10. D N A A N D R N A S Y N T H E S I S
319
B. S T R U C T U R E
Although it was at first thought that the nucleotide bases in D N A
were present in approximately equivalent amounts, Chargaff, Zamenhof and Green (1950) and Chargaff (1955) subsequently demonstrated
that the base composition of D N A in a tissue was characteristic of the
organism from which it was derived; however, in all the DNAs
examined, the adenine content is approximately equivalent to the
thymine content, and the guanine approximately equivalent to the
cytosine content. The X-ray crystallographic work of Astbury (1947)
had demonstrated a repeat distance of 3*4 A in the structure of DNA,
and later work on D N A fibres by Wilkins, Stokes and Wilson (1953) and
by Franklin and Gosling (1953) showed a diffraction pattern indicating a helical structure. O n the basis of the above evidence,
Watson and Crick (1953) proposed that the D N A molecule contains
two intertwined helical chains. These sugar-phosphate chains are held
together by hydrogen bonding between a purine base attached to one
chain and a pyrimidine base attached to the other: adenine is paired
with thymine and guanine with cytosine. At the same time, Watson
and Crick pointed out that the complementary structure of the twinstranded D N A helix suggested a means for self-duplication, in that if
the two chains can unwind and separate, each can serve as a "template"
for the formation of its complementary chain.
Subsequent crystallographic work by Wilkins and collaborators
(Feughelman, Langridge, Seeds, Stokes, Wilson, Hooper, Wilkins,
Barclay and Hamilton, 1955; Hamilton, Barclay, Wilkins, Brown,
Wilson, Marvin, Ephrussi-Taylor and Simmons, 1959) has shown that
the D N A extracted either from cells in a "resting" state (e.g. sperm)
or from rapidly dividing normal and tumour tissues always gives a
similar diffraction pattern consistent with the double-helix model. In
addition, X-ray diffraction patterns of sperm heads, of calf thymus
nuclei and of extracted nucleo-proteins have been obtained: these
patterns are not as well-defined as those for extracted D N A , but it has
been concluded that the double helical structure of D N A exists within
the cell and is not an artefact. The X-ray diffraction patterns for R N A
(Rich and Watson, 1954; Zubay and Wilkins, 1960) are less clear, for
although the photographs contain the elements of a helical diffraction
pattern, they have not the high degree of regularity shown by D N A .
Evidence consistent with the "complementary" mode of replication
of DNA, as suggested by the double helical model (Watson and Crick,
1953) was provided by the experiments of Levinthal (1956) and
Meselson and Stahl (1958). Meselson and Stahl grew a population of
319
B. S T R U C T U R E
Although it was at first thought that the nucleotide bases in D N A
were present in approximately equivalent amounts, Chargaff, Zamenhof and Green (1950) and Chargaff (1955) subsequently demonstrated
that the base composition of D N A in a tissue was characteristic of the
organism from which it was derived; however, in all the DNAs
examined, the adenine content is approximately equivalent to the
thymine content, and the guanine approximately equivalent to the
cytosine content. The X-ray crystallographic work of Astbury (1947)
had demonstrated a repeat distance of 3*4 A in the structure of DNA,
and later work on D N A fibres by Wilkins, Stokes and Wilson (1953) and
by Franklin and Gosling (1953) showed a diffraction pattern indicating a helical structure. O n the basis of the above evidence,
Watson and Crick (1953) proposed that the D N A molecule contains
two intertwined helical chains. These sugar-phosphate chains are held
together by hydrogen bonding between a purine base attached to one
chain and a pyrimidine base attached to the other: adenine is paired
with thymine and guanine with cytosine. At the same time, Watson
and Crick pointed out that the complementary structure of the twinstranded D N A helix suggested a means for self-duplication, in that if
the two chains can unwind and separate, each can serve as a "template"
for the formation of its complementary chain.
Subsequent crystallographic work by Wilkins and collaborators
(Feughelman, Langridge, Seeds, Stokes, Wilson, Hooper, Wilkins,
Barclay and Hamilton, 1955; Hamilton, Barclay, Wilkins, Brown,
Wilson, Marvin, Ephrussi-Taylor and Simmons, 1959) has shown that
the D N A extracted either from cells in a "resting" state (e.g. sperm)
or from rapidly dividing normal and tumour tissues always gives a
similar diffraction pattern consistent with the double-helix model. In
addition, X-ray diffraction patterns of sperm heads, of calf thymus
nuclei and of extracted nucleo-proteins have been obtained: these
patterns are not as well-defined as those for extracted D N A , but it has
been concluded that the double helical structure of D N A exists within
the cell and is not an artefact. The X-ray diffraction patterns for R N A
(Rich and Watson, 1954; Zubay and Wilkins, 1960) are less clear, for
although the photographs contain the elements of a helical diffraction
pattern, they have not the high degree of regularity shown by D N A .
Evidence consistent with the "complementary" mode of replication
of DNA, as suggested by the double helical model (Watson and Crick,
1953) was provided by the experiments of Levinthal (1956) and
Meselson and Stahl (1958). Meselson and Stahl grew a population of