Unit Three. The Continuity of Life
Just as an architect protects building plans from loss or damage by keeping them safe in a central place and issuing only blueprint copies to on-site workers, so your cells protect their DNA instructions by keeping them safe within a central DNA storage area, the nucleus. The DNA never leaves the nucleus. Instead, the process of transcription creates “blueprint” copies of particular genes that are sent out into the cell to direct the assembly of proteins (figure 12.2). These working copies of genes are made of ribonucleic acid (RNA) rather than DNA. Recall that RNA is the same as DNA except that the sugars in RNA have an extra oxygen atom and the pyrimidine base thymine (T) is replaced by a similar pyrimidine base called uracil, U (see figure 3.9).
Figure 12.2. Overview of gene expression in a eukaryotic cell.
The RNA copy of a gene used in the cell to produce a polypeptide is called messenger RNA (mRNA)—it is the messenger that conveys the information from the nucleus to the cytoplasm. The copying process that makes the mRNA is called transcription—just as monks in monasteries used to make copies of manuscripts by faithfully transcribing each letter, so enzymes within the nuclei of your cells make mRNA copies of your genes by faithfully complementing each nucleotide.
In your cells, the transcriber is a large and very sophisticated protein called RNA polymerase. It binds to one strand of a DNA double helix at the promoter site and then moves along the DNA strand like a train engine on a track. Although DNA is double-stranded, the two strands have complementary rather than identical sequences, so RNA polymerase is only able to bind one of the two DNA strands (the one with the promoter-site sequence it recognizes). As RNA polymerase goes along the DNA strand it is copying, it pairs each nucleotide with its complementary RNA version (G with C, A with U), building an mRNA chain in the 5’ to 3’ direction as it moves along the strand (figure 12.3).
Figure 12.3. Transcription.
One of the strands of DNA functions as a template on which nucleotide building blocks are assembled into mRNA by RNA polymerase as it moves along the DNA strand.
Key Learning Outcome 12.2. Transcription is the production of an mRNA copy of a gene by the enzyme RNA polymerase.