Now, you know about the basic structure of DNA. You know it holds the information to build you. Now, we will look into this fascinating molecule more deeply and explore a little more on how this little acid can do what it does.
DNA reproduces, just like organisms. However, its method of duplicating itself is nothing like that of humans. Remember how DNA is made up of two sides? Also, each base must be paired to another specific base? Let's take a look at this example. Going down the left side, you can see the sequence is C A G T. The right side reads G T C A. Cover up the right side. Can you tell, without looking, what the right side will read? That's right, you can, by using the base-pair matchups we discussed earlier. If the left side is C, the right side must be G. Going down, you can fill in each of the missing spots. You can fill in the left side, too, if you had just the right side.
When DNA duplicates itself, it "unzips." This means it unwinds so that it is no longer a spiral, but more like a ladder. Then, enzymes cut the rungs so that each side is now on its own, away from the other side. Enzymes called DNA polymerase come in and they fill in the missing side, much as you just did. Look at the animation. It finds the right base-pair to attach, and attaches a nucleotide with the correct base. Then, the two bind and hold together. It does this for both sides. Do you see what the result will be? There will now be two full strands of double DNA. The animation on the right is showing how the body determines which nucleotide to use. You can see how the nucleotide shapes fit together. On the left, the actual process of replication is represented. The strands unzip, and the enzyme DNA polymerase assists in filling in the blanks. Soon, a full double strand arises from each half.
In bacteria, DNA is identical in structure to ours in all respects but one. The DNA in bacteria occurs in a loop, with no head or tail. The process of replication is still the same.
The enzymes described in Part 1 have to duplicate 3 billion nucleotides. Some DNA polymerase proofreads its own work! It reads the base it places into the DNA and replaces it if it finds an error. This, in addition to the fact that the enzymes make few mistakes in the first place, produces an enviable record of 1 mistake per billion nucleotides! Enzymes may not be perfect, but they're pretty close.
However, some changes enzymes make by mistake are not detected and become a part of a cell's genome, or the total of all genes in a cell. A change could also be brought on by a mutagen, something that causes such a permanent change. A mutagen may be radiation, a chemical, or any other cause of mutation. These mutations can be good, since they could lead to a trait in the organism that makes it more adapted to the environment. For example, a mutation could cause the lengthening of the wool on sheep that live in Canada. This mutation lets the sheep endure the winter more easily, and should therefore survive and be passed on. This is the first step in evolution. Look at the illustration. One sheep becomes long-haired and survives a freeze, while many of its companions do not. The ones left are somewhat weakened. During mating season, the long-haired sheep mates and produces two long-haired offspring, while its flockmates may only produce 1, or maybe no offspring at all. This proportion gradually changes through the generations, and after some time, most of the flock is long-haired -- and well adapted to the environment, thanks to one little mutation.
By now, you should already know that DNA occurs in a double-helix. However, there are different forms of twisting that scientists believe may involve more than an aesthetic adjustment.
There can be many forms of spirals. There are spirals, like a spring, that are smooth and conformed. There are others, like a coil of chained paper clips, that have little zip-zag patterns within the spiral. DNA is like this, only it changes between the two forms. When it is a smooth spiral, it is called B-DNA and when it is zigzagged it is called Z-DNA . Scientists have theories as to why DNA changes from form to form, and how. These questions, however, are not that easily answered. One popular hypothesis is that the shift to Z-DNA causes the genes encoded in the DNA to be "turned on". When a gene is "turned on", it is being read by the cell and the instructions on it carried out. If a gene that makes an enzyme is turned on, that enzyme will be made by the cell's factory. This is called protein synthesis and will be discussed in later levels. However, the B-DNA form of DNA is much more stable, and DNA is normally kept in this state.
There is still much more to the shape of DNA than we've discussed. DNA is 3 billion links long, and obviously cannot just stretch in a line; it has to be wound in a way so that it can fit into a cell's nucleus, when it isn't in active use.
DNA winds itself into chromosomes . However, the winding is much more complex than coiling in a chromosome. The chromosome shown here on the left is a drawing, and is magnified thousands of times. A human has 46 of these in most of the nuclei in the body. In fact, there are many other winding structures beneath this chromosome. If you unwound it, you would find another yarn-like coil of DNA, and that is made of a coiled DNA wound around beads, and this goes on for awhile. Lost? Imagine a thick rope, like ones used in tug-of-war. You can tell, either by gripping it or looking at it, that it is actually made up of smaller components -- smaller ropes. If you unwind the tug-of-war rope, you can get several smaller ropes. Each of these smaller ropes is made of even smaller yarnlike strings, and you can keep unwinding until you get only have one fiber. The tug-of-war rope is made of many, many of these fibers, wound upon itself many times, to form a thick, coarse rope. A chromosome is like this rope; it is made of many smaller "ropes".
When DNA winds into a chromosome, it goes through many steps. First, it winds into nucleosomes. A nucleosome is actually nothing more than a histones protein bead wrapped in the DNA. There are many nucleosomes made by winding DNA. These nucleosomes now wind and stack up. The stacked up nucleosomes are wound into a sprial too, and this new spiral is called chromatin. It hasn't gotten that much tighter yet, so it's still called extended chromatin. The extended chromatin literally scrunches up and winds into yet another helix. This new helix is designated condensed chromatin. Condensed chromatin is now finally wound into a chromosome.
Before you take the quiz, play the DNA polymerase game to sharpen your skills at replicating DNA. Play the part of DNA polymerase and drag and drop to choose the right base-pair.
You then be ready for the quiz.
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