Replication
Replication means doubling of the DNA. In a strictly formal sense have we already discussed it when talking about DNA as a molecule and the cell cycle. A new strand is synthesized at each of the mother strands so that two equal molecules exist after the process. It occurs during the S-phase of the cell cycle. During the subsequent cell division is one of the two new DNAs (a chromatid) distributed onto each of the two daughter cells. The chromatid contains one parental and one new strand (= semiconservative replication). Semiconservative replication was proven to exist in higher plants with Nicotiana tabacum.
It is of no small importance that replication occurs with the highest precision since mistakes can usually not be repaired afterwards. Therefore exist a number of control mechanisms that remove DNA segments containing mistakes and substitute them with correct ones.
All DNA polymerases synthesize from 5' > 3'.
Three different polymerases can be found in all eucaryotic cells ( including plant cells): alpha, beta and gamma. They differ in their molecular weights. Alpha- and gamma-polymerases are said to function in replication while the beta-polymerase is a repair enzyme. One polymerase each exists for the replication of chloroplast and mitochondrial DNA.
To obtain two double strands have the hydrogen bonds between the parental strands to be opened first (melting of the DNA).
We saw already when talking about reciprocal actions that enzymes are not necessarily needed for the opening of hydrogen bonds. But highly organized structures like the WATSON-CRICK-double helix are not only stabilized by a large number of hydrogen bonds but also by van der Waals interactions between the stacked base pairs (= stacking energy). These forces together are strong enough to withstand the thermal movements under physiological conditions. This makes the help of enzymes necessary. They are called unwinding proteins and can be found in large quantities in bacterial cells. They have also been isolated from yeast and animal cells. DNA polymerases do not initiate a new synthesis at any site. They cannot even start by their own and do just elongate existing polynucleotide chains.
A DNA-dependent RNA polymerase that initiates synthesis by forming a short piece of RNA (a primer) that is subsequently elongated with deoxyribonucleic acids is therefore needed. The primer is afterwards removed by a repair enzyme and substituted by deoxyribonucleotides.
The starting site for the replication on a DNA is called origin of replication. While the replication of one of the two strands can be easily explained at least in a strictly formal sense, is that of the respective complementary strand much more complicated. Since the enzyme is unidirectional would it have to be synthesized backwards leading to a number of little fragments.
This is actually the case. After their discovery were these fragments termed OKAZAKI fragments. In plants are they roughly 200 nucleotides long. In a second step are they united in a single long strand by a ligase. Good proof exists for the assumption that a chromatid consists of just one uninterrupted DNA molecule. It is some mm to several cm long. When remembering the DNA polymerase's speed of synthesis it becomes immediately clear that the short period of 10-20 h maximum of the S-phase is never enough to finish the whole replication. A further difficulty is that the DNA double helix cannot be taken apart without constantly rotating around its own axis. Both problems are by-passed in the cell:
Replication starts simultaneously at numerous sites. The replication units' length is species-specific.
To set the torsion energy free is the replicated strand split at regular intervals, unwound and repaired again afterwards.
Usually are replication and cell division coupled processes but
replication does not necessarily cause a cell division. Endopolyploidy
is common in plants, i.e. the DNA and chromosomal numbers are
multiplied but no cell division takes place. The chromosomes can
either stay within one nucleus that becomes polyploid or mitosis
can occur during which two nuclei are formed. Cells with several
nuclei are called polyenergid.
Besides polyploidy occurs also polyteny. This means that the number of chromatids per chromosome is multiplied. The prime example are giant chromosomes.
All statements made until now are based on the assumption that a DNA molecule is replicated simultaneously over its whole length. This is not always the case. In a number of plant and animal species is only the euchromatin replicated during polyploidy or polyteny. In such a case is it spoken of amplification. The cases just mentioned occur frequently in plants. The following conclusions can be drawn based on experimental data:
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Most differentiated cells of nearly all angiosperms (and the green plants of other plant groups as well) contain more DNA than a diploid genome has. |
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The selective multiplication of DNA is either based on the multiplication of a large (though incomplete) portion of the genome or on the enhanced amplification of just a small portion. |
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Proof has accumulated that such selective replications are necessary for cell differentiation, the function of the cell and morphogenesis (organ and tissue formation). As a consequence is a cell modified this way not omnipotent any more. |
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Polyploidisation (both complete and partial) has to be regarded in connection with the DNA evolution and thus with the evolution of species. |
© Peter v. Sengbusch - b-online@botanik.uni-hamburg.de