2-Estrutura do DNA e do RNA

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Estrutura 
do DNA e do 
RNA
Biologia Molecular Princípios e Técnicas, 2012
 Ed. Artmed -Cox, Doudna & O´Donnell
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Composição do DNA
(mononucleotídeos)
H
 Fosfato
base
nitrogenada
 desoxirribose
nucleosídeo
nucleotídeo
Estrutura primária do DNA
H
 Fosfato
base
nitrogenada
 desoxirribose
(2´desoxi-D-ribose)
Ligação glicosídica
Composição do RNA
 Fosfato
base
nitrogenada
ribose
Bases Nitrogenadas do DNA
Pentoses do DNA
Trinucleotídeo (precursores)
Trinucleotídeo
Descoberta da estrutura do DNA
Dados biofísicos
Padrões de difração de raio X
(Permite a exata localização espacial de átomos em uma molécula cristalizada)
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Biophysical data of various kinds. The water content of DNA fibers was particularly important because it enabled the density of the DNA in a fiber to be estimated. The number of strands in the helix and the spacing between the nucleotides had to be compatible with the fiber density. Pauling's triple helix model was based on an incorrect density measurement which suggested that the DNA molecule was more closely packed than it actually is. 
X-ray diffraction patterns (Section 9.1.3), most of which were produced by Rosalind Franklin of Kings College, London, and which revealed the helical nature of the structure and indicated some of the key dimensions within the helix. 
The base ratios, which had been discovered by Erwin Chargaff of Columbia University, New York. Chargaff carried out a lengthy series of chromatographic studies of DNA samples from various sources and showed that, although the values are different in different organisms, the amount of adenine is always the same as the amount of thymine, and the amount of guanine equals the amount of cytosine ( Figure 1.10 ). These base ratios led to the base-pairing rules, which were the key to the discovery of the double helix structure. 
Model building, which was the only major technique that Watson and Crick made use of themselves. Scale models of possible DNA structures enabled the relative positioning of the various atoms to be checked, to ensure that pairs of groups that formed bonds were not too far apart, and that other groups were not so close together as to interfere with one another. 
Difração de raio X
Rosalind Franklin and Maurice Wilkins, ~1950
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X-ray diffraction patterns (Section 9.1.3), most of which were produced by Rosalind Franklin of Kings College, London, and which revealed the helical nature of the structure and indicated some of the key dimensions within the helix. 
The base ratios, which had been discovered by Erwin Chargaff of Columbia University, New York. Chargaff carried out a lengthy series of chromatographic studies of DNA samples from various sources and showed that, although the values are different in different organisms, the amount of adenine is always the same as the amount of thymine, and the amount of guanine equals the amount of cytosine ( Figure 1.10 ). These base ratios led to the base-pairing rules, which were the key to the discovery of the double helix structure. 
Model building, which was the only major technique that Watson and Crick made use of themselves. Scale models of possible DNA structures enabled the relative positioning of the various atoms to be checked, to ensure that pairs of groups that formed bonds were not too far apart, and that other groups were not so close together as to interfere with one another. 
Dados biofísicos
Padrões de difração de raio X
Razão entre as bases – 
regras de Chargaff
Descoberta da estrutura do DNA
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Biophysical data of various kinds. The water content of DNA fibers was particularly important because it enabled the density of the DNA in a fiber to be estimated. The number of strands in the helix and the spacing between the nucleotides had to be compatible with the fiber density. Pauling's triple helix model was based on an incorrect density measurement which suggested that the DNA molecule was more closely packed than it actually is. 
X-ray diffraction patterns (Section 9.1.3), most of which were produced by Rosalind Franklin of Kings College, London, and which revealed the helical nature of the structure and indicated some of the key dimensions within the helix. 
The base ratios, which had been discovered by Erwin Chargaff of Columbia University, New York. Chargaff carried out a lengthy series of chromatographic studies of DNA samples from various sources and showed that, although the values are different in different organisms, the amount of adenine is always the same as the amount of thymine, and the amount of guanine equals the amount of cytosine ( Figure 1.10 ). These base ratios led to the base-pairing rules, which were the key to the discovery of the double helix structure. 
Model building, which was the only major technique that Watson and Crick made use of themselves. Scale models of possible DNA structures enabled the relative positioning of the various atoms to be checked, to ensure that pairs of groups that formed bonds were not too far apart, and that other groups were not so close together as to interfere with one another. 
