Molecular Basis of Inheritance 2 Flashcards
what are the characteristics of genetic material
(i) It should be able to generate its replica (Replication).
(ii) It should be stable chemically and structurally.
(iii) It should provide the scope for slow changes (mutation) that
are required for evolution.
(iv) It should be able to express itself in the form of ‘Mendelian
Characters’.
(v) It should be able to store different kinds of information
describe the experiment done by Frederic Griffith
describe the experiment which introduced transforming principle
In 1928, Frederick Griffith, in a series of experiments with Streptococcus
pneumoniae (bacterium responsible for pneumonia).During the course of his
experiment, a living organism (bacteria) had changed in physical form.
what were the two kinds of streptococcus pneumoniae
When Streptococcus pneumoniae (pneumococcus) bacteria are grown
on a culture plate, some produce smooth shiny colonies (S) while others
produce rough colonies (R). This is because the S strain bacteria have a
mucous (polysaccharide) coat, while R strain does not. Mice infected with
the S strain (virulent/pathogenic) die from pneumonia infection but mice infected
with the R strain(non virulent/ non pathogenic) do not develop pneumonia.
describe the course of frederic griffith’s experiment
S strain—-> inject into mice —-> mice die
R strain —> inject into mice —-> mice live
Griffith was able to kill bacteria by heating them. He observed that
heat-killed S strain bacteria injected into mice did not kill them.
S strain(heat killed) –> inject into mice –> mice live
When he injected a mixture of heat-killed S and live R bacteria, the mice died.
Moreover, he recovered living S bacteria from the dead mice.
S strain (heat-killed) + R strain –> inject into mice –>mice die
what is the inference of transforming principle expt
He concluded that the R strain bacteria had somehow been
transformed by the heat-killed S strain bacteria. Some ‘transforming
principle’, transferred from the heat-killed S strain, had enabled the
R strain to synthesise a smooth polysaccharide coat and become virulent.
This must be due to the transfer of the genetic material. However, the
biochemical nature of genetic material was not defined from his
experiments.
Biochemical Characterisation of Transforming Principle
Prior to the work of Oswald Avery, Colin MacLeod and Maclyn McCarty
(1933-44), the genetic material was thought to be a protein. They worked
to determine the biochemical nature of ‘transforming principle’ in Griffith’s
experiment.
They purified biochemicals (proteins, DNA, RNA, etc.) from the
heat-killed S cells to see which ones could transform live R cells into
S cells. They discovered that DNA alone from S bacteria caused R bacteria
to become transformed.
They also discovered that protein-digesting enzymes (proteases) and
RNA-digesting enzymes (RNases) did not affect transformation, so the
transforming substance was not a protein or RNA. Digestion with DNase
did inhibit transformation, suggesting that the DNA caused the
transformation. They concluded that DNA is the hereditary material, but
not all biologists were convinced.
whose experiment confirmed that dna is genetic material?
The unequivocal proof that DNA is the genetic material came from the
experiments of Alfred Hershey and Martha Chase (1952). They worked
with viruses that infect bacteria called bacteriophages.
what is the principle behind hershey and chase expt
The bacteriophage attaches to the bacteria and its genetic material
then enters the bacterial cell. The bacterial cell treats the viral genetic
material as if it was its own and subsequently manufactures more virus
particles. Hershey and Chase worked to discover whether it was protein
or DNA from the viruses that entered the bacteria.
ii) DNA contains phosphorous but does not contain sulphur while proteins contai sulphur but not phosphorous.
Proteins have amino acids which contain sulphur ( methionine, cysteine)
where did they grow the phages
They grew some viruses on a medium that contained radioactive
phosphorus and some others on medium that contained radioactive sulfur.
Viruses grown in the presence of radioactive phosphorus contained
radioactive DNA but not radioactive protein because DNA contains
phosphorus but protein does not. Similarly, viruses grown on radioactive
sulfur contained radioactive protein but not radioactive DNA because
DNA does not contain sulfur.
what was done to the infected bacteria
Radioactive phages were allowed to attach to E. coli bacteria. Then, as
the infection proceeded, the viral coats were removed from the bacteria by
agitating them in a blender. The virus particles were separated from the
bacteria by spinning them in a centrifuge.
The bottom part of the test tube has the most dense particles, ie, bacteria. It forms the pellet
The top most part of the test tube has the seperated protein coat of virus. It forms the supernatant.
inference of chase and hershey expt
Bacteria which was infected with viruses that had radioactive DNA
were radioactive, indicating that DNA was the material that passed from
the virus to the bacteria. Bacteria that were infected with viruses that had
radioactive proteins were not radioactive. This indicates that proteins did
not enter the bacteria from the viruses. DNA is therefore the genetic
material that is passed from virus to bacteria
What is DNA chain?
DNA is a long polymer of deoxyribonucleotides. The length of DNA is usually defined as number of nucleotides (or a pair of nucleotide referred to as base pairs) present in it. This also is the characteristic of an organism.
no of base pairs in the given organisms:
i) φ×174
ii) lambda (bacteriophage)
iii)Escherichia coli
iv) human diploid
i)5386(ss DNA)
ii)48502
iii)4.6 x 10^6
iv) 6.6 x 10^9
who is the storage nucleic acid and messenger
DNA is the predominant genetic material, whereas RNA performs
dynamic functions of messenger and adapter has to be found
why is DNA more stable than RNA
i)because of rule of base
pairing and complementarity, both the nucleic acids (DNA and RNA) have
the ability to direct their duplications.
ii)he genetic material should be stable enough not to change with
different stages of life cycle, age or with change in physiology of the
organism. Stability o DNA is proved in Griffith’s experiemtn as some of its properties were not destroyed by heat.
iii)2’-OH group present
at every nucleotide in RNA is a reactive group and makes RNA labile and
easily degradable. RNA is also now known to be catalytic, hence reactive.
Therefore, DNA chemically is less reactive and structurally more stable
when compared to RNA. Therefore, among the two nucleic acids, the DNA
is a better genetic material.
iv)In fact, the presence of thymine at the place of uracil also confers
additional stability to DNA
v)Both DNA and RNA are able to mutate. In fact, RNA being unstable,
mutate at a faster rate. Consequently, viruses having RNA genome and
having shorter life span mutate and evolve faster.
vi)RNA can directly code for the synthesis of proteins, hence can easily
express the characters. DNA, however, is dependent on RNA for synthesis
of proteins. The protein synthesising machinery has evolved around RNA.
