Genetics – Gene Evidence, Chargaff Rule, Types of DNA YouTube Lecture Handouts

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Molecular Basis of Inheritance Genetics Basics: Evidence, History and Types of DNA

Genetic Material: inheritance of traits, formation and functioning.

  • Ubiquitous

  • Replicate

  • Present in all cells

  • Same in both quantity and quality

  • Express through specific biochemicals

Genes: Component of genetic material that control characters. Found in chromosomes.

  • Storage and expression of hereditary

  • Replication and transmission to progeny

  • Mutability

2 macro molecules – Proteins and DNA. 20 amino acids met in protein. Protein is found in all organism and cannot replicate. Amino acids different relation with water – lock and key nature.

Image of Earth to Chromosome and Thousands of Genes

Image of Earth to Chromosome and Thousands of Genes

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  • Courtesy of National Human Genome Research Institute

  • DNA – 4 nucleotides – genetic material in most organisms

  • RNA – genetic material in few viruses – act as messenger, adapter, structural and catalytic molecule.

  • In 2000: nucleotide sequence of whole genome was determined

  • In 2006: number and sequence of genes in human chromosomes were determined

Image of RNA and DNA

Image of RNA and DNA

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Indirect Evidences

  • Can replicate prior to cell division

  • Can control cell structure

  • Can show variations / mutations

  • Have inbuilt clock for development

Direct Evidences

Griffith Experiment or Transformation

  • Change in genetic constituent by picking genes in remains of dead relatives. Initially to study bacterium Streptococcus pneumonia with two strains virulent (causes pneumonia – smooth colonies – S type) and non-virulent (no disease – irregular and rough colonies – R Type)

Image of Griffith Experiment or Transformation

Image of Griffith Experiment or Transformation

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  • Transforming principle is a chemical substance released by heat killed bacteria and changed R bacteria into S bacteria. . It is permanent genetic change as new S type bacteria formed only S type progeny.

  • But he could not prove what component of S type bacteria – polysaccharide of mucilage, protein or DNA

  • MacLeod & McCarty – purified biochemical from heat killed S type bacteria. Only DNA of S type can change R type of bacteria into S-type. It proved chemical to be inherited is DNA and forms chemical and molecular basis of inheritance.

Bacteriophage Multiplication or Transduction

  • Bacteriophage are bacterial virus and is bacteriophage that infects E. Coli (present in human intestine) Done by Hershey and Martha Chase with radioactive . Radioactive Sulphur incorporates into Sulphur amino acids (cysteine and methionine) and becomes part of bacterial proteins.

  • Radioactive phosphorous incorporates into nucleotide which for nucleic acid mainly DNA

  • Viruses derived from parents having labelled DNA possessed radioactivity – proves DNA and not protein is genetic material

History of DNA

  • Nucleic acid isolated by Miescher from pus cell and called nuclein

  • Hertwig: Nuclein as carrier of nucleic acid

  • Fisher: discovered purine and pyrimidine base in nucleic acid

  • Levene: found DNA to have phosphoric acid and deoxyribose sugar – characterized 4 nucleotides in DNA

  • Chargaff: purine and pyrimidine content was equal in DNA

  • Astbury: X-ray diffraction showed DNA is polynucleotide

  • Watson, Wilkins & Crick – Double helix model of DNA – awarded Nobel prize – sugar phosphate on outside and nitrogen base on inside with 2 antiparallel chains – helically twisted with each spiral having 10 nucleotides

  • Semiconservative strand: one parent chain and one new but complementary strand

  • Kornberg: Invitro synthesis of DNA in 1959

Types of DNA

Table of Types of DNA




helix sense

Right Handed

Right Handed

Left Handed

base pairs per turn




vertical rise per bp

3.4 Å

2.56 Å

19 Å

rotation per bp




helical diameter

19 Å

19 Å

19 Å

  • B-DNA: Balanced – base pair lie at right angle to axis of helix – more hydrated & frequently found – physiologically and biologically active form but can change to left handed

  • A-DNA: Alternate – 11 base pairs with away from perpendicular to axis

  • C-DNA: 9 base pairs per turn

  • Z-DNA: Zigzag – left handed – 12 base pairs & repeating unit is dinucleotide and unstable

  • Prokaryotes have 2 ends of covalent DNA are linked to form circular DNA (naked, without histone protein)

  • Linear DNA: two ends are free; found in eukaryotic cells and some prokaryotes (like Mycoplasma)

Chargaff Rule

  • In DNA there is always equality in quantity between the bases A and T and between the bases G and C. (A is adenine, T is thymine, G is guanine, and C is cytosine.)

  • Sugar deoxyribose and phosphate occur in equimolar proportions

  • A-T base pairs are rarely equal to C-G base pairs

  • Ratio of is variable but constant for species – used to identify DNA source – ratio is low in primitive organisms and high in advanced organisms