General cytogenetics / J. Sybenga.

  • Sybenga, J. (Jacob), 1926-
Date:
1972
Catalogue details

Licence: Attribution-NonCommercial 4.0 International (CC BY-NC 4.0)

Credit: General cytogenetics / J. Sybenga. Source: Wellcome Collection.

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    §1.1 Background 5 Opposite to cytosine comes guanine (and vice versa). For stereo¬ chemical reasons other combinations are exchided. Hydrogen bonds form between the bases of the old and the new chains; two between thymine and adenine and three between cytosine and guanine. The orientation of the new chain is such that its polarity is reversed compared to that of the old chain. The two chains normally remain together: DNA is double stranded (fig. 1.2). Be¬ fore new synthesis can take place, the two strands must be sepa¬ rated. It might be expected that both strands are synthesized con¬ tinuously starting from one initiation point. Then, because of the opposite polarity of the two strands, synthesis in respect to polari¬ ty would be into one direction in one strand and into the other direction in the other strand. There are strong indications that this is not the case; one strand is synthesized continuously, but the other strand is synthesized in sections (Okazaki fragments) in the same direction in respect to polarity, i.e. into the opposite direc¬ tion when considered from the initiation point. The fragments are subsequently (enzymatically) linked together by hgases (fig. 1.3). Some important aspects of the structure of DNA are: (1) DNA normally is double stranded; (2) The two strands (the old and the new one) are not identical but complementary; (3) The two strands have opposite polarity; (4) In the total DNA there are equal quantities of adenine and thymine, and of guanine and cytidine, on a molecular basis. This is independent of the sequence or of the relative fre¬ quencies of nucleotides in each chain. 1.1.4. Transcription In a comparable fashion but under different conditions and effected by different enzymes the DNA functions as a template for the formation of chains of RNA (transcription). After comple¬ tion these RNA chains are released from the DNA without form- ^ ing H-bonds. The RNA macro-molecule formed on the DNA is called messenger RNA (mRNA). It is carried to the cytoplasm where it is used as a template for the formation of polypeptide chains. This polypeptide synthesis is carried out by small bodies,