Dati sulla struttura del DNA
• Contiene due catene
• Le percentuali G=C e A=T
• Legami fosfodiesterici tra nucleotidi
Lewin, IL GENE VIII, Zanichelli editore S.p.A. Copyright © 2006
Lewin, IL GENE VIII, Zanichelli editore S.p.A. Copyright © 2006
L’appaiamento delle basi
implica la formazione di legami idrogeno
H
citosina
N
H
N
O
N
sugar
H
O
N
N
H
N
CH3
timina
H
O
H
N
sugar
H
N
N
H
O
N
N
N
N
adenina
N
sugar
guanina
N
sugar
Lewin, IL GENE VIII, Zanichelli editore S.p.A. Copyright © 2006
Figura riportata nella
pubblicazione originale
di Watson e Crick
Nature, Aprile 1953
20
Lewin, IL GENE VIII, Zanichelli editore S.p.A. Copyright © 2006
La struttura del DNA (B)
• Il DNA ha una forma ad elica regolare,
diametro 20Å, passo 34Å
• Legami idrogeno tra le basi
• L’impilamento delle basi è determinato da
interazioni idrofobiche
• Ogni coppia è ruotata di 36°
• Solchi maggiore (22Å) e minore (12Å)
• Avvolgimento in senso orario (elica
destrorsa)
Nature , Vol. 171, p.737, April 25, 1953
MOLECULAR STRUCTURE OF
NUCLEIC ACIDS
A Structure for Deoxyribose Nucleic Acid
We wish to s ugge st a structure for the salt of
deoxyribose nucleic acid (D.N.A.). This structure has
novel fea tures which are of considerable biological
interest.
A structure for nucleic acid has alr eady been
proposed by Pauling and Corey (1). They kind ly made
their manuscript availa ble to us in advance of
publication. Their model consists of three intertwined
chains, with the phosphates near the fibre axis, and the
bases on the outside. In our opin ion, this structure is
uns atisfactory for two reasons : (1) We believe that the
material which gives the X-ray diagrams is the salt , not
the free acid. Withou t the acidic hyd rogen atoms it is
not cle ar what forces would hold the structure together,
especia lly as the negatively charged phosph ates near
the axis wil l repel each other. (2) Some of the van der
Waals distances appear to be too small .
Another three-chain structure has also been
sugg ested by F raser (in the press). In his model the
phosph ates are on the outside and the bases on the
inside, li nked together by hyd rogen bonds. This
structure as described is rather ill- defined, and for this
reason w e shall not comm ent on i t.
We wish to put forward a ra dically differ ent
structure for the salt of deoxyribose nuc le ic acid. This
structure has two helical chains each coile d round the
same axis (see diagram). We have made the usual
chemi cal assumptions, namely, that each chain consists
of phosph ate die ster groups
joining
§-Ddeoxyribofuranose residues with 3',5' linkages. The two
chains (but not their bases) are rela ted by a dyad
perpend ic ular to the fibre axis. Both chains follow
right - handed heli ces, but owing to the dyad the
sequences of the atoms in t he two chains run in
opposit e dire ctions. Each chain loosely resemble s
Furberg's2 model No. 1; that is, the bases are on the
inside of the heli x and the phospha tes on the outside.
The configuration of the sugar and the atoms near it is
close to Furberg's 'standard configu ration', the suga r
being rough ly perpend icular to the attached base. There
is a residue on each every 3.4 A. in the z-direction. We
have assumed an ang le of 36΅ between adjacent
residues in the same chain, so that the structure repeats
after 10 residues on e ach chain, that is, aft er 34 A. The
distance of a phosphorus atom from the fibre axis is 10
A. A s the phosphates are on the outside, cations have
easy access to them.
The structure is an open one, and its water content
is rather high. A t lower water contents we would expect
the bases to tilt so that the structure could become more
compact.
The novel feature of the structure is the manner in
whic h the two chains are held together by the purine
and pyrimi dine bases. The planes of the bases are
perpend ic ular to the fibre axis. The are joined together
in pairs, a single base from the other chain, so that the
two lie side by side with identical z-co-ordinates. One
of the pair must be a purine and the other a pyrimi dine
for bonding to o ccur. The hyd rogen bonds are made as
follows : purine posit ion 1 t o pyrimidine position 1 ;
purine po sition 6 to py rim idine position 6.
If it is assumed that the bases only occur in the
structure in th e most plausible tautomeric forms (that is,
wit h the keto rather than the enol configu rations) it is
found that only sp ecifi c pairs of bases can bond
together. These pairs are : adenin e (purine) with
thym ine (pyrimi dine), and guan ine (purine) with
cytosine (pyrimidine).
In other words, if an adenine forms one me mber of
a pair, on either chain, then on th ese assumptions the
other me mber must be thymine ; si mil arly for guan ine
and cytosine. The sequen ce of bases on a s ing le chain
does not appear to be restric ted in any way. However, if
only specifi c pair s of bases can be formed, it foll ows
that if the sequence of bases on one chain is given, then
the sequence on t he other chain is automatically
determined.
