GN701: DNA History_01

1865-1866

• the Laws of heredity, discovered with garden peas
• - not recognized until the early 20th century

• 1. The Law of segregation
• 2. The Law of Independent Assortment
• 3. The Law of Dominance

offspring of the original parents

• a gene can exist in different forms, called alleles
• one allele can be dominant over another (recessive) allele

Each parent carries two copies of a gene

has one copy of each allele

Homozygotes, having two copies of one allele

gametes

haploid (only containing one copy of each gene)

a gamete having either allele

gametes that only have one allele type

1910, chromosome theory of inheritance

1910- chromosome theory of inheritance working with fruit flies

Chromosomes are discrete physical entities that carry the genes

In a given individual autosomes occur in pairs

Sex chromosomes are identified as X and Y (Females have two X, Males have one X and one Y chromosome)

Every gene on a chromosome has its place, or locus

Genes on separate chromosomes behave independently

Genes on the same chromosome behave as if they are linked

This genetic linkage is not absolute

Offspring may show new combinations of alleles not seen in the parents when crossing over/recombination occurs

The frequency of recombination between linked genes indicates the distance separating these genes on the chromosome

• 8
• (2n=8)

1941 – American geneticists George Beadle and Edward Tatum,  working with the bread mold Neurospora crassa, proposed a “One Gene – One Enzyme” hypothesis.

1941 –working with the bread mold Neurospora crassa, proposed a “One Gene – One Enzyme” hypothesis.

Identification of Nutritional Mutants in N. crassa

- wild-type fungi can grow in minimal medium

- radiated spores with Xrays, crossed with wild-type and the resulting offspring were tested in various environments. They found that the resulting mutated fungi could not grow in the minimal medium, but could grow in complete medium.

- different irradiated fungi batches required different addition of substances to thrive (in some cases it was the amino acid arginine)

- Arginine is produced by Neurospora in a metabolic pathway

- when minimal medium + arginine resulted in growth, knew that the knockout mutant must be in the bio-synthetic pathway for arginine

Neurospora can manufacture most of the chemicals it needs to live and grow, provided it has access to a few simple biological substances (also known as a minimal medium)

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• If enzyme 1 is nonfunctional, it can grow given either Ornithine or citruiline

If enzyme 2 is nonfuncitonal, it will not grow in presence of Ornithine, but will grow with citrulilne

If enzyme three is non functional, the mutation is near the end of the pathway that makes Arginine

1.An enzyme may be made of several sub-units, with each sub-unit encoded by a different gene

2.Many genes code for non-enzyme proteins

3.End products of some genes are not polypeptides

4.Some genes can code for multiple protein variants (e.g., due to alternative splicing)

1928- Frederick Griffith developed bacterial transformation working on bacterial pneumococcus: Streptococcus pneumoniae

Griffith worked on the idea of bacterial transformation.

Pneumococci exist in Rough and Smooth colonies.

• (wild S-strain) + mouse = death
• (wild R-strain) + mouse = alive

(heat-killed S-strain) + mouse = alive

(wild R-strain)  + (heat-killed S-strain) + mouse = dead

conclusion: Live R strain cells were transformed to S strain

R-strain is benign and recognized as destroyed by the host's immune system

S-strain is virulent and the polysaccharide capsule around the bacteria prevents detection by the hosts immune system

• dead mouse
• 

both live S and live R bacteria


Some sort of material (“the transforming principle”) had been passed from the dead S strain to the live R strain

1944- demonstrated that the "transforming principle" from the conclusions of the Griffith pneumococci experiment was DNA

They purified the 'transforming principle' :

• Kill virulent S bacteria by heating
• Hydrolyze polysaccharide capsules
• Extract soluble components with saline
• Denature and remove proteins with chloroform
• Precipitate soluble fraction with alcohol

* The resulting fibrous strands (the active portion) retained the ability to transform R strain into S

1952-Alfred Hershey and Martha Chase confirmed that the genetic material was, in fact, DNA using bacteriophage T2

1950- Erwin Chargaff discovered that in the DNA the amounts of bases A and T, and also of G and C are equal.

Chargaff rule: The rule that in DNA there is always equality in quantity between the bases A and T and between the bases G and C


(The amount of cytosine in plants was underestimated as it did not include 5-methyl-cytosine (and plant DNA is heavily methylated on cytosines)!)

(Some bacteriophages (e.g., PhiX174) do not adhere to Chargaff's rules: these phages have single-stranded genomes and, thus, are not constrained by the requirements of a double-stranded structure)

1952- Franklin and Wilkins produced X-ray diffraction on DNA to create a B-form DNA image. Because the X-ray pattern is so regular, the dimensions of the helix must also be consistent.

1953- Watson and Crick solved the structure of the DNA double helix

In vivo is Latin for “within the living.” It refers to studies that are done within living organisms. For example, studies performed on cattle.

In vitro is Latin for “within the glass.” A study performed in vitro is done outside a living organism. For example, in a test tube or lab.

Conservative replication leaves intact the original DNA molecule and generates a completely new molecule.

Dispersive replication produces two DNA molecules with sections of both old and new DNA interspersed along each strand.

Semiconservative replication produces molecules with both old and new DNA, but each molecule is composed of one old strand and one new one.

1958- Meselson and Stahl used E.coli to demonstrate that the mechanism of DNA replication is semiconservative

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- pharmacology (e.g., drug development to target specific pathways)

   - gene therapy (e.g., viral vectors to deliver missing or replace malfunctioning proteins, genome editing to fix mutations)

  - genetic screening (e.g., embryo testing and prenatal screening)

  - regenerative medicine (e.g., induced pluripotent stem cells and in vitro- grown tissues or organs)

- breeding (e.g., marker-assisted breeding for pathogen resistance)

- GMOs (e.g., BT-resistant corn)

- agrichemicals (e.g., pesticides/herbicides to target specific pathways)