Dna Structure and Analysis 2003

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DNA STRUCTURE AND ANALYSIS

I. CHARACTERISTICS OF THE GENETIC MATERIAL

Replication Storage of Information Expression of Information Variation by mutation

II. PROTEIN AS THE GENETIC MATERIAL (until 1944)

Chromosomes have a nucleic acid and a proteincomponent.

Both components were candidates for the role of genetic material

Three factors that favoured protein as the geneticmaterial1.Abundance of protein in cells

Account for 50% of the dry weight of cells

2.Accepted proposal for the chemical structure of nucleic acids

Tetranucleotide Structure of DNA by PhoebusLevene

3.Areas of most active research in genetics Transmission genetics and mutation

TETRANUCLEOTIDE HYPOTHESIS (1910)

Klug, W., Cummings, M., and Spencer, C. 2006. Concepts of Genetics. 8 th ed. Pearson PrenticeHall. Page 1


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o Nucleic acids werethought to besimple repetitivepolymers

o Simple 4-nucleotideunit repeated overand over in DNA

III. EVIDENCE FAVORING DNA AS THE GENETICMATERIAL

Frederick Griffiths Transformation Experiment(1927)



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Transforming Principle: The Avery, MacLeod, andMcCarty Experiment (1944)

o Oswald Theodore Avery (1877 1955)o Colin Munro MacLeod (1909 1972)o Maclyn McCarty (1911 - 2005)



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The molecule responsible for transformationwas DNA (deoxyribonucleic acid).

The Hershey-Chase Experiment (1952)

o Alfred Hershey (1908 1997)o Martha Chase (1927 2003)



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Study of bacterium E.coli and one of its infecting viruses,bacteriophage T2

T2 phages consist of approximately 50%protein and 50% DNA.Infection is initiated by adsorption of the phageby its tail fibers to the bacterial cell wall.

The production of new viruses occurs within thebacterial cell.

They used radioisotopes 32 P and 35 S to follow themolecular components of phages during infection.

o32P effectively labels DNA

DNA contains phosphorus



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o35S effectively labels protein

Protein contains sulfur

IV. NUCLEIC ACID CHEMISTRY

A. Nucleotideso Building blocks of nucleic acidso Components:



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Nitrogenous base Purine (nine-member double ring) Pyrimidine (six-member single ring)

Pentose sugar Ribose Deoxyribose

Phosphate groups

B. Nucleosideso Composed of nitrogenous base and pentose sugar



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The bonding among the nucleotides is highly specific.

C-1 atom of sugar links with the nitrogenous base.o Purine (N-9 atom)o Pyrimidine (N-1 atom)

C-2, C-3 and C-5 atom of sugar links with thephosphate groups

o C-5 phosphate configuration is the prevalentform in biological systems; one found in DNA andRNA



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V. DNA STRUCTURE AND ITS FUNCTION

A. Base Composition Studies

Erwin Chargaff o Used chromatographic methods to separate

the four nitrogenous bases in the DNAsamples from various organisms

o Used quantitative methods to determine theamounts of the four bases from each source

The amount of adenine residues is proportionalto the amount of thymine residues in the DNA of any species.

The amount of guanine residues is proportionalto the amount of cytosine residues.

(A+G) = (C+T)



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The percentage of C+G does not necessarilyequal the percentage of A+T

B. X-ray Diffraction Analysis

The pattern of scatter(diffraction) can be captured asspots on photographic film.

William Astbury o (1947) periodicity within thestructure of the molecule of

3.4 o Bases stacked like coins

Rosalind Franklin o Obtained improved X-raydata from purified samplesof DNA

o Confirmed the 3.4 periodicity

o Suggested that DNA

structure is a helix

Linus Paulingo Used diffraction analysis in

the study of protein structure



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o Analyzed the work of Astbury and otherso Proposed that DNA is a triple helix

C. Watson and Crick Model

Two primary sources crucial to the development of James Watson and Francis Cricks model:

1.Base composition studies of samples of DNA2.X-ray diffraction studies of DNA

Watson-Crick Double Helix



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1. Two long polynucleotide chains are coiledaround a central axis, forming a right-handeddouble helix.

2. The two chains are antiparallel.C-5-to-C-3 orientations run in oppositedirections

3. The bases of both chains are flat structures,lying perpendicular to the axis.

stacked on one another3.4 (0.34 nm) apart

4.Hydrogen bondingA pairs with T (double bond)G pairs with C (triple bond)

5.Each complete turn of the helix34 (3.4 nm) long10 bases per turn in each chain

6. Alternating larger major grooves andsmaller minor grooves

7.Diameter of helix = 20 (2.0 nm)

Accurate Analysis of the form of DNAStructure Recent, moreaccurate

Watson-Crick Model

No. of bases per turn 10.4 10.0Each base pair rotationaround the helical axisrelative to the adjacent

base pair

34.6 36



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Base pairs per turn More than 10 bp 10 bp

VI. ALTERNATIVE FORMS OF DNA

Characteri

stics

A-DNA B-DNA C-DNA D-

DNA

E-DNA Z-DNA P-

DNA

ConditionHigh salt

ordehydrati

on

Low saltcondition

Greaterdehydration (lab)

Heliceslackingguanin

e

Heliceslackingguanin

e-- --

Base pairsper turn 9 bp 10.4 bp 9.3 bp 8 bp 7 bp 12 bp

2.62bp

Diameter 23 20 19 -- -- 18 --Configurati

onRight-

handedRight-

handed -- -- --Left-

handed --

Arrangement of base

pairs

Tiltedand

displacedlaterallyIRT thehelical

axis

Lying flat;Perpendicular to the

helical axis

Samewith A-

DNA -- --

Zigzagconformation (major

groovepresent in

B-DNAnearly

eliminated)

--



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Dna Structure and Analysis 2003