Leading Strand: Priming, Placement, and Primers

[student007]

Question: How does the leading strand work? (I know that it moves in the right direction...i.e. relatively fast) Where is the RNA primer placed on it (i.e. is it just an arbitrary point)? On eukaryotic DNA, about how many total primers will there be on the leading strand relative to the lagging strand?

(This is not a homework question. I have a quiz tomorrow, and i approached these issues while studying)

 

[iansmith]

Question: How does the leading strand work? (I know that it moves in the right direction...i.e. relatively fast)

the rate for eukaryotes DNA polymerase is about 500 to 5000 base pairs per minute and for bacteria it can reach 1,000,000 base pairs per minute.

The source of the movement is debate. It is usually accepted that polymerase and other enzymes, which as a group is called the replisome, are moving along the DNA strand. However, there is evidence in bacteria that there is a central non moving replisome and that the DNA is pushed through the replisome.

Where is the RNA primer placed on it (i.e. is it just an arbitrary point)?

It appears to be random but primers are synthesised at a fairly regular interval.

On eukaryotic DNA, about how many total primers will there be on the leading strand relative to the lagging strand?

there will be a lot. In E. coli, there 1 primer per 1000 base pairs. In eukaryotes, the Okazaki fragments are shorter so the frequency would be lower,it is around 1 every 200 base pair. Just do to math, the human genome has 6 billion base pair, Encephalitozoon cuniculi has the smallest genome with 2,9 million base pair and yeast has 12,5 million base pair. Some amphibiands and some plants can have over 100 billion base pair genome.

 

[student007]

Thanx...one other thing...just wondering...is it EVER possible to have a double-stranded RNA molecule

 

[Monique]

Only some virusses have dsRNA, the human body has Toll-like receptors that recognize dsRNA (TLR-3) and induces the innate immune response to break down cells expressing it.

 

[iansmith]

Yes, it is possible to have double-stranded (ds) RNA; however, the dsRNA are quickly degraded in the cell and dsRNA can be used to regulate gene expression. Some viruses have dsRNA genome. It has been sugested that the machinery that degraded dsRNA has evolved to protect the cell against dsRNA viruses.

 

[Monique]

Interestingly you can knock-down genes in mammals by injecting dsRNA: the normal RNA for that transcript will be down-regulated.