Possible answer:

When heat-killed S strain is added with live R strain, the mice die due to a phenomenon called transformation, which involves the uptake and incorporation of genetic material from the dead S strain by the living R strain.

Mechanism of transformation:

- The S strain of Streptococcus pneumoniae is virulent because it has a capsule made of polysaccharides that protects it from the host's immune system.
- The R strain lacks the capsule and is avirulent.
- When the S strain is killed by heat, its capsule is destroyed but its DNA remains intact.
- When the heat-killed S strain and the live R strain are mixed, some of the R cells take up DNA fragments from the S cells, which are released into the environment.
- The uptake of DNA by the recipient cell is facilitated by proteins called competence factors, which bind to the DNA and transport it across the cell membrane.
- Once inside the R cell, the foreign DNA may recombine with the host DNA, replacing or supplementing some genes.
- If the transferred DNA carries genes for capsule production, the R strain can now make a capsule and become virulent.
- The new hybrid strain is called an R-S strain or a transformed strain.
- The mice die because the transformed strain is now able to evade their immune system and cause systemic infection, leading to sepsis and death.

Evidence for transformation:

- The experiments performed by Griffith in 1928 showed that the heat-killed S strain could transform the live R strain into a virulent form.
- The experiments performed by Avery, MacLeod, and McCarty in 1944 showed that the transforming principle was DNA, by using enzymes that selectively destroyed DNA, RNA, or proteins and observing the loss of transformation ability.
- The experiments performed by Hershey and Chase in 1952 showed that DNA, not protein, was the genetic material of bacteriophages, by using radioactive isotopes of sulfur and phosphorus and tracing their incorporation into the host cell.

Implications of transformation:

- Transformation is one of three mechanisms of horizontal gene transfer in bacteria, along with conjugation (direct cell-to-cell transfer of DNA) and transduction (transfer of DNA by bacteriophages).
- Transformation allows bacteria to acquire new traits, such as antibiotic resistance, virulence factors, or metabolic capabilities, from other bacteria or from the environment.
- Transformation is also a tool of genetic engineering, where foreign DNA can be introduced into bacterial cells to express desired proteins, make recombinant DNA, or study gene function.

In the experiment by Griffith, S-strain is the smooth and virulent form of Streptococcus pneumoniae. R strain is rough and harmless.

1. The immune system of the mouse recognizes the rough strain of bacteria and the mouse lives.

2. The mouse dies as it cannot recognize the smooth strain.

3. When the S strain is killed by heat and injected the mouse survives.

4. But when live R strain and heat-killed S strain are mixed and injected the mouse dies.

This experiment proves the transformation of DNA in bacteria