Why does leading strand not shorten?

[anbuitachi]

So during replication, lagging strand shortens because theres nothing at the 5' end at the end of the strand for it to attach to. So the primer area can't be replaced.

My question is why only lagging strand? Leading strand also has a primer at the beginning if it wants to replicate the entire DNA. That also has nothing to attach to so why doesn't leading strand shorten?

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[tendiw]

Could you word you question differently? If you are talking about the end of the DNA strand where there is no template, the ends are replicated by telomerase. Telomerase make telomeres at the ends on DNA, so DNA is NEVER shortened. If DNA was shortened during replication, genetic information would be lost. To deal with replicating the ends of eukaryotic chromosomes, telomerase is present to make repeating TG units so DNA is protected and information is not lost.

If you mean why is DNA replicated in short fragment on the lagging strand...it is because DNA can only be replicated from 5' to 3'

 

[anbuitachi]

I meant if theres no telomere, lagging would be shortened. Why doesn't this happen to leading strand? The leading strand also has a primer at the 5' end. After that primer gets degraded, how is the cell going to fill up that space?

I understand they both go in 5' to 3' but lagging strand shortens because the last primer can't be replaced since theres nothing to connect it to since its the end of the DNA... but leading strands beginning primer is also at an edge.

 

[tendiw]

Oh I see. In eukaryotes DNA polymerase actually removes the primer and then replicates that region. In prokaryotes, DNAP I, servers that function. Its a process call nick translation that is out of the scope of the MCAT...you learn about this in upper year biochem and genetics. DNA ligase then connect the DNA.

 

[Morsetlis]

I meant if theres no telomere, lagging would be shortened. Why doesn't this happen to leading strand? The leading strand also has a primer at the 5' end. After that primer gets degraded, how is the cell going to fill up that space?

I understand they both go in 5' to 3' but lagging strand shortens because the last primer can't be replaced since theres nothing to connect it to since its the end of the DNA... but leading strands beginning primer is also at an edge.

This kind of detail won't be on the MCAT.

There will of course be telomerases at all ends of the eukaryotic DNA. True, at any one end of a dsDNS there will be a region with primer and a region without. This primer is degraded via 5'-3' exonuclease activity from RnaseH + Fen1. This creates an ssDNA overhang at the 3' end of the parental strand (5' end of the newly synthesized strand). Telomerase comes and uses innate reverse transcriptase activity to synthesize new DNA from an RNA template found within the Telomerase. A new Okazaki fragment is created from this template and then Rnase'd and ligated as usual. After the newest RNA primer is removed, the ssDNA fragment is expendable.

This happens at both ends of the dsDNA strand. This is not a problem at the 5' end of the parental strand (3' end of the newly synthesized strand.)

https://docs.google.com/viewer?a=v&...DctY2NiOC00M2NhLTkyMWMtMWJkYzA5ZDE0ZWQ5&hl=en near the end (Ctrl+Find telomerase)