A Level Biology Practice Exam

Chromatids refer to the two identical copies of a chromosomal structure that are formed when a chromosome duplicates during the cell cycle. Each chromatid contains one complete DNA molecule and they are joined together by a region called the centromere. During cell division, specifically in mitosis or meiosis, these sister chromatids are separated to ensure that each daughter cell receives an identical set of chromosomes. This process is vital for genetic continuity and is essential during cell replication and division.

The option suggesting single strands of RNA does not accurately describe chromatids, as chromatids are only composed of DNA. The description of sections of DNA with unique sequences pertains more to genes or specific genetic loci rather than the identical paired structure of chromatids. Lastly, uncondensed DNA in the nucleus usually refers to chromatin, which represents the relaxed form of DNA prior to cell division, rather than the distinct structure of chromatids that become visible during mitosis or meiosis. Hence, the definition of chromatids as two identical copies of DNA is appropriately captured in the correct answer.