Experiments regarding DNA Replication

Page 1

Wilhelm Johannsen coined the word gene in 1909

Helped with the Chromosome Theory of Inheritance in 1902

Mendelian genes have specific loci on chromosomes

Chromosomes undergo segregation and independent assortment

Formed by observations of Sutton and Boveri and other scientists

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Thomas Hunt Morgan - first solid evidence associating specific chromosome

Experimented with fruit flies and monohybrid crosses

Fruit flies produce many offspring can be brede about every 2 weeks, only have 4 pairs of chromosomes and a variety of characters.

Mated white eyed (mutant) males with red eyed (wild) females

F1 = all red eyes; F2 = 3:1 ratio, only males had white eyes

Determined white eyed chromosome allele must be on X Chromosome

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Fredrick Griffith - 1928 - Discovery of genetic role of DNA while trying to develop vaccine against pneumonia

Worked with 2 strands of bacterium (one pathogenic "S" and one nonpathogenic "R")

Used heat ot kill remains of pathogenic strain with living cells of harmless strain, leading some living cells to become pathogenic

Called it transformation (change in genotype or phenotype due to assimilation of foreign DNA)

Some chemical component of dead pathogenic bacteria caused this change

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Oswald Avery, Maclyn McCarty, Colin Mcleod - 1944 - Transforming substance of Griffith's experiment

Based on experimental evidence saying that only DNA worked in transforming harmless bacteria into pathogenic bacteria

Many were skeptical because little was known about DNA and proteins were considered better candidates for genetic material

Non-pathogenic bateria treated with active DNA led to it becoming pathogenic strain

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Erwin Chargoff - 1950 - Reported DNA composition varies from 1 species to the next

Knew it was a polymer of nucleotides with nitrogenous bases, sugar, and phosphate groups

Analyzed base compositions of DNA from numerous of different organisms

Evidence of molecular diversity made DNA more credible

Noticed regularity in A-T and G-C ratios in all species

Chargoff's Rules

Base composition of DNA varies between species % of A is same in all organisms of a species)

In any species, the number of A = the number of T, same between G and C numbers

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Maurice Wilkins and Rosalund Franklin x-ray crystallography to study molecular structure - produced picture of DNA molecule

Franklin concluded there were 2 outer sugar-phosphate backbones with nitrogenous bases paired in molecular interior

James Watson and Francis Crick - 1953 - proposed double-helical model for DNA structure

Watson also deduced the width of helix and spacing of nitrogenous bases

Built models of double helix to conform to X-rays and chemicals of DNA

Watson built model where they were antiparallel

Trial and error gave way to nitrogenous base pairings

1st thought it was like-like (A-A)

Came up with purine-pyrimidine for uniform width (purine-purine = too wide, pyrimidine-pyrimidine = too narrow)

Reasoned base pairings were more specific

Explained Chargoff's rules

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Matthew Meselson and Franklin Stahl - supported semiconservative model of DNA

Labelled nucleotides of old strands with heavy isotope of nitrogen- any new nucleotides were labelled with lighter isotope

1st round - produced hybrid DNA , 2nd round - produced light and hybrid DNA

More than 12 enzymes and other proteins participate

E-coli - 4.6 million nucleotide pairs

Human cell - 6 billion nucleotide pairs

Copy DNA in few hours with 1 error every 10 billion nucleotides

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Archibald Garrod - 1902 - suggested that genes dictate phenotypes through enzymes that catalyze specific chemical reactions

Believed symptoms of inherited diseases reflected inability to synthesize certain enzymes

Linking genes to enzymes required knowledge that cells synthesize and degrade molecules in a metabolic pathway

George Beadle and Edward Tatum - exposed bread mold to x-rays, creating mutants unable to survive on minimal media

Identified 3 classes of arginine-defiecient mutants, each lacking different enzymes needed for synthesizing arginine

Developed one gene - one enzyme hypothesis (each gene dictates production of specific enzyme)

Some proteins aren't enzymes (one gene - one protein hypothesis)

Many proteins are composed of many polypeptides wach with own gene (one gene - one polypeptide hypothesis)

NOT WHAT IT CURRENTLY IS!!