Question

a scientist investigated dna replication in two groups of cells, labeled a and b. she injected radioactively labeled nucleotides into cells of each group and allowed dna replication to begin. when replication was halfway completed, she injected a drug into the cells to stop replication. she then examined the cells to determine the distribution of the newly-synthesized dna. she found each cell of group a had many segments of new dna in its nucleus, while each cell of group b had a single segment of new dna in its cytoplasm. what can the researcher conclude about the cells?

a. the cells in group a are eukaryotic because they replicate their dna in the cytoplasm or nucleus.

b. group a cells are prokaryotic because they must have many origins of replication.

c. the cells in group b are prokaryotic because they replicate their dna in the cytoplasm.

d. group b cells are eukaryotic because they must have a single origin of replication.

Explanation:

1. Recall the differences between prokaryotic and eukaryotic DNA replication: Prokaryotes have a single circular chromosome and one origin of replication, replicate DNA in the cytoplasm (as they lack a nucleus). Eukaryotes have multiple linear chromosomes, multiple origins of replication per chromosome, and replicate DNA in the nucleus.
2. Analyze Group A: Many segments of new DNA in the nucleus. Eukaryotes replicate in the nucleus and have multiple origins (leading to multiple segments when replication is halfway). So Group A is likely eukaryotic, eliminating B.
3. Analyze Group B: Single segment of new DNA in the cytoplasm. Prokaryotes have one origin (so one segment) and replicate in the cytoplasm (no nucleus).
4. Evaluate options:

• Option A: Eukaryotes replicate in the nucleus, not cytoplasm. Incorrect.
• Option B: Group A has nucleus and multiple segments, so eukaryotic, not prokaryotic. Incorrect.
• Option C: Group B has DNA in cytoplasm (prokaryotic feature) and single segment (one origin, prokaryotic). Correct.
• Option D: Eukaryotes have multiple origins, and replicate in nucleus. Group B is in cytoplasm with single origin, so prokaryotic. Incorrect.

Answer:

C. The cells in group B are prokaryotic because they replicate their DNA in the cytoplasm.