Regras de Chargaff
Erwin Chargaff, 1940’s
Composição de bases do DNA (%)
varia de uma espécie a outra
invariável entre indivíduos da mesma espécie
não muda com idade, nutrição ou ambiente
Em todos os DNAs:
 A = T
 G = C
 A + G = T + C
Dados biofísicos
Padrões de difração de raio X
Razão entre as bases – 
regras de Chargaff
Modelos estruturais
Descoberta da estrutura do DNA
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Biophysical data of various kinds. The water content of DNA fibers was particularly important because it enabled the density of the DNA in a fiber to be estimated. The number of strands in the helix and the spacing between the nucleotides had to be compatible with the fiber density. Pauling's triple helix model was based on an incorrect density measurement which suggested that the DNA molecule was more closely packed than it actually is. 
X-ray diffraction patterns (Section 9.1.3), most of which were produced by Rosalind Franklin of Kings College, London, and which revealed the helical nature of the structure and indicated some of the key dimensions within the helix. 
The base ratios, which had been discovered by Erwin Chargaff of Columbia University, New York. Chargaff carried out a lengthy series of chromatographic studies of DNA samples from various sources and showed that, although the values are different in different organisms, the amount of adenine is always the same as the amount of thymine, and the amount of guanine equals the amount of cytosine ( Figure 1.10 ). These base ratios led to the base-pairing rules, which were the key to the discovery of the double helix structure. 
Model building, which was the only major technique that Watson and Crick made use of themselves. Scale models of possible DNA structures enabled the relative positioning of the various atoms to be checked, to ensure that pairs of groups that formed bonds were not too far apart, and that other groups were not so close together as to interfere with one another. 
Modelo Estrutural
James Watson
Francis Crick
1953
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Biophysical data of various kinds. The water content of DNA fibers was particularly important because it enabled the density of the DNA in a fiber to be estimated. The number of strands in the helix and the spacing between the nucleotides had to be compatible with the fiber density. Pauling's triple helix model was based on an incorrect density measurement which suggested that the DNA molecule was more closely packed than it actually is. 
X-ray diffraction patterns (Section 9.1.3), most of which were produced by Rosalind Franklin of Kings College, London, and which revealed the helical nature of the structure and indicated some of the key dimensions within the helix. 
The base ratios, which had been discovered by Erwin Chargaff of Columbia University, New York. Chargaff carried out a lengthyseries of chromatographic studies of DNA samples from various sources and showed that, although the values are different in different organisms, the amount of adenine is always the same as the amount of thymine, and the amount of guanine equals the amount of cytosine ( Figure 1.10 ). These base ratios led to the base-pairing rules, which were the key to the discovery of the double helix structure. 
Model building, which was the only major technique that Watson and Crick made use of themselves. Scale models of possible DNA structures enabled the relative positioning of the various atoms to be checked, to ensure that pairs of groups that formed bonds were not too far apart, and that other groups were not so close together as to interfere with one another. 
1- Dupla hélice
2- enroladas em torno do eixo da hélice
3- desoxiriboses externas (corrimão)
4- fitas antiparalelas
5- anéis aromáticos no interior (perpendicular ao eixo da hélice)
6-pareamento de bases
7-mecanismo de replicação
Modelo Estrutural
de Watson e Crick
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Interações Hidrofóbicas entre as bases
bases nitrogenadas
(não-covalentes)
Fosfatos
(carga negativa)
Interações de
Wan der Walls
Fitas são antiparalelas
5’
3’
bases nitrogenadas
3’
5’
T e U = ceto (C=O)
A = amino (C-NH2)
G e C =ceto e amino
T, U e C = pirimidinas (P)
G e A= purinas (G)
Entre os N dos 
anéis aromáticos
Não estão 
diretamente opostas
Modelo de Dupla Hélice
Watson e Crick, 1953
Fenda menor
Fenda maior
10 nucleotídeos
por volta
(2 nm)
Hidrólise do RNA
Esqueleto da molécula de DNA
5’
3’
Ligações fosfodiéster
(covalentes)
Interage com Mg2+
Pareamento entre as bases
Timina
Adenina
Citosina
Guanina
2
 ligações
3 
ligações
Ligações de 
Hidrogênio
(não-covalentes)
Não escapou a nossa observação que a especifidade do pareamento entre as bases poderia assegurar a precisão de replicação do DNA
Watson & Crick
Desnaturação do DNA = Fusão (“melting”)
Renaturação = Anelamento (“annealing”)
Desnaturação do DNA
Efeito hipercrômico
DNA fita simples
DNA fita dupla
Temperatura (oC)
Absorção a 260 nm
Tm: temperatura em que 50% das fitas estão desnaturadas
Desnaturação do DNA
DNA fita simples
DNA fita dupla
Temperatura (oC)
Absorção a 260 nm
Desnaturação do DNA
Tm (oC)
Porcentagem de G+C
Tm: depende da relação AT e GC
Obrigada!
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