Strand duplication: Okazaki fragments vs. Looping

Question

I have a question about strand duplication. In class, we learned that the lagging strand can synthesize from 3'-5' by use of okazagi fragments, which will then be connected by DNA ligase. However, I have in my notes that the lagging strand can also form a "loop" and therefore synthesize 5'-3'. In what case with either occur?

Answer 1

This diagram might help.

http://www.biog1105-1106.org/demos/106/u...

Neat, this is new to me.  Always amazing how what you learn 5 years ago is wrong.

Answer 2

There are some very good animations showing the looping out of the lagging strand. Try

http://video.google.com/videoplay?docid=...

or http://www.wehi.edu.au/wehi-tv/dna/index...

Answer 3

the polymerase synthesizes only 5'->3' while reading 3'->5'.  on the lagging strand short okazaki fragments are made 5'->3' (the polymerase only moves one direction.  Lagging strand loops are formed to increase the size of the okazaki fragments.  without a loop only short fragments can be made (because the polymerase moves too fast for anything else).  the loop helps increase the size.   but the direction the polymerase synthesizes is always 5'->3'