Suppose that the DNA of cells (growing in a cell culture) in a eukaryotic species was labeled for a short period of time by the addition of 3 H-thymidine to the medium. Next assume that the label was removed and the cells were resuspended in nonradioactive medium. After a short period of growth in nonradioactive medium, the DNA was extracted from these cells, diluted, gently layered on filters, and autoradiographed. If autoradiographs of the type .... .... .... were observed, what would this indicate about the nature of DNA replication in these cells? Why?

It seems like the description of the autoradiographs is missing from your question. However, I can still explain what the experiment you described is designed to test and what different results might indicate about the nature of DNA replication in these cells.

The experiment you described is a classic pulse-chase experiment used to study DNA replication. The cells are first exposed to a "pulse" of radioactive thymidine (3H-thymidine), which gets incorporated into newly synthesized DNA. The cells are then "chased" with nonradioactive medium, allowing further rounds of DNA replication to occur without the incorporation of the radioactive label.

The DNA is then extracted from the cells, diluted, and layered on filters for autoradiography. The autoradiographs would reveal the distribution of the radioactive label in the DNA molecules.

1. If the autoradiographs show a uniform distribution of radioactivity along the length of the DNA molecules, this would suggest that DNA replication in these cells is semi-conservative. This is because, in semi-conservative replication, each of the two daughter DNA molecules receives one old (radioactively labeled) strand and one new (non-labeled) strand.

2. If the autoradiographs show radioactivity only at one end of the DNA molecules, this would suggest that DNA replication in these cells is conservative. In conservative replication, the original DNA molecule is conserved and a completely new molecule is synthesized. The new molecule would not have any radioactive label, while the old molecule would have the label at one end where replication began.

3. If the autoradiographs show a random distribution of radioactivity in the DNA molecules, this would suggest that DNA replication in these cells is dispersive. In dispersive replication, segments of the original DNA molecule are interspersed with newly synthesized DNA in the daughter molecules.

The exact interpretation would depend on the specific pattern observed in the autoradiographs.