DNA Structure

DNA = Deoxyribonucleic Acid - Nucleotide

• Comprised of 3 parts - 5-carbon sugar (deoxyribose), phosphate, nitrogenous base

• 2 Types Nitrogenous Bases: Purines (2 rings) and Pyrimidines (1 ring)

  • Purines: Adenine, Guanine

  • Pyrimidines: Cytosine, Thymine

  • Adenine pairs with Thymine (A&T)

  • Guanine pairs with Cytosine (G&C)

  • Bases held together by hydrogen bonds

• Percentage of Bases in DNA

  • percentage of A (Adenine) = percentage of T (Thymine)

  • Shape of DNA - Double Helix

    • 2 antiparallel strands - phosphate-sugar backbone

    • Phosphate and sugar held by covalent bonds

  • percentage of C (Cytosine) = percentage of G (Guanine)

DNA Replication

Steps of DNA Replication

• 1. Nucleotide chains separated at the replication fork by enzyme Helicase - breaks hydrogen bonds between bases

• 2. SSBP (Single-Strand Binding Proteins) attach to the separate strands, prevent them from coming back together

• 3. Primase makes RNA primer to signal starting point to DNA Polymerase

• 4. DNA Polymerase III binds to primer, adding nucleotides to the 3’ of the separated strands

  • Synthesis is always bidirectional - always made in the 5’-3’ direction

• Leading Strand

  • Continuous strand that is made in the direction of the replication fork

• Lagging Strand

  • Discontinuous strand made in the opposite direction of the replication fork

  • Okazaki Fragments: sections of DNA about 100-200 base pairs long on the lagging strand

  • Ligase ‘glues’ all the okazaki fragments together

  • Topoisomerase cuts and rejoins the DNA downstream of the replication fork

• Semi-Conservative Replication

  • When replication is finished, you get the original strand and a newly made daughter strand