Transcription: control from the promoter and termination sequence
The promoter’s role is to selectively turn on the ALS gene in cells that will need the ALS enzyme. The promoter accomplishes this by its specific nucleotide sequences. We will discuss the regulation of gene expression in more detail later. What cells in a plant will need the ALS enzyme? All cells that are making proteins. Obviously this will be all cells in the plant at some stage of development. Therefore the ALS gene has a promoter sequence that allows the gene to be turned on in all types of cells.
When the ALS gene is turned on, transcription can take place. Like most chemistry in the cell, enzymes will be needed for gene expression. RNA polymerase is the transcription enzyme (Fig. 9) which will bind to the DNA sequences in the ALS promoter and interact with one strand of the gene (the coding strand).
Figure 9a. The promoter has DNA sequences that signal the cell when to turn on the gene and where to start reading the gene information. (image by d. Namuth-Covert)
Figure 9b. RNA Polymerase binds to the promoter to initiate transcription. (Image by D. Namuth-Covert)
RNA polymerase can literally move along the gene promoter until it encounters a series of A and T nucleotides called the TATA box (Fig 10.). The TATA box signals RNA polymerase that it has reached the end of the promoter. Now the enzyme has the “green light” to read the coding strand DNA and make RNA. The procedure works much like DNA replication. RNA polymerase will read the DNA in a 3’ to 5’ direction and build the RNA 5’ to 3’. Unlike DNA replication though, only one DNA strand is read and the transcription process must end once the RNA polymerase reaches the end of the ALS gene. If RNA polymerase kept going along that strand of the DNA making up the chromosome, other genes that are on the same chromosome might be expressed in the wrong cells or wrong times.
Figure 10. RNA polymerase encounters the TATA box at the end of the promoter. This signals where to begin the reading of the coding region. (Image by D. Namuth-Covert)
How is the RNA polymerase signaled that it has reached the end of the gene? That is the role of a termination sequence in the gene (Fig. 11). The termination sequences signal the end of the gene and can work in a number of ways. One transcription termination strategy will be described here.The nucleotides that make up the termination sequence of the ALS gene could be ordered to form a palindrome. This means that the nucleotides being placed in the RNA can fold back on themselves and form a hairpin loop (Figs. 12-14)
Figure 11. When RNA polymerase reads the termination sequence it is signaled to quit reading the gene’s coding strand. (Image by D. Namuth-Covert)
Figure 12. RNA polymerase has just finished reading the coding region and is starting to read the termination sequence. (dashes are unspecified nucleotides). (Image by D. Namuth Covert)
Figure 13. The termination sequence is palindromic. The RNA being made has a sequence that can form a hairpin loop. (Image by D. Lee)
Figure 14. Formation of the hairpin in the newly made RNA disrupts the RNA polymerase, halting transcription. (Image by D. Lee)
While no geneticist has actually viewed this hairpin loop formation in action, it is believed that the hairpin formation snaps the RNA polymerase off the DNA coding strand and releases the RNA message. Transcription of an RNA that has the coding region information is now completed. Each time an RNA polymerase goes through the process, one copy of the RNA is made by reading the DNA template.The type of RNA made from transcription of the ALS gene is called a messenger RNA or mRNA for short. Some genes can encode transfer RNA (tRNA) or ribosomal RNA (rRNA). These RNA molecules have special roles in the next part of gene expression called translation. Once an mRNA is transcribed some modifications will occur in the nucleus. We will describe some of the post-transcriptional processes later. For now lets follow the ALS mRNA onto the translation step of gene expression.
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