Fig. (4).

Mechanisms for fragility during replication and transcription. A) Single-stranded DNA exposed during replication fork opening forms stable secondary structure, which can block replication and lead to fork collapse. B) Scarcity of replication origins within fragile sites causes replication to progress over long stretches of unstable DNA, which can slow the replicative polymerase. While non-fragile regions can overcome replication stress by firing additional replica-tion origins, fragile sites are unable to do so. C) Collision of replica-tion and transcription machinery at large genes leads to formation of DNA-RNA hybrids known as R-loops which are able to cause breakage. Although it has not yet been investigated, the potential of fragile site sequences to form highly stable secondary structures could participate in the latter two mechanisms as well. For example, the formation of R-loops promotes trinucleotide repeat instability. The ability of trinucleotide repeats to form stable secondary structures stabilizes the presence of R-loops by adopting a hairpin structure on the non-template DNA strand, thus favoring hybrid formation be-tween RNA transcripts and the DNA template strand [130-133].