It has been found experimentall y (3,4) that the ratio
of the amount s of adenine to thymi ne, and the ration of
guanin e to cytosine, are always bery close to unity for
deoxyribose nu cleic acid.
It is probably impossible to build this structure
wit h a ri bose sugar in place of the deoxyribose, as the
extra oxyg en atom would make too close a van der
Waals contact. The previously publ ished X-ray data
(5,6) on d eoxyribose nucleic acid are in sufficient for a
rigorous test of our structure. So far as we can tell, it is
rough ly compatible with the experim ental data, but it
must be regarded as unp roved unt il it has been checked
against more exact results. Some of these are giv en in
the foll owing comm uni cations. We were not aware of
the detail s of the result s presented there when we
devised our structure, which rests mainly though not
entire ly on publi shed experime ntal data and
stereochemical argume nts.
It has not escaped our notice that the specific
pairing we have postulated imm edia tely sugg ests a
possible copying mechanism for the g enetic materia l.
Full details of the structure, includ ing the
condi tions assume d in buil ding it , together with a set of
co-ordinates for the atoms, wil l be published elsewhere.
We are much ind ebted to Dr. Jerry Donohue for
constant advice and criticism, especia lly on interatomi c
distances. We have also been stim ulated by a
know le dge of the general nature of the unpub li shed
experim ental results and ideas of Dr. M. H. F. Wilkins,
Dr. R. E . Franklin and their co-workers at King's
College, London. O ne of us (J. D. W .) has been aided
by a fell owsh ip from the National Founda tion for
Infantil e Paralysis.
J. D. WATSON
F. H. C. C RICK
Medical Research Coun cil Unit for the Study of
Mole cular Structure of Biological Systems, Cavendish
Laboratory, Cambridge.
1. Pauling, L., and Corey, R. B., Nature, 171, 346
(1953); Proc. U.S. Nat. Acad. Sci., 39, 84 (1953).
2. Furberg, S., Acta Chem. Scand., 6, 634 (1952).
3. Ch argaff, E., for refere nces see Zamenho f, S.,
Brawerman, G., and Chargaff , E., Bio chim. et Biophy s.
Acta, 9, 402 (1952).
4. Wyatt, G. R., J. Gen. Phys iol., 36, 201 (1952).
5. A stbury, W. T., Symp. Soc. Exp. Biol. 1, Nucleic
Acid, 66 (Camb. Univ. P ress, 1947).
6. W ilkins, M. H . F., and Rand all, J. T., Biochim. et
Biophys. A cta, 10, 192 (1953).
It has not escaped our notice that the specific
pairing we have postulated immediately suggests a
possible copying mechanism for the genetic material.
Il modello a doppia elica di Crick e Watson
suggerisce un meccanismo di replicazione
OLD
OLD
NEW NEW
OLD
NEW
OLD
NEW
Lewin, IL GENE VIII, Zanichelli editore S.p.A. Copyright © 2006
Lewin, IL GENE VIII, Zanichelli editore S.p.A. Copyright © 2006
Lewin, IL GENE VIII, Zanichelli editore S.p.A. Copyright © 2006
Lewin, IL GENE VIII, Zanichelli editore S.p.A. Copyright © 2006
Strutture alternative del DNA
Gli acidi nucleici possono formare
vari tipi di doppia elica
Gli acidi nucleici possono formare
vari tipi di doppia elica
Elica
Coppie di basi
per giro
Rotazione tra
due basi
Diametro
B
10 (10,4)
36 (34,6)
20 (19)
A
11
32,7
23
Z
12
-30
18
Strutture superiori del DNA
DNA circolare con
superavvolgimento = 0
DNA superavvolto
negativamente
Lewin, IL GENE VIII, Zanichelli editore S.p.A. Copyright © 2006
Lewin, IL GENE VIII, Zanichelli editore S.p.A. Copyright © 2006
Lewin, IL GENE VIII, Zanichelli editore S.p.A. Copyright © 2006
Il superavvolgimento negativo
può convertirsi nella
separazione locale
dei due filamenti
DNA superavvolto
negativamente
struttura Z
(in corrispondenza di
regioni di alternanza Pu/Py)
separazione dei filamenti
(denaturazione di
regioni ricche in A+T)
strutture cruciformi
(in corrispondenza
di palindromi)
Superavvolgimento del DNA
Positivo = DNA superspiralizzato
Negativo = DNA sottospiralizzato
Enzimi che alterano la topologia del DNA
Topoisomerasi: tipo I
tipo II (girasi)
Lewin, IL GENE VIII, Zanichelli editore S.p.A. Copyright © 2006
Lewin, IL GENE VIII, Zanichelli editore S.p.A. Copyright © 2006
Enzimi per gli acidi nucleici
• Polimerasi:
DNA polimerasi
RNA polimerasi
• Nucleasi:
esonucleasi
endonucleasi
Lewin, IL GENE VIII, Zanichelli editore S.p.A. Copyright © 2006
Lewin, IL GENE VIII, Zanichelli editore S.p.A. Copyright © 2006