DNA replication

• All cells in the body contain exact copies of chromosomes that were in the original zygote (1st cell formed when egg and sperm fused)
• DNA Replication is the process of copying DNA before a cell divides into two cells
• DNA Replication ensures that the number of chromosomes in a cell are not halved every time a cell divides
• Except for gametes and red blood cells
• Cell division is the process where one parent cell is split into two daughter cells
• Steps in DNA Replication:

  • Step 1: Double helix of DNA is unwound by the enzyme Helicase, exposing the nitrogenous bases of each strand
  • Step 2: DNA polymerase adds floating nucleotides to the exposed complementary bases on each DNA strand
  • Step 3: Sugar and phosphate molecules from each nucleotide bond to form new sugar phosphate backbones
• Result of DNA Replication:

  • Replication occurs on both exposed strands of DNA, resulting in 2 identical double helices of DNA
  • Each double helix contains one old strand and one new strand
• Chromosome Condensation:

  • Chromosomes normally exist as long strings of DNA that are relatively unravelled or uncoiled
  • After DNA is replicated, it condenses into chromosomes as part of cell division and takes on the distinctive X shape
• Chromosome Structure:

  • After replication, the 2 copies of DNA made are attached at the center named the Centromere
  • Each copy of DNA on either the left or right sides of the X is called a Chromatid
  • Chromosomes usually exist as a single chromatid, but during cell division they double to form 2 sister chromatids
• Homologous Chromosomes:

  • Are 2 similar chromosomes with the same genes at the same location on the chromosome, one version from the mother and the other from the father
• Sister Chromatids are identical copies of the same chromosome and are the result of DNA replication