When cells divide they have a limit on their number of divisions by the telomeres shortening.  Each cell is thought to undergo 50 divisions or so before they die off.  The question I have is when a parent cell undergoes mitosis, do both daughter cell have shortened telomeres, or does one have a shortened telomere and the other a longer telomere.  In other words does one cell keep track of the number of divisions while the other one start from 0 ?

First of all, the relationship between telomere lentgh and the dividing capacity is complex and not as straightforward as one may think. In general shorter telomeres are associated with a longer history of divisions, but the exact role of telomeres in controlling cell division is not fully understood.

That said, each daughter cell has shorter telomeres. The telomere shortening comes from the way DNA is replicated. As you may know, Each chromosome is one DNA molecule (packaged in different forms by proteins) and DNA is a double helix of nucleotide polymer. Each chromosome is duplicated before cell division by replication. The replication is occuring by separating the two strands of the DNA molecule than polymerizing nucleotides using the old strands as template. The replication is complex, as the polymerisation of nucleotides can only happen in one direction, let's just say that both new strands will be shorter, so both chromatides (identical arms of a chromosome that separate during mitosis to form the two daughter chromosomes) will be made by an old and a new strand, the new strand being shorter. So we have two future chromosomes with one short and one long strand, but in fact the long strands are processed to preserve telomere structure, and as a result both future chromosomes are shorter.
I am really sorry about the lack of clarity, but it would take a course to explain properly and in addition all the processes happening at the telomeres during division are not entirely clear, especially in human cells (a bit more is kown in yeast, but it happens differently).

Last edited by Jerome Feldmann (4th Apr 2008 08:34:17)