The Creation Explanation
|Life -- Miracle, Not Accident|
Transcription of RNA on DNA Templates
In order for the information encoded in the DNA molecules of genes to have effective meaning in the life of cells, that information must be transmitted and translated. The first of these two steps begins with a process called transcription.
1. The genetic information contained in the nucleotide sequence of DNA is not directly translated into the amino acid sequences of protein molecules. Instead, DNA serves as a template upon which a similarly constructed RNA (ribonucleic acid) molecule is formed. In eukaryotic cells transcription takes place in the nucleus.
2. The RNA molecule is a chain of ribonucleotides very similar to the deoxyribonucleotides of DNA, except that for the sugar deoxyribose is substituted ribose, and uridine is substituted for thymidine. Thus the four RNA ribonucleotides are:
3. Hydrogen bonded pairs are formed between the deoxyribonucleotides of the DNA template and free ribonucleotides which are to be bonded together to form a complementary RNA chain.
An enzyme called RNA polymerase II stitches the ribonucleotides together. The result is an RNA chain whose nucleotides are complementary to the nucleotides in the DNA template chain or gene. Thus DNA serves as a template to direct the formation of a precisely ordered complementary RNA chain. This RNA chain carries the same information as the DNA gene. Therefore, it is called messenger RNA (mRNA), for it carries the information of a gene to a ribosome which produces a protein molecule.
4. When RNA is produced in a cell, normally one gene or a small group of closely related genes called an operon is transcribed from the DNA chain at one time. A gene is a section of DNA chain containing on the average from; 600 to 1,800 nucleotides. The principal enzyme involved is RNA polymerase. In prokaryotes this is a single complex protein molecule formed of six separate folded amino acid chains containing a total of about 4,700 amino acid molecules. One of these chains, a protein molecule called sigma-factor, reads a start signal on the DNA chain. Then there is an entirely separate protein, the rho-factor, which stops the transcription when one gene or operon has been copied from DNA to RNA. In the eukaryotes which include, of course, the higher animals, plants and humans, there are three different RNA polymerases composed of nine to eleven amino acid chains constructed from perhaps 9,000 amino acid molecules. Transcription in the eukaryotes is catalyzed by polymerase II assisted by at least six other very complex proteins. Fundamental details of this process are the object of continuing research.13
For each gene, just one of the two complementary DNA chains is transcribed to RNA. Somehow the correct chain is transcribed. The process of transcription may be represented as follows:
The DNA double helix is partially unwound and the two strands partially separated so that the strand which is to be transcribed can serve as the template on which the RNA transcript is formed. This RNA is called "messenger RNA" (mRNA), because it transmits the instructions in a DNA gene to the places in the cell where protein molecules are being constructed. Protein molecules are constructed in the cytoplasm, outside of the nucleus where the mRNA is produced.
The description given above may seem complex, but it is actually a highly simplified outline only of major parts of the process of transcription. In fact, many important details are not included and many are still not understood.
13. Drapkin, Ronny and Danny Reinberg, Nature, Vol. 369, 16 June 1994, pp. 523-424.