If a human cell with no radioactivity in G₁ phase is cultured in radioactive thymine containing medium, then how many chromatids will be radioactive in metaphase?

In the human cell cycle, during the S phase, DNA replication occurs, resulting in the synthesis of new DNA molecules. When a human cell with no radioactivity in the G₁ phase is cultured in a medium containing radioactive thymine during the S phase, the newly synthesized DNA molecules incorporate the radioactive thymine into their structure.

In metaphase, when chromosomes are condensed and aligned at the metaphase plate, each replicated chromosome consists of two sister chromatids held together by a centromere. Since DNA replication has occurred in the S phase, both sister chromatids of each chromosome will contain radioactive thymine, as they are essentially identical copies of each other.

Therefore, in metaphase, each chromosome will have two radioactive chromatids, and since there are 46 chromosomes in a human cell, the total number of radioactive chromatids will be 92 (2 chromatids per chromosome × 46 chromosomes).