DNA REPLICATION

  1. DNA (chromosome) replication occurs in the nucleus of __________________ ("true nucleus") cells. The DNA is replicated so that when a cell divides, each daughter cell receives the full complement of genetic instructions.

  2. The strands of DNA are "________________________". One strand runs in the 5' to 3' direction and the other runs in the 3' to 5' direction. Thus, the nucleotides on one original strand are upside down compared to the other original strand.(Diagram 1 in notes)

  3. The enzyme ___________________ begins by breaking the ________________ bonds between complementary bases (_____ with T, _________ with G) on opposite strands of DNA. Often, the enzyme will begin cutting someplace in the middle of the DNA molecule (ie. not at one end). This produces a "replication ___________________" as the strands separate. Two separate _______________ enzymes will be at work such that one is cutting upwards and one is cutting downwards. (Diagram 2 in notes)

  4. New ___________________ are added to each original (parental) strand by the enzyme ___________ _________________. This enzyme can only add nucleotides in the 5' to 3' direction. On the left original strand, one __________ __________________ adds complementary nucleotides in the 5' to 3' direction (with respect to the new strand) through complementary base __________________ in the upwards direction. (Diagram 3 in notes)

  5. On the right original strand, another __________ _______________ (floating around in the nucleus) attaches complementary nucleotides in the 5' to 3' direction (with respect to the new strand) through complementary base pairing in the downwards direction. The two DNA polymerase enzymes are thus moving in opposite directions. (Diagram 4 in notes)

  6. As ___________________ continues to cut in the upwards direction, the replication bubble widens and new unpaired nucleotides become available. The new (daughter) DNA strand on the left side can be continually built as complementary nucleotides (bases) are added by ___________ __________________ moving in the 5' to 3' direction. This is called _____________________ replication because the new DNA strand is made in one piece. (Diagram 5 in notes)

  7. The new DNA strand on the right side cannot be synthesized in the upwards direction by the original DNA polymerase because it was moving downwards in the 5' to 3' direction and cannot turn around. So another DNA polymerase attaches to the right original strand ____________________ from the first enzyme. This enzyme now adds _____________________ in the downwards direction which is 5' to 3'. This means that there are now two fragments of new nucleotides (______________________ fragments). The sugar-______________________ backbones of these fragments are joined together by DNA _____________________ enzyme. This is called ____________________ replication because the new DNA strand is made in pieces. (Diagram 6 in notes)

  8. As ________________ continues to cut in the downwards direction the situation is reversed. A new DNA ___________________ attaches to the left original strand _____________________ from the first enzyme and adds new _______________________ in the upwards direction which is 5' to 3' (creating __________________ fragments which will be joined by DNA ___________________). The original DNA ___________________ simply continues to add nucleotides on the right original strand in the downwards direction. So "continuous" and "discontinuous" replication can occur on both sides as the replication bubble widens until it reaches the top and bottom ends of the DNA. (Diagram 7 in notes)

  9. The DNA is thus completely replicated and now there are 2 genetically ___________________ DNA molecules (ie. chromosomes) for each original (parental) DNA. Depending on the type of cell, _____________ (for somatic cells) or _____________________ (for gametes) will follow.