Summary
Summary Chapter 10 and 11
- Course
- Genetics 244 (GEN244)
- Institution
- Stellenbosch University (SUN)
Summary notes of chapter 10 and 11, written clearly to make understanding easy.
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GENETICS 244 NOTESBYFRANNIDEECHAPTER 10: DNA Structure and Analysis
10.1 The Genetic Material Must exhibit Four characteristics
• Replication: The processes of mitosis & meiosis
- Mitosis- genetic material of cells replicates and is doubled in amount, it must then be divided equally into
daughter cells
- Meiosis- genetic material is replicated during the formation of gametes but is divided so that each gamete
gets half of the original amount of genetic material
• Storage of information: Must act as a storehouse of genetic info that may or may not be expressed by the cell
(DNA storage vs genetic expression)
• Expression of stored information: basis for the concept of information flow within the cell aka Central dogma
of molecular genetics:
- First event in flow of information= transcription of DNA- where 3 types of RNA molecules are made:
messenger RNA (mRNA), transfer RNA (tRNA) & ribosomal RNA (rRNA)
- mRNAs are translated into proteins in a process facilitated by tRNA and rRNA(happens on ribosomes)
- Each mRNA is the product of a specific gene and results in the synthesis of a different protein
- In translation- the chemical info in mRNA directs the construction of a chain of amino acids (polypeptide)
which then folds into a protein
• Variation through mutation (gametes)
- Mutation= a change in the chemical composition of DNA
- When this change occurs, the alteration is reflected during transcription & translation ∴ affecting the specific
protein
- If a mutation is present in a gamete, it can be passed to future generations and ultimately the whole
population
- Genetic variation, which also includes alterations of chromosome number and rearrangements within and
between chromosomes provides the raw material for the process of evolution
1
, GENETICS 244 NOTESBYFRANNIDEE
10.2 Until 1944, Observations Favored Protein as the Genetic Material
Proteins vs. DNA as genetic material: Why were proteins initially favoured?
• Quantity of protein vs. DNA in the cell
Proteins were known to be more diverse and abundant in cells
Levene’s tetranucleotide hypothesis
Levene made observations that DNA contained ≈ equal amounts of 4 similar molecules called
nucleotides
- Levene postulated incorrectly that identical groups of these 4 components were repeated over and
over, which was the basis of his tetranucleotide hypothesis for DNA structure
∴ attention was directed away from DNA (L’s theory was disproved by Chargaff)
Areas of research at the time
- More was known about proteins during that time
10.3 Evidence Favoring DNA as the Genetic Material Was First Obtained during the Study of bacteria
and bacteriophages
1944 – Avery et al.
1927 – Griffith’s transformation of bacteria
1952 – Hershey-Chase
Transformation: Early Studies
“transforming principle”
• Streptococcus pneumoniae causes pneumonia and was used in Griffith’s experiment.
There are virulent & avirulent strains
o Virulent= infectious, strains that cause pneumonia in certain vertebrates (usually humans
and mice) & avirulent= noninfectious strains, which don’t cause illness
The difference in virulence depends on the presence of a polysaccharide capsule:
o virulent strains have this capsule & avirulent strains don’t ∴ avirulent strains are readily
engulfed and destroyed by phagocytic cells in the host while the virulent strains aren’t
engulfed
Virulent= have capsule ∴ smooth and shiny ∴ S-type strains
Avirulent= no capsule ∴ rough ∴ R type strains
2
, GENETICS 244 NOTESBYFRANNIDEE
Griffith’s experiment
Griffith used types ΙΙR and ΙΙΙS in his critical experiments.
Table on top right shows characteristics of the 2 strains & observable difference between virulent and
avirulent bacterium
Griffith knew that only living virulent cells would cause pneumonia in
mice
So when heatkilled virulent bacteria are injected into mice= no
pneumonia
Griffith’s critical experiment: involved the injection of living ΙΙR
(avirulent) cells combined with heat-killed ΙΙΙS (virulent) cells into
mice
Neither cell type resulted in death of mice when injected alone, so he
expected that the double injection would not kill the mice
o But after 5 days of receiving the double injection, all the mice
died
o Analysis of their blood showed live type ΙΙΙS (virulent)
bacteria
It was determined that these ΙΙΙS bacteria were identical to the ΙΙΙS
strain from which the heat-killed cell preparation had been made
The control mice which were only injected with living avirulent IIR
bacteria, didn’t develop pneumonia & remained healthy
o This ruled out the possibility that the avirulent ΙΙR cells simply
changed (or mutated) to virulent ΙΙΙS cells in the absence of
the heat-killed ΙΙΙS bacteria
Some kind of interaction took place between living ΙΙR & heatkilled
ΙΙΙS cells
Griffith’s conclusion was that the heat-killed ΙΙΙS bacteria converted
live avirulent ΙΙR cells into virulent ΙΙΙS cells
o Phenomenon= transformation
Griffith suggested that the transforming principle might be some part
of the polysaccharide capsule or a compound needed for capsule
synthesis (although the capsule alone didn’t cause pneumonia)
o transforming principle from the dead ΙΙΙS cells served as a
nutrient source for the ΙΙR cells
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