dijous, 4 de desembre de 2008

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.), diff
erent 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.