Proteins involved in DNA replication:

 

1. Helicase 

    Open the double helix at replication forks by distrupting the hydrogen bonds that hold the two strands                together.

 

 2. Single-Strand Binding (SSB) Protein

     Binds to single strands of DNA and prevents the helix from reforming before it can be used as a template          for replication.

 

 3. Topoisomerase

     Breaks one or both DNA strands, preventing excessive coiling during replication, and then rejoins them in a      more relaxed configuration.

      

 4. DNA polymerase

     Links nucleotide subunits to form a new DNA strand from a DNA template.

 

 5. DNA primase

     Synthesizes short RNA primers on the lagging strand. Begins replication of the leading strand.

 

 6. DNA ligase

     Links Okazaki fragments by joining the 3' end of the new DNA fragment to the 5' end of the adjoining DNA.

 

 

An overview of DNA replication

 

 

        

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

DNA synthesis begins at origin of replication. Strands are separated at origin of replication and unwound by DNA helicase, which 'walks' along DNA molecule preceding the DNA-synthesizing enzymes. Single-stranded regions are prevented from reforming into double strands by Single Strand Binding Proteins, which bind to single DNA strands. Region of active DNA synthesis is associated with replication fork, formed at junction of single strands and double-stranded region. Both strands are synthesized in vicinity of fork (in 5'--> 3' direction). Completion of replication results in formation of two daughter molecules, each containing one old and one newly synthesized strand. Each double helix is a chromatid of a duplicated eukaryotic chromosome.

 

Leading and lagging DNA stands

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

Leading strand is synthesized continuously in direction toward replication fork; lagging strand is synthesized in direction away from replication fork. Both strands require RNA primer for initiation of synthesis because DNA can be elongated only by addition to 3' end of existing polynucleotide strand. Lagging strand is synthesized as short Okazaki fragments. Okazaki fragment synthesis of RNA primer. Note that first Okazaki fragment synthesized is now at far left. After each Okazaki fragment has been elongated by DNA polymerase, RNA primer is degraded, gaps are filled in with DNA, and adjoining fragments are linked by DNA ligase.

 

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