Genetic structure and function / P.F. Smith Keary.

  • Smith-Keary, P. F.
Date:
1975
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Licence: Attribution-NonCommercial 4.0 International (CC BY-NC 4.0)

Credit: Genetic structure and function / P.F. Smith Keary. Source: Wellcome Collection.

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    # 20 GENETIC STRUCTURE AND FUNCTION the 5' position (the top of the molecule in figure 2.6) then the other end will have a carbon in the З' position. A consequence of this polarity is that if a nucleotide is inverted in relation to the other nucleotides it cannot be attached onto an extending polynucleotide chain. Adenine, guanine, thymine (or uracil in RNA) and cytosine are the bases most commonly found in DNA and RNA, but there are also less common bases which can arise by the addition of methyl or hydroxyl groups to one of the standard bases. For example, in some bacteriophages (these are viruses which are obligate parasites of bacteria) cytosine is replaced by hydroxy-methylcytosine (HMC), while in some higher organisms methylcytosine is found. The Biological Structure of DNA At the time that Avery and his co-workers showed that DNA was the active component of the transforming principle it was generally thought that DNA consisted of a regularly repeating array of the four nucleotides; that is to say, the four bases recurred in a regular sequence so that DNA was of the same composition no matter what its source and so incapable of carrying genetic specificity (this was the tetranucleotide theory). Even after Avery's experiments many contemporary workers still believed that proteins were the genetic material. Then, between 1949 and 1951, Erwin Chargaff used a new technique to make more refined analyses of the base compositions of DNA from various sources and made two very significant observations. Firstly he found that the base composition of DNA varies v^dely according to its biological source, so that DNA was not the uniform polymer as had been wddely believed up to then and it could therefore carry the genetic information. Secondly he found that there were always equimolar amounts (i.e. the equivalent numbers of molecules) of adenine (A) and thymine (T) and of cytosine (C) and guanine (G) so that the ratio A/T = G/C = 1 (table 2.1); we will Table 2.1 Base ratios in different organisms Note that although there are always equivalent amounts of adenine and thymine, and of guanine and cytosine, the overall composition of the DNA varies considerably from one organism to another, whereas DNA from different tissues of the same organism always has the same composition.