NAT URE VOL. 227 A UGUST 8 1970 561
Central Dogma of Molecular Biology
by
FRANCIS CRICK
MRC Laboratory of Molecular Biology.
Hills Road,
Cambridge CB2 2QH
The central dogma of molecular biology deals with the detailed
residue-by-residue t ransfe r of seque ntial information. It states
that such information cannot be transferred from protei n to either
protei n or nucleic acid.
"The central dogma, enunciated by Crick in 1958 and the
keystone of molecular biology ever since. is likely to prove ill
considerable.over-simpl ificat ion."
THIS quotation is taken from t ho beginning of an uns igned
articlel
headed "Central dogma reversedIt. recounting the
very important work of Dr H oward Temin' and others'
showing that an R NA tumour virus can use viral R NA
as a template for DNA synthesis. This is not the first
time that the idea. of tho central .dogma has been misunderstood,
in one way or another. In t his article I
explain why the term was originally int roduced, its true
meaning, and state why I think t hat, properly understood
, it is still an idea of flmdamental import ance.
Tho central dogma was put forward4 at a period when .
much of what we now know in molecular genet ics was not
established. All we had to work on were certain fra.gmontary
experimental results, themselves often rather
uncertain and confused, and a bq,uudless opt imism t hat
t he basic concepts involved were rather simple and
vrobably much the same in all living things. In. such a
situation well constructed t heories can playa really useful
part in stat ing problems clearly and thus guid ing experi-
ment.
The two central concepts which ha.d been produced.
originally without any e:\.-plicit statement of the simplification
being introduced , were those of sequent ial information
and of defined alphabets. Neither of these steps was
trivial. Because it was abundantly clear by that time
that a protein had a well defined t hree dimensional structure,
and that its activity depended crucially on this
structure, it WM necessary to put the folding-up process
on one side, and postulate that, by and large, t he polypeptide
chain folded itself up. This temporarily reduced
tho central problem from a three dimensional one to a
one dimensional one. It was also necessa.ry to argue
that in spite of t he miscellaneous list of amino-acids
found in proteins (as then given in aU biochemical textbooks)
some of them , Stich as phosphoserine, were secondary
modifications; a nd that t here was probably a universal
set of twenty used throughout nature. In t he same way
minor modifications to the nucleic acid bases were ignored;
uracil in R NA was considered to be informationally
nDNA
f'"RNA •
U
" PROTEIN
U
Fig. 1. 'fhe iU'roWS show all the )lO!ISlble simple transfers between the
three famlllea of polymers. 'rhey represent the dlrectJonal flow of
detailed sequence In(ormatlon.
analogous to t hYI'Qine in DNA, t hus giving four standard
symbols fol' the components of nucleic acid.
The principal problem could then be stated as t ho
formulation of the general rules for information tranafor
from one polymer with a defined alphabet to another.
This could be compactly represented by the diagram of
Fig. 1 (which was actually drawn at that time, though I
am not sure that it was ever published) in which all
possible simple transfers were represented by arrows_
The arrows do not, of course, represent the flow of maMer
but the directional flow of detailed, rcsidue-by-roaidue.
sequence information from one polymer molecule to
another.
Now if all possible transfers commonly occurred it
would have been almost impossible to construct useful
theories. Nevertholess, such t heories wore pa.rt of our
discussions. rrhis was becau60 it was being
tacit ly assumed that certain transfers could not occur.
lt occurred to me t hat it would be wiRe to stnte t hese
preconceptions explicit ly.
nDNA
/ ' \ \
RNA - - - " PROTEIN
U:Flg.2. The arrows show the situation as it seemed In 1958. Solid nrrows
represent probnble transfers, dotted arrows possible trll.nafers. T ho
absent arroWl! (compare Fig. 1) represent the Impossible lrllonsfel'll
postulated by thc central dogma. 'rhey nre the three poseiblc arrows
starting from protein.
