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Dna replication

Dna replication. SBI4U0 Ms. Manning. DNA Replication. Produces two identical copies of the chromosome during S phase of interphase Catalyzed by many enzymes including: DNA polymerase, helicase , ligase and primase

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Dna replication

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  1. Dnareplication SBI4U0 Ms. Manning

  2. DNA Replication • Produces two identical copies of the chromosome during S phase of interphase • Catalyzed by many enzymes including: DNA polymerase, helicase, ligaseand primase • Replication takes place at a point where the DNA double helix separates (called the replication fork)

  3. STEPS OF DNA REPLICATION 1. UNWINDING AND SEPARATION OF THE PARENT STRAND -separation is initiated at one or more sites called ORIGINS OF REPLICATION -the structure of 2 unwound DNA strands is referred to as a replication fork

  4. Three enzymes work together to unwind and stabilize the double helix: • DNA HELICASE – breaks the hydrogen bonds that connect the nitrogenous bases which allows the double helix to unwind and separate. • DNA GYRASE – an enzyme that relieves the tension produced by unwinding of DNA • SINGLE STRANDED BINDING PROTEINS (SSBs) – work to keep separated strands of DNA apart

  5. gyrase

  6. 2. DNA SYNTHESIS • Replication begins in 2 directions from the origins as a region of DNA is unwound. Replication proceeds towards the direction of the replication fork on one strand, and away from the fork on the other. • In eukaryotes, more than one replication fork may exist on a DNA molecule. • A replication bubble forms when 2 replication forks are in close proximity to each other

  7. Fig 4 p.220

  8. DNA Synthesis cont’d… • DNA POLYMERASE takes free nucleotides found within the cell and adds them in the 5’  3’ direction to form the new strand • The parent strand is used as a template • i.e. If A is in the parent, then T is inserted into the daughter

  9. LEADING STRAND • The daughter strand that grows continuously towards the replication fork as the double helix unwinds. • This occurs quickly • The new strand grows from 5’  3’

  10. LAGGING STRAND • The other daughter strand that cannot grow towards the replication fork, therefore it grows in the opposite direction • It is built in short segments (in the 5’  3’ direction) away from the replication fork. • This is much slower than the leading strand!

  11. Details about the lagging strand: • The 3’ to 5’ parent strand is a problem for DNA polymerase since it must synthesize in the 5’  3’ direction • Short RNA primer sequences of 10-60 RNA bases are bonded to regions of the lagging strand with the purpose of initiating DNA replication • PRIMASE – an enzyme that binds the RNA primers to the DNA

  12. Lagging strand cont’d… • DNA polymerase builds short sequences of DNA off of the RNA primers called OKAZAKI fragments. • DNA POLYMERASE I – removes the RNA primers once they have been used and replaces them with the appropriate DNA sequence • DNA LIGASE – joins the Okazaki fragments into one strand by the creation of phosphodiester bonds

  13. This lagging strand…is…slow! • The lagging strand takes longer to replicate than the leading strand because it involves more steps and is not continuous replication.

  14. 3) LINKING OF NITROGENOUS BASES • Nitrogen bases of nucleotides of opposite strands (parent and daughter) form new H bonds • Once hydrogen bonds are formed, DNA automatically twists into a double helix

  15. QUALITY CONTROL • DNA POLYMERASE III & I – act as a proof-reader by checking the newly synthesized strand for any incorrectly inserted bases. • If a mistake is found, polymerase acts as an EXONUCLEASE – cutting out the mis-paired base • Errors missed by proofreading can be corrected by one of several repair mechanisms after replication is complete.

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