Really. Just how dna is replicated?
This is quite clear at least in principle by now. It is based on the specific interaction between A and T, and between G and C. That is, take, for example, the double helix in figure below.
Let us label the left strand as “l” strand and the other “r” strand. (This is the complementary strand of “l”). Suppose that you separate the two strands and the “l” strand is isolated. Then you provide a pool of components A, C, G, and T and a means to bind nucleotides (enzyme called DNA polymerase) for the “l” strand.
This enzyme binds nucleotides one by one sequentially. The top bead A on the “l” strand binds a bead T (laterally through hydrogen bond), and next another bead on “l” binds laterally a bead T. Beads T and T are then connected through the phosphate group by the enzyme.
Next the bead G on “l” binds a bead C, and the bead C then is connected to the previous T on the right hand by the enzyme. This is repeated; then you see that an “l” strand will reproduce the complementary “r” strand. The reverse will also be true; i.e., an “r” strand will reproduce the corresponding “l” strand.
Thus, a double strand will have been replicated. How this is accomplished, i.e., mechanics of these chemical reactions are currently very intensely studied, is beyond the level of this book.
Hence, this topic will not be pursued further here. But, the very basic reason why we are like our parents or in other words why a gene molecule (DNA) is (almost) faithfully replicated and transmitted to a progeny can be understood as in the previous paragraph.
This replication mechanism of DNA, however, applies to only cell division. The issue of inheritance in sexual organisms like us is a little more complicated, because we get half of the gene from mother and the other half from father.
But again we are not able to elaborate on this issue here. The issue is more of biology (so-called genetics) than chemistry. The chemical principles are about the same.
We said, “DNA is (almost) faithfully replicated” in the paragraph above. The qualification “almost” implies that replication may not always be exact. In other words, a cell may make mistakes in replicating a DNA. It happens not very often, but frequently enough. If this happens, a wrong DNA may form, which would give
wrong information.
Mistakes can be caused by some factors (some cancer causing factors, for example) or without any particularly cause. The distinction between the right combination A–T/G–C and wrong combinations such as A–C/G–T is not quite definite.
Chemically speaking, the difference in interaction energy between the right and the wrong combination is not very great. Hence, there is some chance that the DNA-making mechanism may simply connect wrong nucleotides occasionally. This may be disastrous to the organism.
Therefore, many DNA-making mechanisms (DNA polymerases) contain in it three functions. One is polymerizing nucleotides (making DNA chain), of course.
The other two are monitoring and repairing mechanisms. It monitors what nucleotides are connected and can identify a wrong one. When it has recognized a wrong one, the repairing mechanism snips off the wrong one.
And then the polymerase portion reconnects another; this time a right one, hopefully. There are many other mechanisms known in organisms that repair “damaged” DNAs. All these are chemical reactions, but too complex to be talked about here. It is also to be noted that these occasional changes in DNA are the ultimate cause of change of species, i.e., evolution.
No comments:
Post a Comment