A little analysi8 showed that t he transfer could be
divided roughly into tlu-ee groups. The first group was
those for which some evidence, direct or indirect, seemed
to exist . These are shown by the solid arrows in Fig. 2_
They were:
I (a) DNA-+DNA
I (b) DNA-+RNA
I (0) RNA-+Protein
I (d) R NA-+R NA
The IllSt of t hese transfers was presumed to occur because
of the existence of RNA viruses.
Next there were two transfers (shown in F ig. 2 as dotted
aTl'o'ws) for which t here was neither any oxperimental
evidence nor any strong t heoretical requiremont. They
were
II (a) RNA_ DNA (see the reference to 'l'c)m in's workl )
II (b)
562
The latter 'vas the transfer postulated by Garnow, from
(double stranded) DNA to protein, though by that time
his particular theory had been disproved.
The third class consisted of the throo tl'o.nsfors th(
arrows of which have been omitted from Fig. 2. Thos
were tho transfers:
III (a)
III (b)
III (0)
Protein---+-Protein
Protoin--+RNA
Protein--+DNA
The general opinion at the time was that class I almost
certainly existed, class II was probably rare or absent,
and that class III was very unlikely to occur. The
decision had to be made, therefore, whethor to assume
that only class I transfers occurred. Thero were, however,
no overwhelming structural reasons why the transfer in
class II should not be impossible. In fact, for all we
knew, the replication of a.ll RNA viruses could have gone
by way of a DNA intermediate. On the other hand, there
were good general reasons against all the three possible
transfers in class III. In brief, it was most unlikely, for
stereochemical reasons, that protein-7protein transfer
could be done in the simple way that DNA-7DNA transfer
was envisaged. Tho transfer protein-7RNA (and the
a.nalogous protein-7DNA) would have required (back)
translation, that is, the transfer from one alphabet to a
structurally quite different one. It was roalized that
forward translation involved very complex machinery.
Moreover, it seemed unlikely on general grounds that this
machinery could easily work backwards. The only reason.
able alternative was that the cell had evolved an entirely
separate set ofcomplicated machinery for back translation,
and of this there was no traco, and no reason to believe
that it might be needed.
I decided, therefore, to play safe, and to state as the
basic assumption of the new molecular biology the non·
existence of transfers of class III. Because these were all
the possible transfers from protein, the central dogma
could be stated in the form "once (sequential) information
has passed into protein it cannot get out again"fo. About
class II, I decided to remain discreetly silent.
At this stage I must make four points about the formulation
of the central dogma which have occasionally produced
misunderstandings. (See, for exa.mple, Commoner5 :
his error has been pointed out by Fleischman' and on
more general grounds by Hershey'.)
(1) It says nothing about what the machinery of
transfer is made of, and in particular nothing about
errors. (It was assumed that, in general, the accuracy of
transfer was high.)
(2) It says nothing about control mechanisms-that is,
about the rate at which the processes work.
(3) It was intended to apply only to present-day
organisms, and not to events in the remote past, such as
the origin of life or the origin of the code.
(4) It is not the same, as is commonly assumed, as the
sequence hypothesis, which was clearly distinguished
from it in the same article4 • In particular the sequence
hypothesis was a positive sta.tement, saying that the
(overall) transfer nucleic acid-,.protein did exist, whereas
the central dogma was a negative statoment, saying that
transfers from protein did not exist.
In looking back I am struck not only by the brashness
which allowed us to venture powerful statements of a
very general nature, but also by the delicate
discrimination llsed in selecting what statements to make.
Time has shown that not everybody appreciated our
restraint.
So much for the history of the subjcct. What of the
present? I think it is clear that the old classification,
though lIseful at the time, could bo improved, and I
suggest that tho nine possible transfers be regrouped
tentath'oly into t.hree cla.sses. I propose that these be
NATURE VOL. 227 AUGUST 8 1970
nDNA
1'\\
RNA - - - " PROTEIN,, ,, ,
'- -Fig. 3. A tentative clRSSlflcntlon for the present day. Solid arrows show
general transfersj dotted arrows show special transrers. Again, the
absent arrows nre the undetected transfers speclfled by the central
dogma.
callcd general transfers, special tra.nsfers and unknown
transfers.
