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It is widely know that each cell cycle during DNA replication some fraction of the telomeres is lost, and this phenomenon is called the end replication problem. Well this is due to the fact that the DNA polymerase only adds nucleotides in 5´--> 3´ direction, thus the synthesis of one of the two DNA strands will need some RNA primers for its polymerization (and DNA polymerase will replicate DNA in a "jumping" pattern). Then, in the very end of the synthesis of this strand (called lagging strand) one last fraction will not be copied from the template as even if the last primer is set in the very end of the chromosome, once this RNA is degraded a void space is going o remain invisible for DNA polymerase.

Well this is all well known, but my question is whether during each DNA replication it is ONLY the telomere of the chromatid whose origin was in the lagging strand which will be shortened? (Thus chromosome shortening is happening in a truly asymmetrical and stochastic pattern)

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Not only one strand is used as lagging strand. In a replication bubble for half the length from the centre one will be used but for the other half other strand will be used.See the wikipedia article for replication. –  biogirl Nov 11 '13 at 17:25
    
you sure?? where do you locate the centre? –  Cobblhistone Nov 11 '13 at 17:42
    
and you cannot use the same strand one "side" as lagging and the other as leading as the direction of a strand is the same from beginning to end –  Cobblhistone Nov 11 '13 at 17:44
    
See this –  biogirl Nov 11 '13 at 18:33
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If you are talking about prokaryotes - yes, if about eukaryotes - no. Eukaryotes have multiple origins of replication to increase speed. So the telomere problem is going to arise at only the "last" origins of replications at both the ends. (No telomeres in middle as there will a 3'end available from the replicated dna from either side. Do you understand this ? It's a little tricky. Try to visualise it with the help of the image I linked) –  biogirl Nov 11 '13 at 18:53

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In replication, both the chromosomal halves (which are simultaneously threaded through the replication complex) have a lagging and a leading strand. A part of the segment will be replicated as leading and a part as lagging.
enter image description here

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i think you are a little confused.. the strand that is in the direction of the fork (5'->3') is leading strand . The other one is lagging. It is so because the DNA polymerase holoenzyme complex has two polymerases in the same orientation. So the 3'->5'strand has to be first bent to make it locally 5'-> 3' and then replicated. –  WYSIWYG Nov 12 '13 at 7:43
    
Since replication can be bidirectional, a strand of a chromosome will contain both leading and lagging elements (was that what you were trying to explain ?). If so, you should elaborate in your answer. The concept of leading/lagging strand is confusing because it is just a local structure, not the entire chromosome. –  WYSIWYG Nov 12 '13 at 7:47
    
@WYSIWYG Yes I meant the second thing. Please feel free to edit it. –  biogirl Nov 12 '13 at 17:33

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