

  • DNA helicase breaks hydrogen bonds between complimentary bases.
  • This unwinds DNA and forms 2 template strands.
  • Free DNA nucleotides are attracted to their complimentary base pair.
  • DNA polymerase forms phosphodiester bonds between adjacent nucleotides.
  • This is semi-conservative replication because each DNA molecule is made from 1 parent strand and 1 new strand.
• DNA replication always occurs in the 5 prime to 3 prime direction.
• DNA replication always occurs in the 5 prime to 3 prime direction because:

  • DNA polymerase is an enzyme with a specifically shaped active site.
  • It is only complimentary to the 3 prime end of a nucleotide.
  • It is not complimentary to the 5 prime end of a nucleotide as the shape is different.
• Watson and Crick theorised and discovered semi-conservative replication and the structure of DNA.
• Meselson and Stahl proved the semi-conservative replication model.
• Meselson and Stahl experiment:

  • Bacterial cells grown in 15N solution so all bases have 15N.
  • Solution was centrifuged and one band settled at bottom of test tube (high density).
  • Cells were transferred to 14N solution and divided once.
  • Centrifuged and band settled in middle (medium density)
  • Cells divide again and two bands form, one in middle and one at top (low density).
• Why DNA is good at carrying genetic information:

  • Very long: So can carry thousands of genes.
  • Helical: Compact.
  • Weak, hydrogen bonds between bases: easily broken for DNA replication
  • Strong, covalent phosphodiester bonds between nucleotides: keeps DNA stable.
  • Sugar phosphate backbone: protects order of bases.
  • DNA bases code for amino acids
  • Complimentary base pairing makes semi-conservative replication accurate.