DNA

In the late nineteenth century, a German biochemist found the nucleic acids, long-chain polymers of nucleotides. Later it was found that the sugar in nucleic acid could be ribose or deoxyribose, giving two forms: RNA and DNA. In 1943, American Oswald Avery proved that DNA carries genetic information. In 1953, scientists Watson and Crick gathered data from several studies of the DNA and postulated a model for the structure of DNA and they deduced that the DNA is a intertwined and extremely long double helix.

Needless to say that all this wouldn’t have been possible if other scientists hadn’t made important discoveries before. Some of the scientists who also made models of the DNA were: Pauling and Corey in 1948 but their model was wrong because as Watson and Crick said in their article in “Nature”: “we believe that the material which gives the X-ray diagrams is the salt, not the free acid and some of the van der Waals distances appear to be too small.”

Fraser suggested other three-chain structure which was wrong too.

Although in the article R.E. Franklin only appears in the acknowledgments, I would like to talk about her because she played an important role in the discovery of the deoxyribose nucleic acid structure.

Franklin, working mostly alone, found that her x-ray diffractions showed that the "wet" form of DNA (in the higher humidity) had all the characteristics of a helix. She suspected that all DNA was helical but did not want to announce this finding until she had enough evidences on the “dry, cristall” as well.

Once Franklin’s results were shown to Watson. He and Crick put forward a radically different structure for the salt of desoxyribose nucleic acid and this structure (the double helix) was worldwide accepted.

This structure has two helical chains each one coiled round the same axis. Each chain consists of phosphate-diester groups joining β-D-deoxyribofuranose residues with 3’ 5’ linkages. Both chains follow right-handed helix and run in opposite directions. The bases are on the inside of the molecule and the phosphates on the outside.

The structure is an open one and its water content is rather high. Moreover the two chains are held together by hydrogen bonds between the purine and pyrimidine bases. The planes of the bases are perpendicular to the fibre axis and they are joined together in pairs. One of the pair must be a purine and the other a pyrimidine. It is important to know that there are specific pairs of bases: adenine with thymine and guanine with cytosine.

The Watson and Crick’s model (also known as the double helix) showed how data could be stored in the DNA molecule.

SARA UROZ

FINDING THE STRUCTURE

Heus ací el meu treball sobre l'ADN.

Tota la informació s'ha extret de la pàgina web: www.dnai.org

Per seguir els passos i fer el puzzle i respondre les qüestions heu d'anar a la secció CODE de la mateixa pàgina i tot seguit entrar en: finding the structure.

Al llarg d'aquesta interessant pàgina podeu veure les diferents hipòtesis dels diferents investigadors, i, en alguns casos, fins i tot es pot escoltar de veu a alguns, com al J.Watson.

Record the specific contribution that each scientist made in the boxes provided:

Friedrich Miescher		Miescher added some simple chemicals to white blood cells from pus-filled bandages that he took at the hospital where he worked. He isolated a white precipitate he called "nuclein", that had lots of proteins. So it was “the first” extract of DNA which was isolated.
Phoebus Levene		He started deconstructing DNA components. He known DNA was a long-chain molecule, made up of four different nucleotides, ribose sugar, and phosphate.
Oswald Avery		He discovered DNA was carrying hereditary information and he showed it was the “transforming principle”. He isolated DNA from one strain of bacteria and then he discovered that it could transfer some features between organisms.

QUESTIONS:

Describe at least two ways that students who selected incorrect puzzle pieces would be able to recognize their mistakes.

If there are more than one peace for each spot, it means that one of it is the incorrect.

Describe why Watson and Crick knew that the triple helix model of DNA was incorrect.

Because Pauling showed a three-chain model hold them together by hydrogen bonds, treating the phosphate groups as un-ionized, as if they still had a hydrogen atom attached. So it was no correct because if there were hydrogen in the phosphate group it couldn’t be an acid, and the reality is that it’s an acid.

Explain why it would not have been possible for Watson and Crick to develop their DNA model without the data from Franklin.

Because X-ray diffraction patterns provide a lot of information about the shape and structure about molecules, and Rosalind showed that X-ray of the crystallized substance had diffracted or scattered because of the encounter with atoms. This was the cause of the helical structure of DNA. Then, Franklin calculated the basic dimensions of DNA, which were constant. Rosalind also knew that phosphate groups were outside the molecule because she covered some of it and the sodium sticking to it with water molecules and she observed the water was going in and out.

What did Watson need to consider when he worked out the DNA base pairing?

He thought it was too much effort to draw them out on paper each time and get the bond angles and bond lengths right. He used cardboard cutouts representing the shapes of the DNA bases. He considered the location of hydrogen atoms and it link bonds. He used Chargaff base pairs (adenine with thymine, guanine with cytosine) although Watson hated him and he didn’t recognized Chargaff’s theory of base pairs until that.

PIECES OF THE PUZZLE: Name of the scientists who:

Developed a three-chain model of DNA: Linus Pauling

Used sperm from different species for his research: Erwin Chargaff.

X-ray showed that the diameter of DNA’s helix is constant: Rosalind Franklin.

X-ray patterns showed Watson and Crick that DNA is helical: Rosalind Franklin.

Triple helix model acted as a catalyst for Watson and Crick: Linus Pauling.

Determined how A and T,G and C. form base pairs: Erwin

Chargaff.

Determined that DNA is crystalline: Maurice Wilkins.

Showed that DNA’s phosphates must be on the outside: Rosalind Franklin.

Determined that the ratio of A:T and G:C. is constant for an organism:
Erwin Chargaff (27% A-T, 52% G-C.), different for each species.

Flipped a coin to decide whose name went first: James Watson and Francis Crick. Finally, Watson name was first.

Obtained X-ray crystallographic patters of DNA: Rosalind Franklin.

Ací podem observar els resultants que la Rosalind Franklin obtingué després de sometre a raigs X la molécula d’ADN, i on s’observà la disposició helicoidal d’aquest.

THE MISTAKE PEACES:

Chargaff determined that all species have the same numbers of A, T,C. and G. It’s false because he determined different proportions of A-T, and C.-T ( it depends of the species).

Pauling worked with Phoebus Levene to develop a protein based a model of hereditary.

False, Phoebus Levene worked with his colleagues.

Franklin cristallyzed white blood cells from pus. False because Miescher isolated white blood cells from pus.

Wilkins determined that protein carries the genetic code.

Watson and Crick developed a triple helix model of DNA. False, Linus Pauling developed it.

Ací podeu veure a l'amic J.Watson, un poc vell, és clar.