(A) Yeast cells with multiple linear chromosomes require the capping protein Cdc13 to protect their telomeres and prevent chromosomes from fusing. Without this protein these cells cannot survive. (B) Cells that have a single linear chromosome can survive without Cdc13 by fusing the ends of their chromosome together to form a circular ring. (C) In the absence of the enzyme telomerase, cells with multiple linear chromosomes are able to survive by employing DNA recombination pathways which can amplify the telomere sequence or the DNA segments that sit between the chromatin and telomere sequence. (D) Cells with a single linear chromosome survive the loss of telomerase by fusing together to form a circular chromosome using homologous recombination, similar to what happens in cells lacking the protein Cdc13.
Image credit: Constance Nugent and Katsunori Sugimoto.