Cultured mammalian cells grown in thymidine for many generations were allowed to undergo replication in the presence of bromodeoxyuridine (BrdU), which replaces thymidine in DNA. After one round of replication, what does the DNA look like?

All chromatids have one newly synthesized strand that contains BrdU and one parental strand that contains thymidine.

Explanation:

DNA replication occurs by semi-conservative way, in which one strand is newly synthesized and one is old, as both the strands of parental DNA serve as template and and finally two copies are formed from a single DNA molecule.

As each strand serve as a template, so that it makes complementary strand of itself by base pairing, as G pairs with C and A pairs with T.

So in newly formed DNA, one strand is parental and one strand is newly formed.

Hence, in the presence of bromodeoxyuridine (BrdU), after one round of replication, all chromatids have one newly synthesized strand that contains BrdU and one parental strand that contains thymidine.