DNA Structure and Replication

• DNA (deoxyribonucleic acid) is the very large molecule in chromosomes that carries the genetic code.
• DNA Structure

  It is arranged in a double helix shape, with two side strands, connected by paired bases.

  
  

  The side strands are called the sugar-phosphate backbone.

  
  

  Each strand of DNA is made up of many repeating units called nucleotides.

  
• Nucleotides and The Nitrogen Bases

  Nucleotides are made up of three parts:

  • A deoxyribose sugar
  • A phosphate group
  • A nitrogen base (adenine, thymine, cytosine and guanine)

  
• Parts of Nucleotide

  A) Phosphate

  B) Nitrogenous Base

  C) Sugar

  
• Nucleotides and The Nitrogen Bases

  The bases always pair according to the base pairing rule.

  • Adenine pairs with thymine (A - T)
  • Cytosine pairs with guanine (C - G)

  The pairs are held together by weak hydrogen bonds, which also holds the two strands of DNA together.
• DNA Replication

  It is essential that DNA can replicate itself, so that the chromosomes can be copied to give the same genetic code to every new cell that is made.

  
  

  The base pairing rule allows DNA to be copied and replicated correctly.
• DNA Replication

  It occurs in a series of steps, each controlled by enzymes, with the energy supplied from ATP.

  
  

  Two identical DNA molecules result from replication, with each having one original strand of DNA and one new strand (semi-conservative replication).
• Key Enzymes

  • Helicase - unwinds the double helix
  • Primase - makes RNA primers
  • DNA Polymerase - adds new nucleotides in 5'-3' direction on leading and lagging strand
  • Ligase - "Glues" the Okazaki fragments together.
  • optional: Topoisomerase - prevents super coiling
• Steps for DNA Replication

  1. The DNA helix unwinds and the base pairs are exposed as the hydrogen bonds are broken (Enzyme = DNA helicase)
• Steps for DNA Replication

  2. An enzyme (DNA polymerase) joins new nucleotides according to the base pairing rules, to the existing strand in the 3’ - 5’ direction with the help of a primer molecule. This is known as the leading strand. DNA replication for this strand occurs in the direction of the replication fork.
• Steps for DNA Replication

  3. The other strand which goes 5’ - 3 ‘ is the lagging strand and nucleotides are added in short fragments called Okazaki Fragments. DNA replication for this strand moves in the opposite direction of the replication fork.
• Steps for DNA Replication

  4. The Okazaki Fragments from the lagging strand are joined together by an enzyme (ligase), and the DNA molecules wind up into their double helix.
• DNA Replication

  A) Lagging

  B) Leading

  C) Helicase

  D) Okazaki Fragments

  E) Polymerase