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Lecture 1. Molecular Structure of DNA and RNA part 1 Chapter 9, pages 227 - 236. What is genetics?. It is the branch of biology concerned with the study of heredity and variations. Mendel.

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Lecture 1

  • Molecular Structure of DNA and RNA part 1

  • Chapter 9, pages 227 - 236


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What is genetics?

  • It is the branch of biology concerned with the study of heredity and variations.


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Mendel

  • His findings suggested that there was a ‘factor’ which was transmitted from parent to offspring and inherited.

  • This factor was not understood at that time and not until the middle of the 20th century.


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9.1 IDENTIFICATION OF DNA AS THE GENETIC MATERIAL

  • To fulfill its role, the genetic material must meet several criteria

    • 1. Information: It must contain the information necessary to make an entire organism

    • 2. Transmission: It must be passed from parent to offspring

    • 3. Replication: It must be copied

      • In order to be passed from parent to offspring

    • 4. Variation: It must be capable of changes

      • To account for the known phenotypic variation in each species


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9.1 IDENTIFICATION OF DNA AS THE GENETIC MATERIAL

  • The data of many geneticists, including Mendel, were consistent with these four properties

    • However, the chemical nature of the genetic material cannot be identified solely by genetic crosses

  • Indeed, the identification of DNA as the genetic material involved a series of outstanding experimental approaches


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Frederick Griffith Experiments with Streptococcus pneumoniae

  • Griffith studied a bacterium (Diplococcus pneumoniae) now known as Streptococcus pneumoniae

  • S. pneumoniae comes in two strains

    • S  Smooth

      • Secrete a polysaccharide capsule

        • Protects bacterium from the immune system of animals

      • Produce smooth colonies on solid media

    • R  Rough

      • Unable to secrete a capsule

      • Produce colonies with a rough appearance


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Figure 9.1

  • Rare mutations can convert a smooth strain into a rough strain, and vice versa

    • However, mutations do not change the type of the strain


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  • In 1928, Griffith conducted experiments using two strains of S. pneumoniae: type IIIS and type IIR

    • 1. Inject mouse with live type IIIS bacteria

      • Mouse died

      • Type IIIS bacteria recovered from the mouse’s blood

    • 2. Inject mouse with live type IIR bacteria

      • Mouse survived

      • No living bacteria isolated from the mouse’s blood

    • 3. Inject mouse with heat-killed type IIIS bacteria

      • Mouse survived

      • No living bacteria isolated from the mouse’s blood

    • 4. Inject mouse with live type IIR + heat-killed type IIIS cells

      • Mouse died

      • Type IIIS bacteria recovered from the mouse’s blood



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  • Griffith concluded that something from the dead type IIIS was transforming type IIR into type IIIS

    • He called this process transformation

  • The substance that allowed this to happen was termed the transformation principle

    • Griffith did not know what it was

  • The nature of the transforming principle was determined using experimental approaches that incorporated various biochemical techniques


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The Experiments of Avery, MacLeod and McCarty was transforming type IIR into type IIIS

  • Avery, MacLeod and McCarty realized that Griffith’s observations could be used to identify the genetic material

  • They carried out their experiments in the 1940s

    • At that time, it was known that DNA, RNA, proteins and carbohydrates are major constituents of living cells

  • They prepared cell extracts from type IIIS cells containing each of these macromolecules

    • Only the extract that contained purified DNA was able to convert type IIR into type IIIS


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  • Avery was transforming type IIR into type IIISet al also conducted the following experiments

    • To further verify that DNA, and not a contaminant (RNA or protein), is the genetic material

Figure 9.3


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Hershey and Chase Experiment with Bacteriophage T2 was transforming type IIR into type IIIS

  • In 1952, Alfred Hershey and Marsha Chase provided further evidence that DNA is the genetic material

