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1)Beadle & Tatum’s Experiment

- Using bread mold neurospora to show each gene encodes the structure of one polypeptide.

2)Meselson & Stahl’s Experiment

- Using the radioisotope labeling technique to show the semi-conservative model of DNA replication

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4.1.3 Gene concept; one gene one polypeptide

• George Beadle and Edward Tatum

experiment: connection between genes & metabolism.

• Neurospora crassa as experimental organism.

- short life-cycle, easily grown.

• Since it is haploid for much of its life cycle, mutations would be

immediately expressed.

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Neurospora crassa able to synthesize all of the amino acids and other

chemicals needed for growth

mutations will affect a single genes and single enzymes in specific metabolic pathways.

Gene concept; one gene one polypeptide

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Chemical reactions occurring in the body mediated by enzymes

(proteins)

Heritable traits- must be a relationship between the gene and proteins.

• Beadle and Tatum proposed the "one gene one enzyme" theory.

Gene concept; one gene one polypeptide

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One gene, one polypeptide

• One gene codes for one protein.

• "One gene one enzyme" modified to "one gene one polypeptide" since many proteins made of more than one

polypeptide

• Biosynthesis of amino acids: complex chemical reactions mediated by enzymes, which if mutated- shut down the

pathway

• no growth.

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4.2 DNA Replication

Watson and Crick Model of DNA suggest that each strand of DNA molecule could serve as a template, for the synthesis of the opposite strand.

Each half-helix could pair with their complementary nucleotides to replace its missing partner.

The result would be two DNA double helices, each identical to the original.

Definition :Process of copying a double stranded DNA strand which is the two resulting double strands are identical and each of them consist of one original and one newly synthesize strand.

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3 hypothesis that told about DNA replication :-

CONSERVATIVEboth parent (or old) strand remain together and an all new copy is made

SEMICONSERVATIVE the 2 strand of the parental molecule separate

and each functions as a template for synthesis of a new complementary strand

DISPERSIVEeach strand of both daughter molecules contains a mixture of old and newly synthesized parts

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In 1958, Meselson and Stahl studied the replication of E.Coli .

Meselson and Stahl Experiment

Hypothesis : DNA replication are take place by

semiconservative mechanism

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Conclusion Hypothesis are accepted. Experiment approved that DNA

replication are take place by semiconservative.