The end replication problem

The “end-replication problem” is one that puzzled many molecular biologists and biochemists for decades. Since a free 3’ OH group is needed for DNA replication and a new strand of DNA to be synthesized, this creates a problem for the ends of linear DNA molecules, such as those we see in eukaryotic cells. Investigation into this mystery began when Blackburn and Szostak sequenced the ends of linear DNA molecules from Tetrahymena and were able to ligate to the ends of yeast cells DNA. These experiments showed evolutionarily conserved properties of Tetrahymena rDNA end sequences with those similar sequences in yeast cells that led to functional telomeres. They were also able to identify repeated C_1-3A sequences in telomeres of both organisms. As a graduate student in Blackburn’s laboratory, Greider was able to identify G-rich rDNA molecules in Tetrahymena that contained the necessary 3-OH group for DNA replication that was complementary to the repeats seen in the telomere sequences. The addition of the G-rich oligonucleotide were also shown in vitro to enzymatically be added to yeast telomeric DNA. At this point, the scientists were able to establish a mechanism for telomere synthesis and maintenance involving the enzyme telomerase as a unique reverse transcriptase with an RNA template and protein components, the mystery had been solved.