资源简介

(共0张PPT)
磷酸
腺嘌呤A
P
含氮碱基
鸟嘌呤G
N
胞嘧啶C
胸腺嘧啶T
脱氧
核糖
H
H
0---H-N/
G
N
N-H----N
C
Sugar
N
N-H----O
Sugar
H
N-H----O
CH3
A
N
N----H一N
T
Sugar
N
N
Sugar
that the two lie side by side with identical z-co-ordinates.One of
the pair must be a purine and the other a pyrimidine for bonding to
MOLECULAR STRUCTURE OF
occur.The hydrogen bonds are made as follows:purine position I
to pyrimidine position I;purine position 6 to pyrimidine position
NUCLEIC ACIDS
6.
A Structure for Deoxyribose Nucleic Acid
If it is assumed that the bases only occur in the structure in the
most plausible tautomerie forms (that is,with the keto rather than
the enol configurations)it is found that only specific pairs of bases
E wish to suggest a structure for the salt of deoxyribose
can bond together.These pairs are:adenine (purine)with thymine
nucleic acid (D.NA.).This structure has novel features
(pyrimidine),and guanine(purine)with cytosine(pyrimidine).
which are of considerable biological interest.
In other words,if an adenine fomms one member of a pair,on
A structure for nucleic acid has already been proposed by
either chain,then on these assumptions the other member must be
Pauling and Corey They kindly made their manuscript available
thymine;similarly for guanine and cytosine.The soquence of
to us in advance of publication.Their model consists of three inter.
bases on a single chain,does not appear to be restricted in any
twined chains,with the phosphates near the fibre axis,and the
way.However,if only specific pairs of bases can be formed,it
bases on the outside.In our opinion.this structure is unsatisfactory
follows that if the sequence of bases on one chain,is given,then
for two reasons:
the sequence on the other chain is automatically determined.
(1)We believe that the material which gives the X-ray diagrams is
It has been found experimentallythat the ratio of the amounts
the salt,not the free acid.Without the acidic hydrogen atoms it is
of adenine to thymine,and the ratio of guanine to cytosine.are
not clear what forces would hold the structure together,especially
always very close to unity for deoxyribose nucleic acid.
as the negatively charged phosphates near the axis will repel cach
It is probably impossible to build this structure with a ribose
other.(2)Some of the van der Waals distances appear to be too
sugar in place of the deoxyribose,as the extra oxygen atom would
small
make too close a van der Waals contact.
Another three-chain structure has also been suggested by Fraser
The previously published X-ray dataon deoxyribose nucleic
(in the press).In his model the phosphates are on the outside and
acid are insufficient for a rigorous test of our structure.So far as
the bases on the inside,linked together by hydrogen bonds.This
we can tell,it is roughly compatible with the experimental data,
structure as described is rather ill-defined,and for this reason we but it must be regarded as unproved until has been checked
shall not comment on it.
against more exact results.Some of these are given in time
We wish to put forward a radically different structure for the
munications.We were not aware of the details of
salt of deoxyribose nuckeic acid.This structure has two helical
the results presented there when we devised our structure,which
chains each coiled round the same axis (see diagram).We have rests mainly though not entirely on published experimental data
made the usual chemical assumptions.namely,that cach chain
and stereo-chemical arguments
consists of phosphate diester groups joining B-D-deoxy-
It has not escaped our notice that the specific pairing we have
ribofuranose residues with 3'5'linkages.The two chains (but not
postulated immediately suggests a possible copying mechanism
their bases)are related by a dyad perpendicular to the fibre axis.
for the genetic material.
Both chains follow nighthanded helices,but owing to the dyad the
Full details of the structure,including the conditions assumed
sequences of the atoms in the two chains run in opposite directions.
in building it,together with a set of co-ordinates for the atoms.
Each chain loosely resembles Furberg's'
will be published elsewhere
model No.1;that is,the bases are on the
We are much indebted to Dr.Jerry Donohue for constant
inside of the helix and the phosphates on
advice and criticism,especially on interatomic distances.We have
the outside.The configuration of the sugar
also been stimulated by a knowledge of the general nature of the
and the atoms near it is close to Furberg's
unpublished experimental results and ideas of Dr.M.H.F.
standard configuration',the sugar being
Wilkins,Dr.R.E.Franklin and their co-workers at
roughly perpendicular to the attached
King's College,London.One of us (J.D.W.)has been aided by a
base.There is a residue on each chain
fellowship from the National Foundation for Infantile Paralysis.
every 3-4 A.in the z-direction.We have
assumed an angle of 36"between adjacent
JD.WATSON
residues in the same chain.so that the
F.H.C.CRICK
structure repeats after 10 residues on each
chain,that is,after 34 A.The distance of a
Medical Research Council Unit for the Study of the Molecular
phosphorus atom from the fibre axis is 10
Structure of Biological Systems,Cavendish Laboratory,
A.As the phosphates are on the outside,
Cambridge.Aprl 2
cations have casy access to them.
The structure is an open one,and its
water content is rather high.At lower
Pauling.L,and Corey.R.B.mature,171,346 (1953k Proc,U.S.Nat Acad.Sci,39,84
water contents we would expect the bases
1953
to tilt so that the structure could become
Furberg.S..Acta Chem Scand,6.634 (1952)
more compact.
Chargaft E,for references see Zamenhof,S.,Brawerman,G.,and Chargaf,E.
The novel feature of the structure is the Biochim et Bopbys Ae 9402 (1952)
manner in which the two chains are held Wyan,GR.J.Gen.Pyiol,36 201 (1952)
together by the purine and pyrimidine Aubury.W.T.Syme Soe.Exp.Biol 1.Nuclei Acid,66 (Cami Uiv.Pres,1947)
bases.The planes of the bases are perpendicular to the fibre axis.wilkins M.IF.and Randal J.T.Biochim et.nioplys.Acta 10.102 (1953)
They are joined together in pairs,a single base from one chain
being hydrogen-bonded to a single base from the other chain,so
This figure ts purely diagrammatic
heoi止onss)n
horontal rods the pairs of bases
holding the chains together.The
:靶点社
vertical line marks the fibre ais

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