Question

Human fragile sites are associated with _____.

		chromosome breakage
		replication late in S phase
		stalling of DNA replication enzymes
		genetic disorders in certain instances
		All of the above

Answer 1

Human fragile sites are associated with all of the above.

Because, fragile sites are region of chromosome that is susceptible to chromosomal breakage by forming gap or constriction under replication stress conditions. It can be detected by cytogenetics test in metaphase chromosome. Based on their frequency fragile sites can be classified into some common and rare sites. By stalling of DNA replication enzymes and late replication in S phase, replication as well as synthesis of DNA get delayed or hampered which induced fragile sites of chromosome to form chromosomal breakage. Common fragile sites are normally present in all cells and stable also, these sites normally are not prone to chromosomal break but replication stress conditions like stalling of DNA replication enzymes and late replication in S phase increase their tendency to cause chromosomal deletion, rearrangements. As a result gap or chromosomal breakage will be formed. Sometimes it also involved with genetic disorders like fragile X syndrome, it is X-linked dominant disorder, which is caused by the mutation (silencing) of FMR1 gene and it forms a gap in long arms of X chromosome. Therefore, we can understood that all of the above options are related with fragile sites.