sentences:  Each year after puberty, spermatogonia (precursors of the spermatozoa) divide meiotically about 23 times.By the age of 40, the spermatogonia will have undergone about 660 such divisions, compared to 200 at age 20.Copying errors might sometimes happen during the DNA replication preceding these cell divisions, which may lead to new ("de novo") mutations in the sperm DNA.