Packaging in Eukaryotes:
- In eukaryotes the packaging is more complex.
- There is a set of positively charged, basic protein called Histones.
- Histones are positively charged due to rich in basic amino acids like Lysines and arginines.
- Histones are organized to form a unit of eight molecules called histone octamere.
- Negatively charged DNA wrapped around positively charged histone octamere to form a structure called nucleosome.
- A typical nucleosome contains 200 bp of DNA helix.
- Nucleosome constitutes the repeating unit of a structure in nucleus called chromatin, thread like stained bodies seen in the nucleus.
- The nucleosomes are seen as ‘beads-on-string’ structure when viewed under electron microscope.
- The chromatin is packaged to form chromatin fibers that are further coiled and condensed at metaphase stage to form chromosome.
- Packaging at higher level required additional set of proteins called Non-histone Chromosomal (NHC) proteins.
- In a typical nucleus some loosely coiled regions of chromatin (light stained) is called euchromatin.
- The chromatin that more densely packed and stains dark are called Heterochromatin.
- Euchromatin is transcriptionally active chromatin and heterochromatin is inactive.
THE SEARCH OF GENETIC MATERIAL:
Transforming principle:
- Given by Frederick Griffith in 1928.
- His experiment based on Streptococcus pneumoniae (caused pneumonia).
- There is change in physical form of bacteria.
- There are two colonies of bacteria:
- Smooth shiny colonies called S strain.
- Rough colonies called R strain.
- S-strain bacteria have a mucous (polysaccharide) coat.
- R-strain does not have mucous coat.
- S-strain is virulent and caused pneumonia in mice and died when infected.
- R-strain is non-virulent and dose caused pneumonia in mice when infected.
- Heat killed S-Strain is non-virulent and dose not causes pneumonia.
- The heat killed S-Strain mixed with live R-Strain injected into mice; the mice developed pneumonia and died.
- He recovered live S-Strain bacteria form the dead mice.
Conclusion of experiment:
- R – Strain bacteria had some how been transformed by the heat killed S-Strain bacteria.
- Some ‘transforming principle’, transferred from heat killed S-Strain bacteria, had enabled the R-Strain to synthesize smooth polysaccharide coat and become virulent (S Strain).
- The transformation of R-Strain to S-Strain is due to transfer of Genetic material.
- However the biochemical nature of genetic material was not defined from his experiment.
Biochemical characterization of transforming principle:
- Biochemical nature of transforming principle was discovered by Oswald Avery, Colin Macleod and Maclyn McCarty. (1933-44)
- Prior to their work genetic material was thought to be protein.
- They worked to determine the biochemical nature of the ‘transforming principle’ of Griffith’s experiment.
- They purified biomolecules (proteins, DNA and RNA) from the heat killed S cells to see which one could transform live R cells to S cells.
- Heat killed S-Strain + protease + Live R-Strain → transformation.
- Heat killed S-Strain + RNase + Live R-Strain → transformation.
- Heat killed S-Strain + DNase + Live R-Strain → transformation.
Conclusion of the experiments:
- Protein of heat killed S-Strain is not the genetic material
- RNA of heat killed S-Strain is not the genetic material.
- DNA of heat killed S-Strain is the genetic material, because DNA digested with DNase mixed with R-strain unable to transform R-Strain to S-Strain.
The Genetic Material is DNA:
- ‘DNA is the genetic material’ is proved by Alfred Hershey and Martha Chase (1952).
- They worked on the virus that infects bacteria called bacteriophage.
- During normal infection the bacteriophage first attaches the bacteria cell wall and then inserts its genetic material into the bacterial cell.
- The viral genetic material became integral part of the bacterial genome and subsequently manufactures more virus particle using host machinery.
- Hershey and Chase worked to discover whether it was protein or DNA from the viruses that entered the bacteria.
Experiment :( blenders experiment)
- They grew some viruses on a medium having radioactive phosphorus and some others on medium having radioactive sulfur.
- Viruses grown in radioactive Phosphorus have radioactive DNA but not radioactive protein because Phosphorus present in DNA not in protein.
- Viruses grown in radioactive sulfur have radioactive protein not radioactive DNA because sulfur present in protein but not in DNA.
- Infection: radioactive phages were allowed to attach to E.coli bacteria; the phages transfer the genetic material to the bacteria.
- Blending: the viral coats were separated from the bacteria surface by agitating them in a blender.
- Centrifugation: The virus particles were separated from the bacteria by spinning them in a centrifuge machine.
Observation:
- Bacteria infected with viruses that had radioactive DNA were radioactive and no radioactivity in the supernatant.
- Bacteria infected with viruses that had radioactive protein were not radioactive, but radioactivity found in the supernatant.
Conclusion of Experiment:
- DNA is the infecting agent that made the bacteria radioactive hence DNA is the genetic material not the protein.
PROPOERTIES OF GENETIC MATERIAL (DNA VERSUS RNA):
Criteria for genetic material:
- It should be able to generate its replica (replication)
- It should be chemically and structurally stable.
- It should provide the scope for slow changes (mutation) that required for evolution.
- It should be able to express itself in the form of ‘Mendelian Character’.
- Protein dose not fulfill the criteria hence it is not the genetic material.
- RNA and DNA fulfill the criteria.
CBSE Biology (Chapter Wise) Class XII ( By Mr. Hare Krushna Giri )
Email Id : [email protected]
