dna replication

[nycfella]

After the primer first attaches to the template to initiate the creation of the new strand (the leading) it is removed by rnaseh. Why can't the last segment of okasaki fragment have the primer removed in the same way. That is, why is there a problem on the lagging strand (it makes the strand shorter I know, but how come the leading strand isn't cut short even though it has a primer on its tip) but no the leading strand?

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[Sol Rosenberg]

BOTH strands are cut shorter because of the primer at the end of the leading strand, even though most Biochem/Genetics courses seem to just focus on the lagging strand for erosion of telomeres.

 

[nycfella]

BOTH strands are cut shorter because of the primer at the end of the leading strand, even though most Biochem/Genetics courses seem to just focus on the lagging strand for erosion of telomeres.

thanks

 

[greatnt249]

BOTH strands are cut shorter because of the primer at the end of the leading strand, even though most Biochem/Genetics courses seem to just focus on the lagging strand for erosion of telomeres.

Why would there be a primer at the end of the leading strand if that's the direction DNA polymerase is moving toward?

 

[Sol Rosenberg]

Why would there be a primer at the end of the leading strand if that's the direction DNA polymerase is moving toward?

Sorry, should have been more specific....at the 3' end of the leading strand. That's the direction that the DNA polymerase is moving away from.

EDIT: In ASCII art from (because I'm slightly bored):

D = LeaDing Strand DNA
P = Primer
N = New DNA
X = Empty (Things don't align right with spaces)

When elongation is finished on the leading strand, you have:

3'-DDDD...DDDDD-5'
5'-PPNN...NNNNN-3'

After the primer is removed:

3'-DDDD...DDDDD-5'
5'-XXNN...NNNNN-3'

There is no way to fill in the Xs, so the new complement of the leading strand is shorter.

For an explanation of the problem with the complement of the lagging strand, consult any other reference, because that's what they all seem to focus on.

 

[greatnt249]

Sorry, should have been more specific....at the 3' end of the leading strand. That's the direction that the DNA polymerase is moving away from.

That would still be at the origin of the replication fork, not necessarily at the end of the strand of linear DNA, though.

 

[greatnt249]

Sorry, should have been more specific....at the 3' end of the leading strand. That's the direction that the DNA polymerase is moving away from.

EDIT: In ASCII art from (because I'm slightly bored):

D = LeaDing Strand DNA
P = Primer
N = New DNA
X = Empty (Things don't align right with spaces)

When elongation is finished on the leading strand, you have:

3'-DDDD...DDDDD-5'
5'-PPNN...NNNNN-3'

After the primer is removed:

3'-DDDD...DDDDD-5'
5'-XXNN...NNNNN-3'

There is no way to fill in the Xs, so the new complement of the leading strand is shorter.

For an explanation of the problem with the complement of the lagging strand, consult any other reference, because that's what they all seem to focus on.

What you've shown is the lagging strand, not the leading strand (in reference to the new DNA being added). Lagging/leading refer to the orientation of the strand being built on the original set of DNA.