Replication of DNA

The Replication of DNA

Written & Illustrated by: Athena Roberts

Cover by: David Read

Written: May 1985

DNA:

Basic Information

Scientists often refer to DNA as "the wondrous ladder." Without this precious molecule, no life as we know it would be possible. It causes all life on Earth to exist. It contains the information on how to build the tree this paper was made out of, and it's in every cell in the human body. It's in every cell because of its ability to replicate itself. The idea of replication is simple, but a few things need to be known to understand how it takes place.

DNA is an abbreviation for Deoxyribonucleic Acid. It is two long strands of molecules, held together by hydrogen bonds. It is so thin that its details can't be seen even with an electron microscope. Nevertheless, the average DNA molecule is about 16 inches long, and if you tied four of them together, it would be long enough for a you to use as a jump rope.

DNA has a double helix shape with hydrogen bonds joining the two strands together. The two outer edges of DNA are made of alternating units of phosphoric acid and deoxyribose, (see figure 1.1) a sugar, and contain no information about how to build anything. Between the strands are the little "steps" that contain all the information. Pairs of four different types of base molecules make up the steps. They are: Adenine, Thymine, Cytosine, and Guanine.

The molecules can pair with each other as follows: A-T, T-A, C-G, G-C (see figure 1.2). No other combination is possible since A and T only have two places to form hydrogen bonds, and C and G have three places. The molecules pairing like this is "base pairing" and forms the hydrogen bonds that hold the DNA together. Each base attaches to one of the "ladder's" side pieces. One base, phosphate, and deoxyribose molecule put together is a nucleotide (see figure 1.3). There are different types of nucleotides. They are Adenosine phosphate, Thymidine phosphate, Guanosine phosphate, and Cytidine phosphate.

Replication

During mitosis (when a cell divides), DNA duplicates itself. There are three main ideas (see figure 1.4) about how it does this. Conservative Replication is one idea. That means that after the replication is over, there are an old DNA molecule and a new one with neither containing parts from the other. There is some evidence that supports that, however it is not widely accepted. The second one is Dispersive Replication. That is the theory that each new molecule has mixed old and new strands in each side. The dispersive idea actually occurs to a limited extent during genetic repair. The last idea, and probably the most likely, is the one proposed by Watson & Crick (the scientists who discovered that DNA is a double helix). Its name is Semi-Conservative Replication. That is the idea that the new molecules each contain one old and one new strand. The experiments of Taylor, Meselson, and Stahl (other genetic research scientists) support that idea. Other experiments later proved it as a fact, so it will be the one I write about.

When a cell starts to make preparations to divide, the DNA duplication starts. Two complete sets of chromosomes must form before the cell divides so each new cell has a complete set. The DNA strands move to a replisome for duplication. Replisomes are tiny packs of protein that occur in a large number of fixed points inside the nucleus. The DNA slips through the replisome forming a series of loops (see figure 1.5) in and out of it. As it moves through replisome, it splits in half (see figure 1.6) thus breaking its hydrogen bonds. The enzyme, DNA polymerase, (see figure 1.7) then connects the nucleotide molecules, in the replisome, to one of the two strands. Guanine joins to cytosine and thymine joins to adenine. This joining happens at a one-at-a-time pace in a specific sequence. After a while, a new chain starts to build up on each side. The new chain is a ploynucleic chain (see figure 1.8). This operation goes on in several different places at once until two exact duplicates of the original strand exist. Now the process is complete and two new and complete strands of DNA exist for each new cell. Duplication is very accurate because of base pairing and special replicating enzymes. How the duplication takes place without everything getting tangled up in the nucleus is not fully known. Each chromosome goes through that process until they are all duplicated. Now, after everything else is taken care of, the cell will divide and form two daughter cells. Then, the process can start all over again.

End Notes

(1) Figures 1.1, 1.3, 1.5-1.8 are from "Elements of General and Biological Chemistry" by John R. Holum. Copyrighted in 1983.

(2) Figure 1.2 is from "General, Organic, and Biological Chemistry" by M. Lynn James and James O. Schreck. Copyrighted in 1982.

(3) Figure 1.4 is from "Molecular Genetics" by A. Gib DeBusk. Copyrighted in 1968.

Bibliography

(1) DeBusk, A. Gib. Molecular Genetics. New York: The Macmillan Company, 1968.

(2) James, M. Lynn & Chreck, James O. General, Organic, and Biological Chemistry. Lexington, MA: D.C. Health and Co., 1982.

(3) Hollum, John R. Elements of General and Biological Chemistry. New York: John Wiley & Sons, 1983.

(4) Light, Robley J. A Brief Introduction to Biochemistry. New York: W.A. Benjamin, 1968.

(5) Rubenstein, Irwin, "Cell," World Book Encyclopedia., 1980 edition, Vol. 3, pg. 250-251.