General and Special Transfers
A general transfer is one which can occur in aU cells.
The obvious cases are '
RNA-7Protein
Minor exceptions, such as the mammalian reticulocyte,
which probably lacks the first two of these, should not
exclude.
A special transfer is one which does not occur in most
cells, but may occur in special circumstances. Possible
candidates are
DNA-7Protein
At the present time the first two of these have only been
shown in certain virus-infected cells. As far as I know
there is no evidence for the third except in a special ceIlfree
system containing neomycin', though by a trick it
could probably be made to happen, using neomycin, in an
intact bacterial cell.
Unknown Transfers
These are the threo transfers which the central dogma
postulates never occur:
.Protein-7Protein
Protein-7DNA
Protein-7RNA
Statcd in this way it is clear that the special transfers
are those about which there is the most uncertainty. It
might indeed have "profound implications for molecular
biology"l if a.ny of these special transfers could be shown
to be goneral, or-if not in all cells-at least to be widely
distributed. So far, however, there is no evidence for the
first two of these except in a cell infected with an RNA
virus. In such a cen the central dogma demands that at
least one of the first two special transfers should occurthis
statement, incidentally, shows the power of the
central dogma in making theol'etical predictions. Nor, e.s
I have indicated, is there any good theoretical reason why
the transfer RNA-7DNA should not sometimes be used.
I ha.ve never suggested that it cannot occur, nor, as far as
I know, have any of my colleagues.
Although the details of the classification proposed here
are plausible, our knowledge of molecular biology, even
in one cell-let alone for all the organisms in nature---
is still far too incomplete to allow us to assert dogmatically
that it is correct. (There is, for exa.mple, the problem or
the chemical nature of the agent of the disease scrapie:
NATURE VOL. 227 AUGUST 8 1970
600 tho articles by Gibbons and and by Gl'iffith lO•
Nen'l'theless, wo know enough to say t hat a non-trivial)
oxample showing that the classification was wrong could
be an important discovery. It would certainly be of great
interest to find a cell (as opposed to a virus) which had
lOL\. as its gonetic material a nd no DNA, 01' a coli which
used single-stranded DNA 0.8 messenger mthor t han RNA.
Perhaps the so-called repetitive DNA is produced by an
R.NA-7DNA transfor. Any of these would be of t ho
greatest interest" but t hey could be accommodated into
our t hinking without undue strain. On t he other hand.
the discoV01'Y of just one type of present day cell which
could carry out any of the t hroe un.known transfors would
shake the whole intellectual basis of molecular biology,
563
and it is for t his reason that the central dogrnfl is as
important today as when it W [l S first proposed.
Received July S, 1070.
I NalllTe, 226,1108 (1970).
I Temin, n. M., and 1\lizutnni, S., Nalure, 228, 1211 (1070). ThlsnrUcle con·
tains the rerenmccs to Dr 'l'emln's earlier work dating back to 1903,
I Baltimore, D., Natllre, 226, 1200 (1970). See al.so the brier or
Spiegelman's reccnt work 011 page 1202.
• Crick, F , H, C.. [n Symp. Soc. Rxp. Biol., The Biological Replication of
M acromolecules, XU, 138 (HISS).
'Commoner, n., Nattlre, 220, 334 (1968).
• Fleischman, P., Nature, 225 , 30 (HI70),
, Hershey, A, D., NaWre, 226, G07 (1970).
I :McCnrlhr, ]3" and Holland, J . J., Proc. US Na/,. .Acad. Sci., 54, 8S0 (1965).
• Gibbons, R. A., and Runter, G. D., Nalure, 215, 1041 (1967).
II Griffith, J. S., l\'atuTe, 215, 10-13 (HI67).
