1. Telomeres and senescence: The history, the experiment, the future
Carol W. Greider 
Current Biology 
Volume 8, Issue 5, Pages R178-R181 (February 1998)
DOI: /S (98)

2. Figure 1
The telomere hypothesis of cell senescence and immortalization proposes that: (1) Telomeres are maintained in the germline because of the presence of telomerase. (2) Telomeres shorten due to the end replication problem in many somatic cells that do not express telomerase. (3) Agents such as oncogenes which extend cell life span bypass the senescence signal. (4) Cultures containing cells with short telomeres undergo a growth crisis where many cells die. (5) Immortal cell clones that emerge from crisis have activated telomerase and so maintain, or even lengthen, telomeres.
Current Biology 1998 8, R178-R181DOI: ( /S (98) )

3. Figure 2
(a) In cell strains that do not express telomerase, telomere repeats are lost during cell division. When telomeres reach a certain length, a signal is sent to initiate a program of cellular senescence. (b) In cell strains that are forced to make telomerase by expression of hTERT, telomere length does not shorten (it even lengthens, but this is not shown). These cells do not enter senescence, even after undergoing more cell doubling than their telomerase-negative cousins.
Current Biology 1998 8, R178-R181DOI: ( /S (98) )