DNA REPLICATION - HOW IS DNA REPLICATED? BASIC INFORMATION AND TUTORIALS

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.

MERCURY AND MERCURY POISONING - THE CONTROVERSIAL ELEMENT (THE APPLICATIONS AND DANGERS OF MERCURY)

Virtually all metals exist as solids at room temperature. Mercury is the only metallic element that is a liquid under normal conditions. If cooled to 39°C, it does freeze to a solid. Liquid mercury is shiny and metalliclooking.

You have probably heard a fair amount about the toxicity of mercury. As a liquid, it is not especially toxic when swallowed since most of it passes through the body unchanged.

However, mercury vapor is highly toxic, as are all compounds of mercury that dissolve in water to form solutions. Once they enter the body, these forms of mercury can attack the brain and produce mental and physiological disturbances.

An incident in Texarkana, on the Texas–Arkansas border, illustrated the hazards of handling mercury. Two teenagers stole 40 pounds of liquid mercury from a site where it had been used to make neon lights.

They poured it over themselves and on floors in their homes, gave it out to friends, and even dipped cigarettes into the liquid and smoked them. Within days they began to exhibit the signs of mercury poisoning: coughing up blood, vomiting, breathing difficulties, and seizures.

The end result was that eight contaminated homes were evacuated, a family dog was killed by the vapors, and more than 170 people in the town and surrounding areas received medical treatment for mercury exposure.

Mercury poisoning was much more common in the 19th century when workers who used mercury to cure felt hats developed twitches, spoke incoherently, and drooled as a result of long-term exposure to mercury vapors.

These workers provided Lewis Carroll with a model for the Mad Hatter in Alice in Wonderland. These days, most of the mercury that enters the environment comes from the incineration of waste and sewage sludge, and the burning of coal.

Recently, scrapping cars without removing the elemental mercury used in light switches and other components was identified as a significant source of the element to the environment.

MERCURY DANGERS IN DENTAL CARE?
Until quite recently, an alloy that most people had an intimate acquaintance with was the material used to fill cavities in decayed teeth. You may be surprised to learn that mercury was one of the metals used to fill teeth.

Although mercury is a liquid at room and body temperatures, it forms many alloys, called amalgams, that are solid at normal temperatures. Those having melting points in the 60°C range are useful for fillings, since they can be placed in the decay cavity as a warm liquid metal without causing the patient pain.

The liquid assumes the cavity shape as it cools and solidifies in place. Dental amalgam combines mercury with silver, which imparts resistance to tarnishing and mechanical strength, and about half as much tin, which readily amalgamates with mercury.

When first placed in a tooth, and whenever the filling is involved in the chewing of food, a tiny amount of the mercury is vaporized. Some scientists believe that mercury exposure from this source causes long-term health problems in some individuals, but an expert panel of the U.S. National Institute of Health concluded that dental amalgams do not pose a health risk.

A recent study of adults found that no measure of exposure to mercury—whether the level of the element in the urine or the number of dental fillings—correlated with any measure of mental functioning or fine motor control.