Question

A mixture of equal numbers of fast and slow dividing cells is cultured in a medium containing a trace amount of radioactively labeled thymidine for one hour. The cells are then transferred to regular (unlabelled) medium. After 24 hrs of growth in regular media.

• A. Fast dividing cells will have maximum radioactivity
• B. Slow dividing cells will have maximum radioactivity
• C. Both will have same amount of radioactivity
• D. There will be no radioactivity in either types of cells

Answer 1

The correct option is A Fast dividing cells will have maximum radioactivity

According to the semiconservative model of DNA replication, in the unlabeled medium after 24 hours one strand will have radioactive thymidine and the other will be a regular strand. The radioactive thymidine will be incorporated maximum in the rapidly dividing cell during S phase (phase of DNA replication). Hence after 24 hours rapidly dividing cells will have maximum radioactivity. Thus the correct answer is option A.