C1-3A Short overhang
Figure 17.1. Yeast telomere end structure. (A) Yeast telomere repeat tracts have 250-400 bp of the irregular sequence TGi_3 at their very ends. The terminus of the chromosome has a 3' overhang in the G1 phase of the cell cycle which is 12-14 nt long (Larrivee et al., 2004). The telomeres of human cells also possess 3' overhangs (McElligott and Wellinger 1997). (B) The 3' overhang is elongated in S phase in a telomerase-independent manner. At the end of S-phase after telomere replication, the overhang is expanded to greater than 50 nt and then returns to a shorter overhang which is presumed to have the G1 structure. Because this long overhang is detected in cells lacking in vivo telomerase activity, it is thought that the overhang is produced by a nuclease acting on replicated telomeres (Wellinger et al., 1996).
In yeast and human germ cells, this arrest is prevented by the synthesis of new telomere repeats by the activity of the enzyme telomerase.
Telomerase is the name given to the core enzyme consisting of a protein (the TERT subunit for TElomerase Reverse Transcriptase) and RNA (the TER subunit for TElomerase RNA, also called TERC and TR). The RNA serves as an internal template for the synthesis of new telomere repeats (reviewed by Smogorzewska and De Lange (2004)). A number of other protein subunits that make up the telomerase holoenzyme are known in several organisms, but most conserved components are TERT and TER. While growing cultures of human and yeast cells that lack telomerase eventually senesce and/or die from loss of telomere function, cultures of cells that express telomerase activity form cultures that continuously double (Bodnar et al., 1998). Consequently, the presence of telomerase can prevent the short telomere-mediated cell senescence or death of all cells in the culture.
When human and yeast cells express telomerase, the size of their telomere repeat tracts is usually kept within a certain range (as opposed to elongating continuously). A great deal of work on telomerase and the genetics of telomere length control has yielded a model in which much of the control of telomere length is due to telomeric chromatin regulating the access of telomerase to the 3' end of the chromosome (reviewed by Smogorzewska and De Lange (2004)). The proteins that bind directly to the telomere repeats, or the co-factors that they recruit, are thought to form a structure that sequesters the chromosome end from telomerase and other cellular activities. Short telomeres are preferentially elongated over longer telomeres (Hemann et al., 2001; Ouellette et al., 2000; Teixeira et al., 2004), and it is thought that shorter telomeres bind fewer of these proteins, which makes them more likely to be lengthened. This interplay between telomerase and the many telomere chromatin components is an area of intensive research, and a greater understanding of the intricacies of these processes is likely in the coming years.
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