In all cells, the genetic message encoded in the DNA is first transcribed into a faithful RNA copy of the DNA molecule. The two key differences between DNA and RNA molecules are that the sugar in RNA is ribose instead of deoxyribose, and the base uracil replaces thymine (figure 4.1). In addition, RNA molecules are not double helices like DNA. We say that RNA is a single-stranded molecule, not a
Figure 4.2 DNA and its mRNA Copy. A short piece of double-stranded DNA, represented by letters, and its mRNA copy.
double helix (or a double-stranded molecule) like DNA. However, the base sequence found in messenger RNA (mRNA), as RNA copies of DNA are known, has the exact same base sequence as that found in DNA, with U replacing T (figure 4.2).
The hardware that copies genes present in the DNA is called RNA polymerase. This enzyme reads the base sequence of DNA and builds an RNA copy of each gene in the DNA molecule. To do this, RNA polymerase binds tightly to DNA, moves along it, and makes an RNA copy using RNA building blocks in the cell. This process is like copying DNA, except that only one of the two DNA strands is copied, thus the RNA product is released as a single-stranded RNA molecule (figure 4.3). Also, each gene is copied separately by RNA polymerase. For this, the base sequence of each gene has at the beginning what is called a "promoter region." This is the part of the DNA that RNA polymerase recognizes as the "start" signal for a gene. Think about a promoter as being a DNA base-pair sequence that tells RNA polymerase to "begin here." Transcription of the gene can then proceed. When RNA polymerase reaches the end of the gene, a terminator region, also made of a special base sequence, tells RNA polymerase to "stop transcribing here." The RNA produced is called messenger RNA (or mRNA). RNA polymerase then falls off the DNA and is ready to perform another round of transcription (figure 4.4).
Thus, a cell contains a large collection of different messenger RNA molecules, each a copy of a particular gene. Bacteria typically harbor a few thousand genes, while humans have approximately 35,000. This means that bacterial cells can contain a few thousand different messenger RNA molecules, while human cells can contain tens of thousands. There may be just a few messenger RNA for one gene and
Figure 4.3 The Beginning of Transcription. A diagram illustrating how transcription starts with the DNA double strand opened up and single-stranded mRNA copied from one of the DNA strands. The lighter-shaded line represents the sugar-phosphate backbones of the DNA and RNA. The rectangular blocks hanging from these backbones represent the bases. The bases of mRNA are complementary to the DNA bases from which they are being copied.
hundreds of messenger RNAs for another gene. The lengths of messenger RNA molecules also vary from about 1,000 bases in bacteria to tens of thousands of bases in humans. This is because bacterial genes are in general much shorter than human genes.
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