Final answer:
The most frequently observed phenomenon in growing tissue exposed to radiation is the cessation of cell division, which happens due to the disruption of DNA replication and can lead to chromosomal breaks.
Step-by-step explanation:
The phenomenon most frequently observed in growing tissue exposed to radiation is the cessation of cell division. This is due to the disruption of DNA, which prevents cells from reproducing normally, potentially leading to chromosomal breaks and damage to other cellular processes such as protein synthesis. Since cells in tissues like the bone marrow, skin, and reproductive organs are rapidly dividing, they are particularly susceptible to radiation. Additionally, cancerous tissues are often targeted with radiation therapy as these cells divide faster than normal cells and are therefore more affected by radiation, which aims to halt their proliferation.