Sept. 20, 2007 Lecture 4 BGYA01
In the last class we were talking about DNA replication.
An origin of replication
a) occurs at the end of the double stranded DNA
b) has two RNA primers/origin
c) occurs in the interior of the double stranded DNA
d) has one RNA primer per origin
e) answers b and c are both true
We know that DNA replication is initiated at origins of replication (which are in the interior of
the double stranded DNA molecule) and that replication proceeds bi-directionally from the
origin. Figure 11.13, page 244 and homemade figure all
Bidirectional replication creates a growing replication bubble, but the textbook is lazy and only
shows events for half of the bubble – one replication fork.
We also know that the first nucleic acid synthesized at the origin is actually RNA. An RNA
primer is made for each fork (ie two RNA primers per origin). homemade figure frame d and
Figure 11.16, page 246
Once the RNA primers are laid down at the origin, things go smoothly on one half of each fork.
For each fork Figure 11.18, page 247- One newly synthesized strand has the correct polarity to
readily continue replication from the original RNA primer; each time a nucleotide is added on,
there is a free 3’OH group on that nucleotide to add on to.
We call such a newly created strand a leading strand.
We also say DNA is create 5 to 3 (from the 5’ end to the (growing) 3’ end.
For this terminology the reference is the newly synthesized DNA.
As helicase continually opens up the DNA helix of this fork, replication occurs continuously on
the leading strands. We say replication on the leading strands is continuous.
The other portion of the same fork (the one that uses the antiparallel strand of DNA as the
template) has a more difficult time. Why?
It is because that region has a template, but no free 3’OH group (to add onto) in the direction the
fork is opening. ). textbook figure 11.18, page 247 top panel So at the moment no replication
can occur there (study guide replication figure, part D
Instead replication lags on this side of the fork until the DNA helix gets more opened up. We
call the strand that will eventually be made on this portion of the fork, the lagging strand.
If you look at the homemade figure you should be able to see that there are actually two leading
and two lagging strands per origin of replication.