DNA Part I The History and Discovery of the Structure and Role of DNA - PowerPoint Presentation

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DNA Part I

The History and Discovery of the Structure and Role of DNA Chapter 16.1

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Life’s Operating InstructionsIn 1953, James Watson and Francis Crick introduced an elegant double-helical model for the structure of deoxyribonucleic acid, or DNAHereditary information is encoded in DNA and is found in the nucleus of every cell in the bodyDNA contains the instructions to build every trait in an organism. This includes biochemical, anatomical, physiological, and(to some extent) behavioral traits.

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DNA is the genetic materialEarly in the 20th century, the identification of the molecules of inheritance loomed as a major challenge to biologists

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Identifying DNA as a unique

molecule

1869- Friedrich

Miescher

, a

Swiss chemist

was

the first to identify DNA as a unique molecule

.

S

howed

that when

the enzyme pepsin was

used on the nucleus of

blood cells, a

strange phosphorous-containing material remained.

He

called this molecule

nuclein

, but still

thought that proteins were the molecules of heredity.

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Genes are Located on chromosomesIn 1920, Thomas Hunt Morgan showed that genes are located on chromosomes. The two components of chromosomes—DNA and protein—became candidates for the genetic material

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Evidence That DNA Can Transform BacteriaThe discovery of the genetic role of DNA began with research by Frederick Griffith in 1928Griffith worked with two strains of a bacterium, one pathogenic and one harmlessWhen he mixed the heat-killed pathogenic strain with living cells of the harmless strain, some living cells became pathogenicHe called this phenomenon transformation. A “transforming factor” turned the harmless strain into a pathogenic one

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Can a genetic trait be transferred between different bacterial strains?https://www.youtube.com/watch?v=MsFM-hvLtsE

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In 1944, Oswald Avery, Maclyn McCarty, and Colin MacLeod performed a similar experiment using a different bacteria and announced that the “transforming factor” was DNAMeaning that the instructions for being “pathogenic” was contained in its DNA and could be transferred to the harmless strainMany biologists remained skeptical, mainly because little was known about DNAMost scientists still believed that protein contained genetic material

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Evidence That Viral DNA Can Program CellsMore evidence for DNA as the genetic material came from studies of viruses that infect bacteriaSuch viruses, called bacteriophages (or phages), are widely used in molecular genetics researchA virus is a nucleic acid (DNA sometimes RNA) enclosed by a protective protein coat

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Structure & Replication Cycle Of T2 Bacteriophage Virus

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In 1952, Alfred Hershey and Martha Chase showed that DNA is the genetic material of a phage known as T2They designed an experiment showing that only one of the two components of T2 (DNA or protein) enters an E. coli cell during infectionThey concluded that the injected DNA of the phage provides the genetic information

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Animation: Hershey-Chase Experiment

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Additional Evidence That DNA Is the Genetic MaterialIt was already known that DNA is a polymer of nucleotides, each consisting of a nitrogenous base, a sugar, and a phosphate groupIn 1950, Erwin Chargaff reported that DNA composition varies from one species to the nextThis evidence of diversity made DNA a more credible candidate for the genetic material

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Based on the observations above,

three rules can be deduced%A =%T; %C=%GA+G=C+T=50%The percentages of the nucleotide vary for different speciesThe basis for these rules was not understood until the discovery of the double helix

Chargaff’s Rules

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Using Chargaff’s Rules

If thymine makes 40% of the nucleotides in a species. What percentage of nucleotides will be cytosine?

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Building a Structural Model of DNAAfter DNA was accepted as the genetic material, the challenge was to determine how its structure accounts for its role in heredityMaurice Wilkins and Rosalind Franklin were using a technique called X-ray crystallography to study molecular structureFranklin produced a picture of the DNA molecule using this technique

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Figure 16.6

(a) Rosalind Franklin

(b) Franklin’s X-ray diffraction

photograph of DNA

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Franklin’s crystallographic images enabled Watson to deduce that DNA was helical The X-ray images also enabled Watson to deduce the width of the helix and the spacing of the nitrogenous basesThe pattern in the photo suggested that the DNA molecule was made up of two strands, forming a double helix

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Franklin had concluded that there were two outer sugar-phosphate backbones, with the nitrogenous bases paired in the molecule’s interiorWatson and Crick built models of a double helix to conform to the X-rays and chemistry of DNAWatson built a model in which the backbones were antiparallel (their subunits run in opposite directions)

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At first, Watson and Crick thought the bases paired like with like (A with A, and so on), but such pairings did not result in a uniform width Instead, pairing a purine with a pyrimidine resulted in a uniform width consistent with the X-ray data

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Figure 16.UN02

Purine + purine: too wide

Pyrimidine + pyrimidine: too narrow

Purine + pyrimidine: width

consistent with X-ray data

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Watson and Crick reasoned that the pairing was more specific, dictated by the base structuresThey determined that adenine (A) paired only with thymine (T), and guanine (G) paired only with cytosine (C); called complimentary base paringA & G are pyrimidines; C & T are purinesThe Watson-Crick model explains Chargaff’s rules: in any organism the amount of A = T, and the amount of G = CAnd it is the sequence of nucleotides in DNA that determines an organism’s traits

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Figure 16.7b

(b) Partial chemical structure

3

′

end

5

′

end

3

′

end

5

′

end

Hydrogen bond

T

A

G

C

A

T

C

G

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Figure 16.7a

(a) Key features of DNA structure

0.34 nm

1 nm

3.4 nm

T

T

T

A

A

A

C

C

C

G

G

G

A

T

T

A

C

G

C

G

A

T

C

G

C

G

C

G

C

G

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Figure 16.5

5′ end

Thymine (T)

Adenine (A)

Cytosine (C)

Guanine (G)

Nitrogenous

bases

Sugar–

phosphate

backbone

3

′

end

Nitrogenous

base

Sugar

(

deoxyribose

)

DNA

nucleotide

Phosphate

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Staining of DNA reveals somatic cells have the same amount of DNA and twice as much as gametes.

1914-Robert Feulgen, a German chemist, found a

technique that determines the relative amount

of DNA

present in a cell.

Found

that all cells in an organism had the same amount of DNA except gametes, which had half the normal amount.