Lecture 6. DNA, The Molecule of Heredity

I.Importance

In order to understand fully the very basic mechanisms of heredity, how genes control phenotype, the evolution of organisms, we must understand the molecular basis of genes

II. Early History

Friedrich Miescher (1869) extracted a substance from nuclei of white blood cells:

deoxyribo (sugar) + nucleic (from the cell nuclei) + acid (acidic)

Feulgen (1923): Schiff reagent stains DNA red; Chromosomes reddened.

Thus DNA is found within chromosomes.

Is DNA the molecule of heredity?? But chromosomes are almost equally protein

How can only four bases combine to provide all the variation needed to make different genes. Compare to

over 20 amino acids

III. Bacterial Transformation Implicates DNA as the Hereditary Material

A.Griffith’s monumental studies 1928: able to make harmless bacteria virulent

Smooth- has polysaccharide capsule

Rough- missing polysaccharide capsule and thus is attacked efficiently by immune system

B. Bacterial Transformation is Caused by DNA Oswald Avery’s Laboratory (1931)

Transformation without animals Grow R form of Bacteria in presence of components of S form and got Living S!!! Living R Form + Heat-killed S cell components

Avery, MacLeod and McCarty (1944): Protease, Rnase, No polyscacch – still transforms but

DNase results in component that CANNOT transform R to S

Physical and Chemical Analyses indicate a predominance of DNA

Component that was able to transform R to S was active at dilution of 1/600,000,000. Contaminants??

C. Hershey-Chase Experiments

Bacteria and bacteriophages (phages/viruses)

What allows the phage to parasitize the replication machinery of bacteria??? 1939: Invention of EM

IV. The Watson-Crick

A.Rosalind Franklin and Maurice Wilkens

X-ray crystallographers

X-ray diffraction patterns revealed crosswise pattern and therefore DNA is helical

Spacing between repeating units along the axis of the of the 3.4 angstroms

The helix makes a complete turn every 34 ang.

The diameter of the molecule is 20 ang.

Must contain more than one polynucleotide chain

B. Erwin Chargraff

Data on nucleotide composition:

Organism %A %T %G %C A:T G:C

Staphyloccus 12.8 12.9 36.9 37.5 0.99 0.99

E. Coli 26.0 23.9 24.9 25.2 1.09 0.99

Yeast 31.3 32.9 18.7 17.1 0.95 1.09

C. Elegans 31.2 29.1 19.3 20.5 1.07 0.96

Arabidopsis 29.1 29.7 20.5 20.7 0.98 0.99

Fruit Fly 27.3 27.6 22.5 22.5 0.99 1.00

Human 30.7 31.2 19.3 18.8 0.98 1.03

C. The Watson-Crick model

Two DNA chains spiral around an axis

Sugar-phosphate on outside and pairs of bases meeting in the middle

Chains are anti-parallel: 5’-3’ up and 5’-3’ down

Base pairs are flat and at rt. Angles to the helix axis

The chains wrap around each other every 34 Ang.

It is approximately 20 Ang. Wide

The two chains are held together by weak H-bonds, but because of the immense number of bonds the DNA

molecule is very stable

Purine with a Pyrimidine are 20 Ang

Adenine Thymine

Guanine Cytosine

Some viruses have single stranded DNA, or RNA, Some DNA molecules are circular

Alternative forms of DNA are formed but there significance is not known

DNA’s sequence specific information is mostly accessible only when the double helix is unwound

Some proteins can recognize sites even in intact double-stranded DNA

Secrets Revealed:

How does the molecule carry information?

How is that information copied for transmission to future generations?

What mechanisms allow the information to change?

How does the information govern the expression of the phenotype????????

V. DNA replication

How is done at the molecular level?

Each strand can act as a template during complementary base pairing.

Semiconservative replication, Conservative replication or, Dispersive replication

Review and then the Experimental Evidence:

Further details on recombination

Gene Conversion reveals mechanism of RECOMBINATION

VI. Recombination and Replication

(Remember, Recombination is a NEARLY UNIVERSAL Phenomenon- How and WHY!!)

First: DNA molecules break and rejoin

Messelson’s and Weigle’s experiments:

Lambda phage grown in either heavy or light C and N isotopes, followed by CsCl density gradient.

Further details on recombination

Gene Conversion reveals mechanism of RECOMBINATION

Recombination Models-Meselson, Radding and Holiday:

1. Homologs physically break, exchange parts and rejoin

2. Breakage and repair create reciprocal products of recombination

3. Recombination events can occur anywhere along the DNA molecule

4. Precision in the exchange- no gain or loss of nuclotide pairs- prevents mutations from occurring during the process

5. Gene Conversion- small segment of information is transferred from 1 homolog to the other.

50% recombination and 50% no recombination

VII. Conclusion

The helical, complementary structure of DNA provides a simple explanation for how genetic information

is stored, how replication occurs and why DNA is so stable.

Replication is associated with Recombination

Terms/Concepts to Know:

DNA structure, Classic Experiments: Griffith’s, Avery et al., Hershey-Chase, Chargaff, Franklin and Wilkins,

Watson and Crick, Messelson and Stahl and Holliday, Conversion, semiconservative replication, mechanism of replication and recombination, Figs 3, 5, 7, 9, 13, 14, 15, 16, 17, 19, 20, 21, 22 and Table 1