BIO 104
Instructor: Avis Gibons
Outline of DNA Replication
1)
Replication begins at multiple origins of replication, specific sequences of nucleotides at
specific sites on the DNA molecule.
2)
An enzyme complex recognizes each origin of replication on the double-stranded DNA.
3)
One of the enzymes, helicase, begins to unwind and unzip the strands by breaking the hydrogen
bonds between the bases, causing a “replication bubble” or opening between the strands.
4)
The strands separate where the bubble forms. The two Y-shaped ends of the bubble are called
replication forks.
5)
Single-stranded DNA binding proteins keep the strands apart by binding to the DNA. They
also prevent degradation of the separated strands.
6)
Free nucleotides present in the nucleus form hydrogen bonds with their complementary bases on
unzipped portions of the two original, or parent, strands of DNA.
7)
Primase makes two primers, short chains of RNA, each complementary to a parent strand of
DNA. Primers give the next enzyme that acts a place to start.
8)
That next enzyme, DNA polymerase, binds to the RNA primers and starts hooking nucleotides
together to form the backbone of each new, or daughter, strand.
DNA polymerase can join nucleotides only at the 3’ end, so the strands can grow only in a 5’ to
3’ direction. This works fine for the daughter strand that will use the 3’ to 5’ parent as a
template, since it will be made in the 5’ to 3’ direction. This daughter strand is made
continuously and is called the leading strand (elongates toward the replication fork).
However, the strand that will go in the 3’ to 5’ direction is made discontinuously as a series of
segments about 100 base pairs long. This strand is called the lagging strand (elongates away
from the fork). The segments are called Okazaki fragments.
To make the 3’ to 5’ lagging strand, primase periodically jumps ahead (toward the replication
fork), making primers that allow DNA polymerase to work in the 5’ to 3’ direction (backwards).
Whether the daughter strand elongates toward or away from the direction of replication fork
advancement determines its role as the leading or lagging strand. Because replication is
bidirectional, the daughter strand’s role changes from one side of the origin to the other.
9)
DNA polymerase proofreads the new molecules for incorrect pairings, corrects any errors
found, and replaces RNA primers with DNA nucleotides.
10)
Another enzyme, DNA ligase, joins Okazaki fragments or other gaps to complete each strand.
11)
When the replication forks meet, replication ends and the two DNA molecules separate.