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

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Watch Video Lecture on YouTube: Molecular Basis of Inheritance Genetics Basics: Evidence, History and Types of DNA

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

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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

  • 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

DNA

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

  • 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
Table of Types of DNA

B-Form

A-Form

Z-Form

helix sense

Right Handed

Right Handed

Left Handed

base pairs per turn

10

11

12

vertical rise per bp

3.4 Å

2.56 Å

19 Å

rotation per bp

+36°

+33°

-30°

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

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