TRANSCRIPTION
Learning Objectives for this Section
During transcription (def), messenger RNA (mRNA) (def) is synthesized by complementary base pairing (def) of ribonucleotides (def) with deoxyribonucleotides (def) to match a portion of one strand of DNA called a gene (def). Subsequently, ribosomal subunits attach to the mRNA and amino acids are joined to form a polypeptide or a protein through a process called translation.
A. Transcription in Prokaryotic Cells
DNA is transcribed by the enzyme RNA polymerase (def). This enzyme initiates transcription, joins the RNA nucleotides together, and terminates transcription. To initiate transcription in bacteria, a variety of proteins called sigma factors bind to RNA polymerases. This complex can then bind to a specific sequence of usually about 40 deoxyribonucleotide bases called the promoter (def) located along the DNA prior to the coding region of the gene. The promotor determines what region of the DNA and which strand of DNA will be transcribed into RNA. Like DNA polymerase, RNA polymerase can only synthesize nucleic acid in a 5' to 3' direction while "reading" a DNA template in the 3' to 5' direction. As mentioned earlier in this unit, the 3' end (def) of a strand of nucleic acid has a hydroxyl (OH) group on the 3' carbon of the deoxyribose or ribose and is not linked to another nucleotide. The 5' end (def) of that strand has a phosphate group attached to the 5' carbon of the sugar and is not linked to another nucleotide (see Fig. 1).
Once the RNA polymerase/sigma factor complex recognizes the correct promoter, the sigma factor dissociate from the RNA polymerase and the enzyme begins to unwind the helix of the DNA creating a region of nonpaired deoxyribonucleotides that serve as a template for RNA synthesis (see Figs. 2 and 3).
While the RNA polymerase does not transcribe the promoter itself, it does transcribe a short noncoding leader sequence (def) just prior to the coding sequence (def) of the gene. The leader sequence is the portion of DNA that is transcribed into the ribosome binding site of the mRNA (see below under translation.) The coding sequence contains the actual message for protein synthesis.
Once the actual transcription begins, ribonucleotides containing 3 phosphate groups hydrogen bond through the process of complementary base (def) pairing with the exposed deoxyribonucleotides on the unwound strand that is to be transcribed (see Fig. 4). The ribonucleotides are then covalently bonded together by phosphodiester bonds (def), the energy being supplied by the cleavage of two phosphate groups from the ribonucleotide triphosphate (see Fig. 5). (The phosphodiester bond refers to the phosphate on the 5'C of the newly inserted nucleotide covalently bonding to the 3'C of the last ribonucleotide in the mRNA chain.) The mRNA polymerizes at a rate of about 30 nucleotides per second.
As the RNA polymerase moves down the DNA, the previous stretch of DNA again pairs with its complementary strand. This process continues until the RNA polymerase encounters a "stop" signal or transcription terminator (def) at the end of the gene. This causes the completed mRNA to drop off the gene.
Once the RNA polymerase moves beyond the promotor region, a new molecule of RNA polymerase can bind to the promotor and start a new round of transcription. In this way, a single gene can be transcribed multiple times.
Transcription is summarized in Figs. 6 and 7.
B. Transcription in Eukaryotic Cells
Transcription (def) is more complex in eukaryotic cells than in those that are prokaryotic. Activator proteins (def) bind to genes known as enhancers (def) which help determine which genes are switched on and speed up transcription. Repressor proteins (def) bind to genes called silencers (def) which interfere with activator proteins and slow down transcription. Coactivators (def), adapter molecules which coordinate signals from activator and repressor proteins, relay this information to basal factors (def) which then position RNA polymerase (def) at the start of the coding region of the gene to begin transcription.
Once the actual transcription begins, ribonucleotides containing 3 phosphate groups form hydrogen bonds through the process of complementary base (def) pairing with the exposed deoxyribonucleotides on the unwound strand that is to be transcribed. The ribonucleotides are then covalently bonded together by phosphodiester bonds (def), the energy being supplied by the cleavage of two phosphate groups from the ribonucleotide triphosphate (see Fig. 5). (The phosphodiester bond refers to the phosphate on the 5'C of the newly inserted nucleotide covalently bonding to the 3'C of the last ribonucleotide in the mRNA chain.)
Unlike prokaryotes, most genes in higher eukaryotic cells contain large amounts - often 50-75% - of noncoding regions called interons (def) that are not part of the code for the final protein. These are interspersed among the coding regions or exons (def) that actually code for the final protein. RNA polymerase copies both the exons and the interons to form what is called precursor mRNA or pre-mRNA (def). Early in transcription, a cap in the form of an unusual nucleotide, 7-methylguanylate, is added to the 5' end of the pre-mRNA. This cap helps ribosomes attach for translation (def). As transcription is nearly completed, a series of 100-250 adenine ribonucleotides called a poly-A tail is added to the 3' end of the pre-mRNA. This poly-A tail is thought to help transport the mRNA out of the nucleus and may stabilize the mRNA against degradation in the cytoplasm. After transcription of the precursor mRNA, noncoding regions (interons) are excised and coding regions (exons) are joined together by complexes of ribonucleoproteins called spliceosomes to produce what is termed mature mRNA (def) as shown in Fig. 8. This process is called RNA processing (def).
The mature mRNA then passes through the pores in the nuclear membrane to be translated (def) into protein by tRNA (def) on 80S (def) ribosomes (def) (composed of 60S and 40S subunits) in a manner similar to prokaryotes.
The mRNA molecule is divided up into codons. A codon (def) is a series of three consecutive mRNA bases coding for one specific amino acid. The various codons and the amino acids for which they code are shown in Fig. 9. There are 64 codons. One codon, AUG, also serves as a start codon (def) to initiate translation, and three codons, UAG, UAA, and UGA, function as stop or nonsense codons (def) to terminate translation.
In addition to the genes that are transcribed into mRNA to be translated into polypeptides and proteins, there are also specific genes in the DNA from which each of the different transfer RNAs (tRNAs) (def) and the ribosomal RNAs (rRNAs) (def) are transcribed.
Once transcribed, the mRNA can be translated into protein.
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© Gary E. Kaiser
All Rights Reserved
Updated: June 28, 2001