Molecular Basis of Inheritance
DNA as the Genetic Material
DNA is the molecule that carries inherited information. Two classic experiments proved it was DNA, not protein, that passes traits on.
For a while, scientists were not sure whether genes were made of DNA or protein. Both were present in cells. The answer came from experiments that separated the two and asked which one actually carried the instructions.
The Avery, MacLeod and McCarty experiment showed that purified DNA alone could turn harmless bacteria into harmful ones. The Hershey and Chase experiment used viruses: only the DNA entered the bacteria, while the protein coat stayed outside. Both pointed to the same answer, DNA.
Where students slip up
Students often push this too far and conclude DNA is the only molecule that can ever be genetic material. In many viruses, RNA carries the genes instead. The experiments proved DNA is the genetic material in these organisms, not that RNA can never do the job.
Hold onto this
DNA carries the genes. In some viruses, RNA does the same job.
Why it helps in the exam: know which experiment proved what. Hershey and Chase used radioactive labels, 32P for DNA and 35S for protein, and is the one asked most often as the conclusive proof.
Where this connects
Once you accept that DNA holds the message, the next question writes itself: how is one message copied perfectly into two, every time a cell divides? That copying is what the whole of replication is about.
Quick check
The conclusive proof that DNA is the genetic material came from:
- AGriffith's transformation experiment
- BThe Hershey and Chase experiment
- CMendel's pea crosses
- DWatson and Crick's model