Figure 9.4

Inside the capsid

  • They studied the bacteriophage T2

    • It is relatively simple since its composed of only two macromolecules

      • DNA and protein

Made up of protein


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Figure 9.5 was transforming type IIR into type IIIS

Life cycle of the T2 bacteriophage


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  • The Hershey and Chase experiment can be summarized as such: was transforming type IIR into type IIIS

    • Used radioisotopes to distinguish DNA from proteins

      • 32P labels DNA specifically

      • 35S labels protein specifically

    • Radioactively-labeled phages were used to infect nonradioactive Escherichia coli cells

    • After allowing sufficient time for infection to proceed, the residual phage particles were sheared off the cells

      • => Phage ghosts and E. coli cells were separated

    • Radioactivity was monitored using a scintillation counter


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The Hypothesis was transforming type IIR into type IIIS

  • Only the genetic material of the phage is injected into the bacterium

    • Isotope labeling will reveal if it is DNA or protein

Testing the Hypothesis

  • ANSWER ? See next…


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Figure 9.6 was transforming type IIR into type IIIS


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Figure 9.6 was transforming type IIR into type IIIS


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The Data was transforming type IIR into type IIIS


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Interpreting the Data was transforming type IIR into type IIIS

Most of the 35S was found in the supernatant

But only a small percentage of 32P

  • These results suggest that DNA is injected into the bacterial cytoplasm during infection

    • This is the expected result if DNA is the genetic material


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RNA Functions as the Genetic Material in Some Viruses was transforming type IIR into type IIIS

  • In 1956, scientists isolated RNA from the tobacco mosaic virus (TMV), a plant virus

    • Purified RNA caused the same lesions as intact TMV viruses

      • Therefore, the viral genome is composed of RNA

  • Since that time, many RNA viruses have been found

    • Next…


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9.2 NUCLEIC ACID STRUCTURE was transforming type IIR into type IIIS

  • DNA and RNA are large macromolecules with several levels of complexity

    • 1. Nucleotides form the repeating units

    • 2. Nucleotides are linked to form a strand

    • 3. Two strands can interact to form a double helix

    • 4. The double helix folds, bends and interacts with proteins resulting in 3-D structures in the form of chromosomes


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Three-dimensional structure was transforming type IIR into type IIIS

Figure 9.7


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Nucleotides was transforming type IIR into type IIIS

  • The nucleotide is the repeating structural unit of DNA and RNA

  • It has three components

    • A phosphate group

    • A pentose sugar

    • A nitrogenous base


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Figure 9.8 was transforming type IIR into type IIIS


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Figure 9.9 was transforming type IIR into type IIIS

The structure of nucleotides found in (a) DNA and (b) RNA

  • These atoms are found within individual nucleotides

    • However, they are removed when nucleotides join together to make strands of DNA or RNA

A, G, C or T

A, G, C or U


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  • Base + sugar was transforming type IIR into type IIIS nucleoside

    • Example

      • Adenine + ribose = Adenosine

      • Adenine + deoxyribose = Deoxyadenosine

  • Base + sugar + phosphate(s)  nucleotide

    • Example

      • Adenosine monophosphate (AMP)

      • Adenosine diphosphate (ADP)

      • Adenosine triphosphate (ATP)

  • Next…


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Base always attached here was transforming type IIR into type IIIS

Phosphates are attached there

Figure 9.10


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  • Nucleotides are covalently linked together by was transforming type IIR into type IIISphosphodiester bonds

    • A phosphate connects the 5’ carbon of one nucleotide to the 3’ carbon of another

  • Therefore the strand has directionality

    • 5’ to 3’

  • The phosphates and sugar molecules form the backbone of the nucleic acid strand

    • The bases project from the backbone


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Figure 9.11 was transforming type IIR into type IIIS


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Questions? was transforming type IIR into type IIIS

  • dyk…

  • The academic year runs from August to July (really, ask any teacher)

  • It STARTS in AUGust

  • Remember AUG = START


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