DNA Synthesis, Germination, and Growth

There exists some confusion in the published literature because some workers have used the terms DNA synthesis and mitotic cell division synonymously. This is incorrect for as we shall see some cells which never divide synthesize DNA others synthesize DNA many hours before cell division commences. The latter case is easily explained if we consider the cell cycle, which is simplified in the following diagram. 

A number of reports have been published which suggest that DNA synthesis in the cytoplasm precedes that in the nucleus. For example, in castor bean 

Thymidine must first be converted to its triphosphate (TTP) before incorporation as the deoxyribonucleotide (dTTP) into the DNA molecule. Thus two enzymes must be present for thymidine to be utilized (1) thymidine kinase (ATP thymidine 5 phosphotransferase), for phosphorylation of thymidine and (2) DNA polymerase, for incorporation of deoxyribonucleotides on the DNA template. Limiting levels of either of these enzymes in seeds could obviously restrict DNA synthesis and, as a corollary, DNA synthesis could be regulated by either of these enzymes. 

Thymidine kinase activity is very low in dry wheat embryos, but is markedly active by the 36th h after imbibition has started [135]—it is not known when enzyme activity starts to rise. The enzyme consists of two components (P and T), both of which are required for its activity. In corn embryos the T (but not the P) component is absent during the first 36 h after imbibition. Then it starts to be synthesized and the activity of the enzyme rises. Thus, thymidine kinase activity during the early hours after imbibition starts could be limiting DNA synthesis by restricting the utilization of the endogenous pool of thymidine precursors. In germinated peanut axes the activities of thymidine, deoxyuridine and deoxycytidine kinases increase 36 h after imbibition, which also appears to coincide with an increase in DNA synthesis [110]. 

The activity of a soluble DNA polymerase in wheat embryos starts to increase during the first six hours after imbibition commences and continues to do so for at least the 1st day [91]. DNA replication occurs after radicle expansion, some 15 h after the initial hydration of the embryo [90]. The polymerase appears to be synthesized de novo during germination, for if protein synthesis is inhibited during the first nine hours then subsequent DNA synthesis is inhibited. Inhibition of protein synthesis at times after nine hours is progressively less effective in preventing DNA synthesis, presumably indicating that critical levels of the polymerase are synthesized prior to the ninth hour. 

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