dna

• DNA is made up of nucleotides, which are composed of three parts - deoxyribose sugar, phosphate group, and nitrogenous base.
• The four types of bases found in DNA are adenine (A), thymine (T), cytosine (C), and guanine (G).
• The two types of nitrogenous bases found in DNA are purines (adenine and guanine) and pyrimidines (cytosine and thymine).
• Ribonucleic acid (RNA) differs from DNA as it contains ribose instead of deoxyribose, uracil instead of thymine, and has a single strand rather than double-stranded.
• Nucleic acids can be classified into two main groups based on their structure: Ribonucleic acid (RNA) and Deoxyribonucleic acid (DNA).
• DNA replication

  1. Occurs in preparation for mitosis

  2. Occurs during the S phase of the cell cycle (which occurs during interphase, when a cell is not dividing)
• Helicase


  Unwinds the DNA double helix by breaking the hydrogen bonds between the base pairs on the two antiparallel polynucleotide DNA strands to form two single polynucleotide DNA strands
• Template


  Each single polynucleotide DNA strand acts as a template for the formation of a new strand made from free nucleotides attracted to the exposed DNA bases by base pairing
• DNA polymerase

  1. Joins new nucleotides together by catalysing condensation reactions to form a new strand

  2. Cleaves the two extra phosphates and uses the energy released to create phosphodiester bonds between adjacent nucleotides

  3. Synthesises new DNA strands from the two template strands
• Semi-conservative replication


  Method of replicating DNA where half of the original DNA molecule is kept (conserved) in each of the two new DNA molecules
• Nucleoside triphosphates


  Free nucleotides in the nucleus containing three phosphate groups, which activate the nucleotides for DNA replication
• Leading & lagging strands

  1. DNA polymerase can only build the new strand in one direction (5’ to 3’ direction)

  2. DNA polymerase synthesises the leading strand continuously

  3. DNA polymerase synthesises the lagging DNA strand in short segments called Okazaki fragments

  4. DNA ligase is needed to join the lagging strand segments together to form a continuous complementary DNA strand
• In DNA, the nitrogenous base pairs are always adenine with thymine and guanine with cytosine.
• DNA replication

  1. Segments (called Okazaki fragments) are formed

  2. DNA ligase joins the lagging strand segments together to form a continuous complementary DNA strand

  3. DNA ligase catalyses the formation of phosphodiester bonds between the segments to create a continuous sugar-phosphate backbone

  4. The synthesis of the complementary strands occurs slightly differently on the leading and lagging template strands of the original DNA molecule being replicated
• There are five different types of nitrogenous bases found in nucleic acids: adenine, guanine, cytosine, thymine, and uracil.
• The nucleotides that make up the backbone of both RNA and DNA are composed of three components: a pentose sugar molecule, a phosphate group, and one or more nitrogenous bases.
• Cell Structure

  • The Cell Theory
  • Structure of Eukaryotic Cells
  • Specialisation of Eukaryotic Cells
  • Eukaryotic Cell Organisation
  • Structure of Prokaryotic Cells
  • Prokaryotic v Eukaryotic Cells
  • Viruses
• Cell Recognition & the Immune System

  • Cell Recognition
  • Antigens
  • Phagocytosis
  • The T Lymphocyte Response
  • The Role of Antigen-Presenting Cells
  • The Role of Helper T cells
  • The B lymphocyte Response
  • Antibodies
  • Antigen-Antibody Complex
  • Plasma & Memory Cells
• Vaccines, Disease & Monoclonal Antibodies

  • Vaccines
  • Active & Passive Immunity
  • HIV
  • AIDS
  • Monoclonal Antibodies
  • Ethical Issues with Vaccines & Monoclonal Antibodies
  • ELISA
• Genetics, Variation & Interdepend


  • Topics related to genetics, variation, and interdependence