Document Type: Research Paper
Genetics group, Biology Department, Faculty of Sciences, Isfahan University, Isfahan, I. R. Iran.
Biomolecular Sciences Department, UMIST, Sackvill St., P.O. Box: 88, Manchester, M60 1QD, UK.
In the budding yeast of Saccharomyces cerevisiae the tandem repeated of rDNA genes are located on
chromosome XII, which is in the nucleolus. There are different types of proteins in the nucleoluskeleton,
silencing proteins have got important role in nucleolus.
It is shown that meiotic recombination between nonsister chromatids in the rDNA genes are strongly
suppressed, and suggested that silencing proteins such as SIR2 are involved in silencing state. It is also
shown that nucleolus shows some changes during ageing process. It is claimed that intrachromosomal
recombination within the rDNA repeated sequences; producing 3-μm rDNA circles are accumulated with old
cell. This study looked at the rDNA breakage in two different types of strain ORD 1181 according to their
age. The fine analysis of the rDNA array was performed using restriction endonuclease enzymes, which
do not cleave within the rDNA array. The results suggest that there are only meiotic hot regions for chromosome breakage in the old cells within the rDNA array.