Each human cell contains 23 pairs of chromosomes, corresponding to 46 long chains of DNA.
This gives about six billion base pairs (A—T or C—G) in each cell.
If this were written out, it would fill 900 Biology, 5th Edition books.
When each cell divides, it must copy all six billion base pairs—all 900 books—worth of information.
This is done in less than two hours with very few errors.
About one error per billion base pairs
About six errors in 900 books worth of text!
Imagine yourself at the fastest copy machine in the world, trying to copy 900 books in less than two hours with only six errors in the whole project.
A series of specialized molecules work together to make the process more efficient.
The Copy Process
The Rule
Only one rule matters: DNA only copies in one direction.
New nucleotides are only added at the 3' end of the new DNA strand.
We thus say that DNA is copied in the 5' → 3' direction, referring to the building of the new strand.
Viewed from the perspective of the old strand, the process moves in the 3' → 5' direction—but we never view it from this perspective.
The Team
Helicase
Unwinds the DNA and separates the DNA for copying at the replication fork.
Single-strand binding proteins
Holds the two DNA strands apart during copying
Primase
Gets the copying process started by adding RNA nucleotides to make a short stretch of RNA called a primer.
Note: It is very strange to use RNA when making DNA—we have no idea why this is done!
DNA polymerase III
Adds DNA nucleotides beginning at the RNA primers.
Problem: it can act continuously on one strand (the leading strand), but must act in short segments on the other strand (the lagging strand) as the DNA is unwound.
This is because of the 5' → 3' rule.
Thus, there is a continual “dance” on the lagging strand between primase and DNA polymerase in order to continue the copying process.
The short segments of DNA are called Okazaki fragments after the Japanese scientist who discovered them.
DNA Polymerase I
Removes RNA primers and replaces them with DNA.
Ligase
Joins all the individual strands of DNA (stitches everything together)
DNA Replication in Context
Remember that DNA replication occurs during the S phase of the cell's life cycle.
