Foundations of a replicative organism

1. Foundations of a replicative organism

2. DNA Discovery by Friedrich Miescher (Swiss, 1844-1895) He discovered a substance containing both phosphorus and nitrogen, made up of molecules that were apparently very large, in the nuclei of white blood cells Named the substance nuclein because it seemed to come from cell nuclei. In 1874 when Miescher separated it into a protein and an acid molecule. It is now known as deoxyribonucleic acid (DNA)

3. Phoebus Levene He worked with Albrecht Kossel and Emil Fischer, the nucleic acid and protein experts at the turn of the 20th. century

4. He conducted experiments that in 1931 suggested that the four components of DNA occur in approximately equal ratios He suggested the possibly that DNA was made of a repeating tetramer If so, the implication was that the structure of DNA was too simple and too regular to contribute to genetic variation: attention thereafter focused on protein as the probable hereditary substance

5. Not only did Levene identify the components of DNA, he also showed that the components were linked together in the order phosphate-sugar-base to form units. He called each of these units a nucleotide, and stated that the DNA molecule consisted of a string of nucleotide units linked together through the phosphate groups, which are the 'backbone' of the molecule. Scientist thought that Proteins (made from 20 aa) were Needed to encode life, not the 4(?) forms of Nucleotides

6. The Tetranucleotide Hypothesis

7. The extracts of the separated compounds are then studied in the ultraviolet spectrophotometer. The measurement of complete absorption spectra permits the determination of the purity of the solutions and at the sameThe extracts of the separated compounds are then studied in the ultraviolet spectrophotometer. The measurement of complete absorption spectra permits the determination of the purity of the solutions and at the same time the quantitative estimation of their contents. The details of theThe extracts of the separated compounds are then studied in the ultraviolet spectrophotometer. The measurement of complete absorption spectra permits the determination of the purity of the solutions and at the same time the quantitative estimation of their contents. The details of the procedures employed have been published a. In this manner, adenine, guanine, uracil, cytosine, and thymine (and also hypoxantlfine, xanthine, and 5-methylcytosine 4) can be determined quantitatively in amounts of 240 y. The precision of the method is ｱ4% for the purines and even better for the pyrimidines, if the averages of a large series of determinations are considered. procedures employed have been published a. In this manner, adenine, guanine, uracil, cytosine, and thymine (and also hypoxantlfine, xanthine, and 5-methylcytosine 4) can be determined quantitatively in amounts of 240 y. The precision of the method is ｱ4% for the purines and even better for the pyrimidines, if the averages of a large series of determinations are considered. time the quantitative estimation of their contents. The details of the procedures employed have been published a. In this manner, adenine, guanine, uracil, cytosine, and thymine (and also hypoxantlfine, xanthine, and 5-methylcytosine 4) can be determined quantitatively in amounts of 240 y. The precision of the method is ｱ4% for the purines and even better for the pyrimidines, if the averages of a large series of determinations are considered. Erwin Schrödinger published in 1945 a book titled What is Life? that planted the idea for searching “the secret of life”

8. Chargaff noted the publication of Avery, MacCleod and McCarty paper and realized that DNA was the key to life and set out to prove Levene wrong

9. In 1944 Consden et al. showed that it was possible to separate individual amino acids and to determine the amino acid composition of protein hydrolysates by partition chromatography on paper strips. The method was, in principle, readily adapted for the separation and identification of a large number of other substances, including the purines and pyrimidines of the nucleic acids (Figure 1), a task carried out in Chargaff’s laboratory by the Swiss post-doctoral fellow Ernst Vischer [12], and independently at the Rockefeller Institute by Rollin Hotchkiss

10. Note C not always equal to G

11. Apart from not demonstrating equal amounts of the four bases and thus casting doubt on the validity of the tetranucleotide hypothesis, certain other unexpected patterns also emerged: the amounts of purines seemed always to equal those of pyrimidines (that is, A + G = C + T, or (A + G)/(C + T) = 1). This had been found by Alfred Mirsky in 1943, but seems to have been overlooked by the Chargaff laboratory. More curiously, the ratios of A:G and T:C were always similar to each other whether they were greater or less than 1

12. The significance of these relationships was puzzling and a constant source of comment. At the end of 1949 Chargaff noted that ‘‘A comparison of the molar proportions [of the bases] reveals certain striking, but perhaps meaningless, regularities’’. Early in 1950, he wrote ‘‘It is noteworthy, although possibly no more than accidental, that in all desoxypentose nucleic acids examined thus far the molar ratios of total purines to total pyrimidines were not far from 1. More should not be read into these figures.’’ Later in 1950, apparently as a last-minute insertion in the paper, Chargaff wrote ‘‘It is noteworthy – whether this is more than accidental, cannot yet be said – that in all desoxypentose nucleic acids examined thus far the molar ratios of total purines and total pyrimidines, and also of adenine to thymine and of guanine to cytosine [ratios curiously not actually presented], were not far from 1’’ [2]. The following year, he wrote ‘‘As the number of examples of such regularity increases, the question will become pertinent whether it is merely accidental or whether it is an expression of certain structural principles that are shared by many desoxypentose nucleic acids, despite far-reaching differences in their individual composition and the absence of a recognizable periodicity in their nucleotide sequence’’. He then added ‘‘It is believed that the time has not yet come to attempt an answer’’, although clearly the subject was very much on his mind.

13. Why didn’t Chargaff predict The Double Helix? -He didn’t want to be wrong like Levene!

14. Two reasons why he would have been wrong 1) 2) What if it was some other reason that A/T G/C ratio was the same???? 3) H and X nucleotides?

15. Accounting in Saccharomyces cerevisiae

16. One of Chargaff’s Rules