DNA Replication and Repair - INTRO SBI 4U: Metablic Processes DNA Replication and Repair - INTRO Mitosis: division of the nucleus into two daughter nuclei with identical chromosomes. Followed by cytokinesis (division into 2 cells). Essential for growth, regeneration, repair of cells and tissue DNA replication: The hydrogen bonds between complementary base pairs break, the DNA helix unzips and each single strand acts as a TEMPLATE to build the complementary strand resulting in two identical DNA molecules. Meselson and Stahl, 1958 use radiolabelled nitrogen and Escherichia coli to prove DNA replication is SEMICONSERVATIVE – each new DNA molecule is comprised of one old strand and one new strand. Section 1.3

DNA REPLICATION: SEPARATING THE STRANDS SBI 4U: Metablic Processes DNA REPLICATION: SEPARATING THE STRANDS Special proteins (origin of replication complex or ORC) bind at a specific site on the DNA known as the replication origin (many repl. origins on a strand) DNA helicase (enzyme) unwinds the double helix by breaking the H- bonds between the base pairs. Single stranded binding proteins (SSBs) attach to each strand and block H-bonding to prevent them coming back together (anneal: pairing of complementary base pairs through H-bonds). DNA gyrase (bacterial enzyme) relieves the tension produced by the unwinding (similar enzymes in eukaryotes) Section 1.3

SEPARATING THE STRANDS cont'd SBI 4U: Metablic Processes SEPARATING THE STRANDS cont'd DNA cannot be fully unwound – it is much bigger than the cell it is in. DNA is copied in two directions from the origin(s) as DNA is unwound. Replication fork: part of strand that is still joined Replication bubble: unwound strand between two forks that are close together (eukaryotic only). EXTRA INFO: 50 nucleotides are added per second when DNA replicates It would take 700 days to replicate the whole human genome if not for many time saving devices we have (multiple replication points), which speed it up to around 5-10 hours Section 1.3

BUILDING THE COMPLEMENTARY STRANDS SBI 4U: Metablic Processes BUILDING THE COMPLEMENTARY STRANDS There are 5 different types of DNA polymerase that help to build the complementary strands. DNA polymerase III synthesizes DNA in the 5' to 3' direction. An RNA primer must first anneal to the template (by primase) to give DNA polyIII a place to start, the primer will be removed later. DNA polyIII adds deoxyribonucleoside triphosphates to the 3' end of the elongating strand (starting at the primer). The bond between the first and second phosphate groups is broken and the energy released is used to drive the condensation reaction that attaches the nucleotide to the new strand. The extra two phosphates are recycled back to the cell. Section 1.3

BUILDING THE COMPLEMENTARY STRANDS cont'd SBI 4U: Metablic Processes BUILDING THE COMPLEMENTARY STRANDS cont'd Leading strand: synthesized continuously from 5' to 3', toward the replicating fork. Lagging strand: built on the 3' to 5' strand in small 5' to 3' sections moving away from the replication fork, called Okazaki fragments, (he discovered them). RNA primers are constantly being added so DNA polymerase III adds deoxyribonucleosides in short sequences DNA polymerase I removes the primers and replaces them with the appropriate deoxyribonucleosides DNA ligase attaches the Okazaki fragments together with phophodiester bonds. As the two strand are completed they immediately twist into their helical shape. Section 1.3

SBI 4U: Metablic Processes DNA REPAIR Both DNA polymerases (I and III) read over the completed strands to ensure there are no mistakes. They can each act as an exonuclease and remove and replace any wrong nucleotides with the correct ones. DIAGRAMS: Semi-conservative: http://www.animalgenome.org/edu/doe/fig4.html Replication: http://distancelearning.ksi.edu/demo/bio378/DNA_files/image017. gif http://www.bio.davidson.edu/Courses/Molbio/MolStudents/spring2 005/Durnbaugh/dna_replication.gif http://www.andrew.cmu.edu/user/berget/Education/TechTeach/repl ication/repfork.gif Summary: http://library.thinkquest.org/04apr/00217/images/content/74- Summary-DNA-Replication.jpg Section 1.3