Francis Crick

The Origin of the Genetic Code

Abstract The general features of the genetic code are described. It is considered that originally only a few amino acids were coded, but that most of the possible codons were fairly soon brought into use. In subsequent steps additional amino acids were substituted when they were able to confer a selective advantage, until eventually the code became frozen in its present form.

Genetical implications of the structure of deoxyribonucleic acid.

J. D. Watson, F. H. C. Crick: Genetical Implications of the Structure of Deoxyribonucleic Acid

Codon-Anticodon Pairing: The Wobble Hypothesis

It is suggested that while the standard base pairs may be used rather strictly in the first two positions of the triplet, there may be some wobble in the pairing of the third base. This hypothesis is explored systematically, and it is shown that such a wobble could explain the general nature of the degeneracy of the genetic code.

A structure for deoxyribose nucleic acid

What is the purpose of this ar2cle? (Why did Watson and Crick write this ar2cle?) _________________________ _________________________ _________________________ _________________________ _________________________ _________________________ _________________________ Summarize Watson and Crick’s descrip2on of the DNA molecule. ________________________________________________________ ________________________________________________________ ________________________________________________________ ________________________________________________________ ________________________________________________________ ________________________________________________________ ________________________________________________________ ________________________________________________________ ________________________________________________________ ________________________________________________________ ___________________________________________________

The structure of DNA.

THE STRUCTURE proposed for DNA (deoxyribonucleic acid) has been described before (for a rerien : see Jordan, 21) and will only be outlined here. It consists of two polynucleotide chains running in opposite directions and twined round one another. The two chains are held together by hydrogen bonds between the bases, each base being joined to a companion base on the other chain. This pairing of bases is specific, odenine going with thymine, and guanine wit.h cytosine. The structure is not only found in extracted DX4 from a wide variety of sources, but is also present in intact biological material such as sperm heads and bacteriophage (36). The x-ray work up to 1954 has already been briefly summarized, with references (6). Since then, the group at King’s College, London, under Dr. N. H. F. Wilkins have published an interim report (17) on their studies of the B form of DK-4 and on their work on nucleoprotamine. Two reviews (9, 37) have also appeared; the one by Wilkins (37) touching on the very recent work of his group on nucleohistone. Some studies of the B form of DB-4 have also been carried out by Wykoff (38), who has shown that the B structure obtained by stretching the -4 form is slightly different from that produced by swelling it. The King’s College group have obtained the B form in a crystalline state, and in particular, ha\-e shown that the lithium salt gives a good lattice (37). According to Wilkins (37:~. rhe structure is now firmly established. The detailed structure described by Crick and Watson (8) has been shown to have too large a diameter, and a drawing has been given of an improved model which is in fairly good agreement with the s-ray data. No coordinates ha\-? 50 fnr been published.* (It should be noted

The molecular structure of collagen.

This paper describes in detail our work on the structure of collagen which we have already outlined elsewhere ( Rich & Crick, 1955 ). The main substance of the paper is: (1) a demonstration that, given certain assumptions, only two basic types of structures are possible for collagen; (2) detailed work on the coordinates and Fourier transforms of one of these models (collagen II), and a comparison between these predictions and the observed X-ray diffraction data.

The molecular structure of polyadenylic acid

The structure of fibers of polyadenylic acid at acid pH has been studied by X-ray diffraction. A model is proposed consisting of two parallel intertwined helical chains, each having a screw of 3·8 A and 45° and related to each other by a dyad axis parallel to the fiber axis. Coordinates, bond distances and angles and the calculated Fourier transform are given for this model. Reasons are given why the quite different model of Morgan & Bear is thought to be wrong.

Diffraction by helical structures

T h e s y m m e t r y of hel ical s t r u c t u r e s a n d t he i r d i f f r ac t i on p a t t e r n s is d i scussed , a n d a l is t is g iven of t h e l ine g r o u p s for e n a n t i o m o r p h i c hel ical s t r u c t u r e s . T h e m a i n b o d y of t h e p a p e r c o n c e r n s t w o specia l k i n d s of p r o j e c t i o n of a hel ical s t r u c t u r e t h e r ad ia l p r o j e c t i o n a n d t h e hel ical p r o j e c t i o n . I t is s h o w n t h a t t he se p r o j e c t i o n s p r o v i d e a v e r y c o n v e n i e n t w a y of t h i n k i n g a b o u t a hel ical s t ruct u r e a n d a n a l y s i n g i ts d i f f r a c t i on p a t t e r n . The t h e o r y of t hese p r o j e c t i o n s is g iven in de ta i l , a n d the i r uses are d i scussed .

Constraints on cortical and thalamic projections : the no-strong-loops hypothesis

The many distinct cortical areas of the macaque monkey visual system can be arranged hierarchically, but not in a unique way. We suggest that the connections between these cortical areas never form strong, directed loops. For connections between the visual cortex and particular thalamic nuclei, we predict that certain types of connections will not be found. If strong, directed loops were to exist, we suggest that the cortex would go into uncontrolled oscillations.

The Complementary Structure of Deoxyribonucleic Acid

This paper describes a possible structure for the paracrystalline form of the sodium salt of deoxyribonucleic acid. The structure consists of two DNA chains wound helically round a common axis, and held together by hydrogen bonds between specific pairs of bases. The assumptions made in deriving the structure are described, and co-ordinates are given for the principal atoms. The structure of the crystalline form is discussed briefly.