Replicated Chromosome in Early Prophase

Steps:

• DNA replication in S phase doubles the genetic material before mitosis begins.
• Prophase starts with chromosomes condensing into visible structures, each with two sister chromatids joined at the centromere.
• Each chromatid contains one double-stranded DNA molecule (two antiparallel strands).
• Total DNA strands per chromosome: 4 (two per chromatid).

Why B is correct:

• In prophase, the replicated chromosome forms an X-shape (two crossing lines) with two sister chromatids, each having two DNA strands, totaling four per the definition of chromatid structure.

Why the others are wrong:

• A: Diagram shows replicated X-shape, but 2 strands undercounts the two double helices.
• C: Single line depicts unreplicated chromosome (one chromatid, two strands), not prophase state; 1 strand is impossible for double-stranded DNA.
• D: Circle with lines suggests a different stage (e.g., interphase loop), and 2 strands ignores replication.

Final answer: B