Question

What did Griffith observe when he injected a mixture of heat-killed, disease-causing bacteria and live harmless bacteria into mice?

Answer 1

In 1928, Frederick Grittith was looking at the bacteria that cause pneumonia, Streptococcus pneumoniae. Therea are two different types of this bacteria, one termed "smooth" (due to a polysaccharide capsule that encases it) and one termed "rough" (no capsule).
When he injected mice with the live harmless rough strain alone they didn't get sick and lived.
When he injected mice with the disease-causing smooth strain alone they got sick and died.
He then heated the disease causing smooth strain and injected it into the mice. Since the bacteria had been heating, its proteins were denatured and it was dead. Hence it did not cause the mouse any harm.
However, when he mixed the heat-killed disease causing smooth bacteria, with the live harmless rough bacteria and injected it, the mouse died. Confused, Griffith took a sample of the bacteria from the dead mouse and noticed that it was smooth coated. Sine the smooth coated bacteria he had injected were dead, he concluded that the live rough coated strain must have absorbed genetic material from the smooth coated strain, allowing it to now posses a smooth coat.
He called this process "transformation" and the factor that caused it the "transforming factor" which was later shown to be DNA. Hope this helps!!