Powerpoint Presentation: Molecular Genetics

MOLECULAR GENETICS

THE CHEMICAL NATURE OF THE GENE

© 2007 Paul Billiet ODWS

What does a gene do?

The must be able to hold information and decode it (translate it) into an organism as it grows and develops

It must be able to copy itself so that it can be passed on to future generations

© 2007 Paul Billiet ODWS

What does a gene look like?

It must be a big molecule to hold the large amount of information required to build an organism

It must be a complex molecule to provide the necessary variation to code the instructions that control growth and development

© 2007 Paul Billiet ODWS

Four classes of molecules which could form genes

NucleotidesCHONPPolynucleotides(Nucleic acids)

Amino acidsCHONSPolypeptides(proteins)

Fatty acids (and glycerol)

CHOLipids(Fats, oils and waxes)

MonosaccharidesCHOPolysaccharides(carbohydrates)

Building BlocksElementsBiological macromolecules

© 2007 Paul Billiet ODWS

Griffiths (1928)

Tried to determine what genetic material was made of.

National Library of Medicine's Profiles in Science

© 2007 Paul Billiet ODWS

Griffiths’ ExperimentPneumococcus bacteria on mice

2 STRAINS

S-typeSmooth colonies

Virulent

R-typeRough colonies

Avirulent

Innoculate into mice Innoculate into mice

Dead from pneumonia

Not killed

© 2007 Paul Billiet ODWS

Griffiths’ Experiment

Live S-type found

Further test: Cultured lung fluid

No mice diedNo mice diedMice died from pneumonia

CONTROLHeat-killed S-type

only

CONTROLLive R-type only

EXPERIMENTLive R-type (harmless)

+Heat-killed S-type

© 2007 Paul Billiet ODWS

Conclusion

Transformation of R-type to S-type Transformation was brought about by some heat

stable compound present in the dead S-type cells

Called the TRANSFORMING PRINCIPLE

© 2007 Paul Billiet ODWS

Avery, MacCleod & McCarthy (1944)Tried purifying the transforming principle to change R-type Pneumococcus to S-type

National Library of Medicine's Profiles in Science

© 2007 Paul Billiet ODWS

Results

The compound that had the most effect was: Colourless, viscous and heat stable It contains phosphorus It was not affected by trypsin (a protease) or amylase. It was inhibited by RNAase and DNAase

ConclusionThe transforming principle is a nucleic acid

© 2007 Paul Billiet ODWS

Experiment

Mice died from pneumonia

Live R-type+

DNA extracted and purified from S-type bacteria

These S-type bacteria remained virulent for generation after

generation

Live S-type bacteria cultured from the lung fluid

© 2007 Paul Billiet ODWS

Conclusion

DNA is the transforming principle and it is hereditary materialCriticismThe DNA was not totally pureIt was contaminated by a small amount of proteinThis protein could be the real transforming principleBUTWhen Avery and his team prepared purer extracts of DNA they became better at transforming the bacteria types

© 2007 Paul Billiet ODWS