5.1 DNA Structure

DNA

Scientists now know:

• DNA carries genetic information

• DNA defines many traits and

predisposition for certain

diseases

Experiment 1a:

Identifying DNA as the Material of Heredity

Frederick Griffith uses Streptococcus pneumoniae (1928)

S-strain: highly pathogenic; after heating non-pathogenic

R-strain: non-pathogenic

Transformation

Griffith believed that something from the S-strain

transformed the R-strain into a deadly bacteria

What is that ‘something’ ???

Experiment 1b:

Identifying DNA as the Material of Heredity

Avery, MacLeod, and McCarty

The only extract that did not transform R-strain is the one treated with DNAse (destroys DNA)

CONCLUSION

DNA was responsible for the transformation

DNA was the hereditary material

Experiment 2:

Identifying DNA as the Material of Heredity

Alfred Hershey and Martha Chase (1952)

Used T2 bacteriophage strain of virus

• DNA in the viral core with a protein capsid

• When infecting a bacterial cell, virus attaches to cell and injects genetic information into it

• Genetic information then instructs cell to replicate many viral particles

Experiment 2:

Identifying DNA as the Material of Heredity

Experiment to determine virus’ ability to replicate

was due to the protein in the capsid or the DNA in

the viral core

Used radioactive isotopes to trace

• DNA contains phosphorus 32P

• Protein contains sulfur 35S

Conclusion

Viral DNA held the genetic material needed for

viruses to reproduce

Determining the Chemical Composition

and Structure of DNA

Fredrich Miescher discovered DNA (1869)

• Isolated nuclei of white blood cells and extracted an acidic molecule

• Named ‘nuclein’

Phoebus Levene isolated 2 types of nucleic acids (early1900s):

• RNA and DNA

• Proposed both types are made up of nucleotides

• Composed of: one of four nitrogeneous bases, a sugar, and a phosphate group

Determining the Chemical Composition

and Structure of DNA

Other scientists confirmed and extended Leven’s work

Determining the Chemical Composition

and Structure of DNA – Chargaff’s Rule

Erwin Chargaff concluded (late 1940s):

1. There is variation in the composition of nucleotides in

different species

2. Nevertheless, DNA maintains certain nucleotide proportions:

amount of A = amount of T

amount of C = amount of G

(Chargaff’s Rule)

Pauling Discovers Helical Structure for Proteins

Linus Pauling discovered that many proteins have helix-shaped structures (1951)

This information was used by scientists including James Watson and Francis Crick to deduce structure of DNA

Franklin Discovers Helical Structure for DNA

Rosalind Franklin and Maurice Wilkins used X-ray diffraction to analyze DNA sample

Franklin captured high-resolution photograph: the central x-shaped pattern

With mathematical theory, it was determined that:

• DNA has a helical structure

• The nitrogen bases are on the inside of the DNA helix, and the sugar-phosphate backbone is on the outside

Watson and Crick Build a 3-D Model for DNA

Using results from their peers, Watson and Crick proposed a structure for DNA:

• A twisted ladder called double-helix, with sugar-phosphate molecules making up the ‘sides’ and the bases making up the ‘rungs’ by protuding inwards

• Distance between sugar-phosphate backbones remains constant; base pairs sit across each other

• Different sequences of base pairs account for differences between species

Modern DNA Model:

The Double Helix

2 nucleic acid strands twist around one another

(backbone double helix)

alternating phosphate groups and sugar

Constant distance between backbones (2nm)

Antiparallel (5’ end of one strand is across from

the 3’ end of the other strand)

Bases are attached to each sugar and face inward

Complementary bases (A-T; C-G)

Hydrogen bonds between bases hold strands together 5’phosphate

3’ hydroxyl