Module 7 Lesson 2 - 4

DNA Replication Summary

1. Initiation: The replication origin is identified.


2. Unwinding: Helicases bind and uncoil the DNA double helix.


3. Elongation:
   
   
   1. DNA primase builds RNA prime
      
   2. RNA primer is the starting point for the DNA polymerase
   3. DNA polymerase adds complementary nucleotides in 5' to 3' direction
      
      
      1. Leading strand - DNA polymerase adds complementary nucleotides in 5' to 3' direction until reaches the end of the DNA molecule
         
      
      
      2. Lagging strand - DNA polymerase adds complementary nucleotides in 5' to 3' direction in short segments called Okazaki fragments.  Okazaki fragments are spliced by DNA ligase.


4. Termination: DNA replication ends at the telomere where DNA codes for termination.
   

Topoisomerase and Single Strand Binding Proteins

What are topoisomerase and single strand binding (SSB) proteins?

DNA replication is an incredibly rapid process. More than 1000 nucleotides are added per second. This causes the replication site called replication fork to rotate about 100 times per second. To stabilize the replication bubble by relieving the twist, an enzyme called topoisomerase attaches near the replication fork. After helicase unwinds and separates the DNA helix, the single strand binding proteins bind to the leading and lagging strands to prevent them from rewinding.

Quality Control

How can DNA polymerases process more than 1000 nucleotides per second without making a single mistake? They make about one mistake for every 10 million nucleotides.

DNA polymerase proofreads for mistakes and removes mismatched nucleotides. After proofreading, the error rate is about one in 1 billion nucleotides. With more than 6 billion nucleotides in our genome, that is still an alarmingly high rate of errors. After replication, mismatch repair processes reduce the error rate further. However, replication errors can remain and can result in mutations.

Mutation is studied later in the module.

Self-Check

1. Indicate whether each event occurs on the leading, lagging strand or both during DNA replication.
   A.    Leading strand
   B.    Lagging strand
   C.    Both leading and lagging strands
   
   1.  Elongation occurs continuously.
   2.  Elongation occurs in “chunks”
   3.  DNA polymerase elongates nucleotide chain
   4.  Nucleotides are added to the nucleotide’s 3’ end
   5.  Elongation occurs away from the unwinding section of DNA
   6.  Process involves Okazaki fragments
   7.  Elongation occurs towards the unwinding section of DNA
   8.  Elongation occurs in the 5’ to 3’ direction 
   
2. What would the complementary RNA primer be if the DNA single strand had the following nucleotide sequence → CGA ?