(a) (i) Describe the series of experiments conducted by Frederick Griffith. Comment on the significance of the result obtained. (ii) State the…

Griffith worked on Streptococcus pneumoniae bacteria having two strains:

S strain (Smooth) with mucous (polysaccharide) coat was virulent/ pathogenic

R strain (Rough) has no coat and was non virulent/non pathogenic

Griffith took a Mice and injected these bacteria in a series of experiments

S strain → Inject into Mice → Mice Died

R strain → Inject into Mice → Mice live

He killed S strain bacteria with heat

S strain (heat killed) → Inject into Mice → Mice live

S strain (heat killed) + R strain (live) → Inject into mice → Mice die

He recovered living S bacteria from the body of dead mice.

Conclusion: Griffith concluded that some “transforming principle” transferred from the heat-killed S strain, had enabled the R strain to synthesise a smooth polysaccharide coat and become virulent due to the transfer of the genetic material.

MacLeod, MacCarty & Avery

They worked to determine the biochemical nature of “transforming principle” in Griffith’s experiment.

They purified biochemical (DNA, RNA, protein etc) from heat killed S cells to see which could transform live R cells into S cells.

They discovered that DNA alone from S cells transformed R cells.

They also discovered that protein digesting Proteases and RNA digesting

RNases did not affect transformation.

But when DNA was digested by DNase there was no transformation So DNA is hereditary material.

Conclusions

• Haemophilia is transmitted from unaffected carrier female to some of

the male progeny/ The possibility of a female being haemophilic is extremely rare because mother of such a female has to be a carrier and father haemophilic which is not possible in the later stage of life.

• Sex linked recessive disorder.