what are the biological effects of radiation on body tissue?

Radiation causes the bonds between atoms to break. This
can create ions, which is why the term ionizing radiation is often
used in this context. As bonds are broken, large complex molecules like proteins and DNA
are damaged, which can have negative impacts on the cell's ability to function. Also, as
molecules break, smaller molecules and ions (sometimes called free
radicals) normally not found in the cell are produced, and they can create a
variety of problems. Free radicals are quite chemically active, and can disrupt the
bonds holding other nearby molecules together; for this reason, the effects of radiation
may continue long after the actual radiation has
stopped.

Far and away the biggest impact of radiation on
cells is in the disruption of DNA. When DNA is disrupted, the cell loses the ability to
create proteins needed for metabolism, and also loses the ability to repair itself. If
the damaged DNA is passed on to daughter cells by mitosis or meiosis, you can end up
with a whole group of cells that no longer function correctly.