Transcription of RNA [take a quiz on this topic]

Chapter Six: DNA, RNA, and Protein Synthesis Early hypotheses regarding the genetic material The famous DNA experiments DNA's chemical composition and structure The packing of DNA in eukaryotes DNA replication Transcription of RNA Types of RNA and the mRNA code Translation Mutations

George Beadle (image courtesy of Encylopædia Britannica)

Research by such scientists as George Beadle and Edward Tatum suggested that DNA in some way dictates the production of proteins in a cell. However, amino acid molecules cannot simply line up against a DNA molecule as the bases do in DNA replication; amino acids and nitrogenous bases are too unlike one another to possibly have a matching system. Therefore, biologists hypothesized that the ordering of the bases in a DNA molecule indirectly determined what proteins would be produced, and that certain specific patterns of bases would code for different amino acids which would then be bonded together to form proteins.

Edward Tatum (image courtesy of Encylopædia Britannica)

After a great deal of research, scientists determined the general method by which proteins were synthesized based upon the DNA code. First, a molecule of what is called messenger RNA would be synthesized through a process known as transcription based upon the nitrogenous bases in the DNA molecule. The messenger RNA would then move to ribosomes in the endoplasmic reticulum, where the ribosomes would translate the code on the messenger RNA into amino acids. We will discuss this process in detail in the following sections, but we must first discuss the process of RNA transcription. Althought there are several types of RNA, the process of transcription is the same for all of them.

RNA transcription is actually almost identical to the process of DNA replication. The enzyme involved in transcription is called RNA polymerase, and just as a new strand of DNA is formed by matching the corresponding base pairs along side of the original DNA, so an RNA molecule is synthesized by stringing together the appropriate bases as the DNA is unzipped. However, instead of thymine, RNA contains a different base, uracil. So, if a DNA strand read AGGTCG, the RNA strand produced by transcription would read UCCAGC.

You may be wondering how the RNA polymerase knows where to begin and end the transcription of RNA; after all, only certain parts of the DNA molecule need to be copied at certain times. The answer to this question is actually very satisfying; there is a certain sequence of bases on the DNA molecule called the promoter which the RNA polymerase enzyme must attach to in order to begin transcription. The enzyme moves down the DNA molecule, moving toward the DNA's 3' end, until it reaches a second sequence called the termination signal.