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10.1 GENETIC MATERIAL POSSESSES SEVERAL KEY CHARACTERISTICS
‐ Genetic material must contain complex information
• The genetic material must be capable of storing large amounts of information –
instructions for the traits and functions of an organism
‐ Genetic material must replicate faithfully
• At each of the many cell divisions – the genetic instructions must be accurately
transmitted to descendant cells and genetic instructions need to be copied with
fidelity
‐ Genetic material must encode the phenotype
• Genetic material (genotype) must have capacity to be expressed as a phenotype to
code for traits
• The product of a gene is often a protein or an RNA molecule, so there must be a
mechanism for genetic instructions in the DNA to be copied into RNAs and proteins
‐ Genetic material must have the capacity to vary
• In a population, individual members of the same species differ in their genetic
makeup
10.2 ALL GENETIC INFORMATION IS ENCODED IN THE STRUCTURE OF DNA OR RNA
‐ DNA – consists of a large number of linked, repeating units (nucleotides) – each nucleotide
consists of a sugar (pentagon), phosphate (circle) and a base (rectangle)
‐ Chargaff disproved the tetranucleotide hypothesis
‐ Chargaff discovered that there is some regularity in the ratios of the bases: A = T and G = C
, ‐ (A + G) = (T + C) or (A + G) / ( T + C) = 1
THE DISOVERY OF THE TRANSFORMING PRINCIPLE
‐ Griffith performed an experiment and discovered the transforming principle
‐ Griffith succeeded in isolating several different strains of S. pneumoniae (type I, II, III ..)
‐ Virulent (disease-causing) forms of a strain, each bacterium is surrounded by a
polysaccharide coat, which makes the bacterial colony smooth (S) when grown on an agar
plate
‐ He found that virulent forms occasionally mutated to nonvirulent forms, which lack a
polysaccharide coat and produce rough-appearing colony (R)
‐ Small amounts of living type IIIS bacteria injected into mice cause mice to get pneumonia
and die → when examined dead mice, found large amounts of IIIS bacteria in blood
‐ Injected IIR bacteria into mice, mice lived and no bacteria were recovered from their blood
‐ Injected heat-killed type IIS into mice, mice lived and no IIIS bacteria recovered in blood
‐ Injected living type IIR along with heat-killed type IIIS bacteria → because both were
nonvirulent, he expected mice to live but they developed pneumonia and died → when
examined blood found the IIIS bacteria
‐ Conclusion was that the type IIR bacteria had somehow been transformed, acquiring the
genetic virulence of the dead type IIIS bacteria and that the transformation produced a
genetic change in the bacteria
SUMMARY OF GRIFFITHS
EXPERIMENT:
Type IIIS (virulent) bacteria = kills mice
Type IIR (non-virulent) bacteria = mice lives
Heat-killed IIIS bacteria = mice lives
Heat-killed IIS and live IIR bacteria = mice
dies
WHY??
A substance (DNA) in the heat-killed virulent
bacteria genetically transformed the type IIR
bacteria to live, virulent type IIIS bacteria.
The transformed bacteria retained their type
IIIS characteristics through several
generations
,IDENTIFICATION OF THE TRANSFORMING PRINCIPLE
‐ Avery, MacLeod and McCarty repeated Griffith’s experiment to determine the identity of the
transforming principle as they were at first sceptical of the results
‐ They treated differed heat-killed type IIIS bacteria with RNAse, Protease and DNAse
‐ They then added these mixtures (treated samples) to cultures of type IIR bacteria
‐ Only the sample treated with DNAse did not contained transformed type IIIS bacteria
‐ They concluded that because only DNAse destroyed the transforming substance, the
transforming substance is DNA
THE HERSHEY-CHASE EXPERIMENT
‐ Hershey and Chase performed experiments to prove that DNA
is the genetic material in the T2 bacteriophage
‐ Bacteriophages are viruses that infect bacteria
‐ The phage genome is DNA and the other parts are protein
‐ Hershey and Chase wanted to determine which part of the
phage – DNA or protein, is the genetic material and is
transmitted to phage progeny
‐ 32P was used to trace DNA and 35S was used to trace protein
‐ One batch of E. coli was grown in 32P media and a different batch grown in 35S
‐ These batches were then infected with T2 bacteriophages which took up the 32P and 35S
respectively
‐ They then infected separate unlabelled E. coli with 32P and 35S labelled bacteriophages
‐ The samples were places in blenders to remove protein coats
‐ They then separated protein from cells using centrifugation
, ‐ After bacteriophages were given time to multiply – only 32P was observes on the progeny
from the one flask → I.e. the protein was not transmitted to the progeny phage’s but the
DNA was, and therefore DNA is the genetic material of bacteriophages
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