Characterization of the Products of RNA-directed DNA
Polymerases in Oncogenic RNA Viruses
by
S. SPIEGELMAN
A. BURNY
M. R. DAS
J. KEYDAR
J. SCHLOM
M. TRAVNICEK
K. WATSON
Several RNA tumour viruses contai n an enzyme that synthesizes a
DNA-RNA hybrid using the single stranded viral RNA as template.
Hybridization experiments confirm that the DNA strand is complementary
to the viral RNA.
Institute of Cancer Research,
Columbia University, and
Colle:e of Physicians and Surgeons.
99 Fort Washington Avenue, New York, NY 10032
TEMIN'S DNA provirus hypothesisl, according to which
the replication of t he RNA or RNA tumour viruses takes
place tlu'ough a DNA intermediate, explained the following
unique features of infections with R NA oncogenic viruses :
(a) the heritably stable transformation of normal cells
induced with t hese viruses; (b ) the appa.rent vert ical
transmission of high leukaemia frequency in reciprocal
crosses betwecn high and low frequency strains of mice!! ;
and (c) the requirement for DN A synthesis3 in the early
stages of infection.
The hypothesis makes two specific predictions amenable
to experimental test. DNA complementary to viral
H.NA should appear after infection and therefore should be
detectable by molecular hybridization. Suggestive but
not decisi\'o cxperiments supporting t his prediction have
been reported·l
. s. FUl'ther, Tcmin invokes t ho existence
of an enzy me t hat can carry out a revcrsal of transcription
by catalysing t ho synthesis of DNA on un IlNA template.
Evidence for such an enz.yme has been presented recently
by Baltimore' and Temin and l\iizutanii , who found a
DNA·polymerizing activity in both avian and mm'ine
t umour virllses. Tho enzyme was detected by the incorporation
of tritium-labelled thymidine t riphosphate
(3H·TTP) into all. acid -insoluble product that can be
destroyed by deoxyribonuclease. Maximal activity
required the presence of all four deoxyribosid e triphos.
phates and magnesium. The fact t hat the activity is
inhibited by ribonuclease implies that t he RNA of t he
virion is necessary for the reaction .
These findings are clearly pregnant with implications
for t he molecular details of viral oncogcnesis. Their
potential importance demands quick confirmation and
extension, a task t he present wOl'k undertook to fulfil.
" re report hero tho finding of DNA polymerase activity
in all of t he seven tumour viruses we havo examined and
establish by physical and chemical characterization that
the product is in fact a DNA heteropolymer. Further, we
show that the DNA synthesized is complementary to
v iral RNA by demonstrating its ability to hybridize
specifically with homologous viral H.NA. Finally, we
find t he expected nascent RN A- DNA complexes in the
rcaction. These have been detected and characterized
in glycerol and Cs2SO, gradients and shown to be sensitive
to denaturation procedures which disrupt RNA-DNA
hybrids.
Preparation of Viruses for Enzyme Test
R a uscher murine leukaemia virus (RLV) was oLtained
as a ten-fold mouse plasma concentrate. Virus lot
RPV-HL-67-5 (infectivity tit,·o of 3·9 Jog spleen weight
enlarging units per mI.) prepared from CF\V·S mice was
used. All procedures following the origi nal tha"'ing of
t he plasma weJ'e conducted at 0°_4° C. P lasma witS first
clarificd at 16.000g for 10 min. The rcsulti ng supcrnatant
was layered on a 100 pel' cent glycerol cushion and
centrifuged at 95,000g for 70 min. Tho material obtained
on and just above tho glycerol cushion was then layered
over a preformed 25- 50 pel' ccnt sucrose gradient and
centrifuged at 95,000g for 3 h. T he I'esulting virus band
(1·16 g/cm3
was diluted in 0·0 1 M Tris·HCI (pH 8'3),
0·1 1\1 NaCl, 0·002 :i\I EDTA buffer (TNE) a nd recentrifuged
for 2 h a t 95,OOOy. Tho resulting pellet was rcsus·
pended in TNE and assayed for protein content. A
similar pl'ocedure served to purify R LV hnl've-sted fl'om
JLS-V5 tissue culture supernatants grown in OU I' labora ·
tory.