DNA Replication - Mrs. Tuma's Biology Page



DNA Replication

• DNA must be duplicated if cells are to undergo division

• replication is “semi-conservative”

• occurs in nucleus

• each strand is used as a template for a new, complementary strand

Several Key Steps:

1. DNA strand is unwound by enzyme called helicase, using ATP as energy source

2. Gyrase (topoisomerase) enzyme assists by making sure twists and knots don’t form in other regions of the DNA

-- may make small breaks to relieve stress and then rejoin the ends

3. DNA polymerase catalyses formation of sugar-phosphate bond between nucleotides

-- needs already existing DNA

-- needs to attach to short region of double stranded DNA

(primer strand)

4. Nucleotides can only be added to the 3' OH end of the chain so growth goes from 5' to 3' direction

-- region of unwinding DNA is called replication fork

-- individual nucleotides coming in have 3 phosphate groups attached, but two are lost, supplying energy and the last remains with the nucleotide on the strand.

5. One chain is the leading chain and is synthesized continuously. The other chain is lagging chain and is synthesized discontinuously

-- lagging chain must grow in direction opposite to unwinding

6. A primer of RNA is made (10 nucleotides long) by primase enzyme

7. Gaps on lagging strand are filled in by synthesis of DNA by DNA polymerase and RNA primers are removed.

8. Segments of lagging chain are sealed by DNA ligase

Overall Summary

Enzymes in DNA Replication

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