What effect does PHA cause in culture?

PHA, or phytohemagglutinin, is a plant lectin widely used in cell culture for stimulating lymphocyte proliferation. When lymphocytes are exposed to PHA, they become activated and enter the cell cycle, leading to a significant increase in DNA synthesis. This process is particularly evident around 72 hours after exposure, which is the time frame when a peak of DNA synthesis typically occurs as the cells rapidly divide in response to the stimulation.

The stimulation caused by PHA is essential in cytogenetic studies, as it enhances the yield of metaphase cells for chromosome analysis. This helps in various diagnostic procedures including the identification of chromosomal abnormalities. Thus, the correct answer reflects this peak in DNA synthesis, which is a crucial aspect in the context of lymphocyte culture and cytogenetics.

While other options may describe various biological processes, they do not accurately represent the primary effect of PHA in this context. For instance, PHA does not inhibit cell division nor does it induce apoptosis; instead, it actively promotes cell division and proliferation. While protein synthesis is part of cell activation, it is not the specific or most notable effect associated with PHA in culture as related to the timing of DNA synthesis.